diff --git a/LICENSE b/LICENSE
index c71f04c..e47911e 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 BSD 3-Clause License
 
-Copyright (c) 2025, Lixen Wraith
+Copyright (c) 2026, Lixen Wraith
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:
diff --git a/README.md b/README.md
index 56c6e68..bd2a772 100644
--- a/README.md
+++ b/README.md
@@ -53,5 +53,14 @@ server.AddCredential(cred)
 ## Testing
 
 ```bash
-go test -v ./
+go test ./... -race -count=1
+go test ./... -run '^$' -bench . -benchmem
+
+# fuzz targets (run individually)
+go test -run '^$' -fuzz FuzzParsePHC -fuzztime 60s
+go test -run '^$' -fuzz FuzzVerifyPassword -fuzztime 60s
+go test -run '^$' -fuzz FuzzImportCredential -fuzztime 60s
+go test -run '^$' -fuzz FuzzValidateHS256Token -fuzztime 60s
+go test -run '^$' -fuzz FuzzParseBasicAuth -fuzztime 30s
 ```
+
diff --git a/argon2.go b/argon2.go
index 50089c7..7e0c900 100644
--- a/argon2.go
+++ b/argon2.go
@@ -1,9 +1,9 @@
 package auth
 
 import (
-	"crypto/hmac"
 	"crypto/rand"
 	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/base64"
 	"fmt"
 	"strings"
@@ -25,6 +25,17 @@ const (
 	MaxPHCHashLen   = 256
 )
 
+// Execution budget for KDF parameters taken from an encoded record.
+// parsePHC accepts the full PHC range (m <= 4 GiB, t <= 1000, p <= 255) because
+// those values are well-formed; running them is a different decision. One
+// crafted record would otherwise allocate 4 GiB. Applies to any Argon2 run
+// whose parameters were chosen by a peer rather than by this process.
+const (
+	MaxVerifyArgonMemory  = 256 * 1024 // KiB
+	MaxVerifyArgonTime    = 16
+	MaxVerifyArgonThreads = 16
+)
+
 // argonParams holds configurable Argon2id parameters
 type argonParams struct {
 	time    uint32
@@ -86,9 +97,7 @@ func HashPassword(password string, opts ...Option) (string, error) {
 	}
 
 	salt := make([]byte, params.saltLen)
-	if _, err := rand.Read(salt); err != nil {
-		return "", fmt.Errorf("%w: %v", ErrSaltGenerationFailed, err)
-	}
+	rand.Read(salt) // cryptographically secure random bytes
 
 	hash := argon2.IDKey([]byte(password), salt, params.time, params.memory, params.threads, params.keyLen)
 
@@ -139,117 +148,128 @@ func credentialFromSaltedPassword(username string, saltedPassword, salt []byte,
 // PHC format for Argon2id. It validates structure, parameters, and encoding,
 // but does not verify a password against the hash.
 func ValidatePHCHashFormat(phcHash string) error {
-	// Cap total input before any splitting or base64 decoding
-	if len(phcHash) > MaxPHCHashLen {
-		return fmt.Errorf("%w: encoded hash exceeds %d bytes", ErrPHCInvalidFormat, MaxPHCHashLen)
-	}
-
-	parts := strings.Split(phcHash, "$")
-	if len(parts) != 6 {
-		return fmt.Errorf("%w: expected 6 parts, got %d", ErrPHCInvalidFormat, len(parts))
-	}
-
-	// Validate empty parts[0] (PHC format starts with $)
-	if parts[0] != "" {
-		return fmt.Errorf("%w: hash must start with $", ErrPHCInvalidFormat)
-	}
-
-	// Validate algorithm identifier
-	if parts[1] != "argon2id" {
-		return fmt.Errorf("%w: unsupported algorithm %q, expected argon2id", ErrPHCInvalidFormat, parts[1])
-	}
-
-	// Validate version
-	var version int
-	n, err := fmt.Sscanf(parts[2], "v=%d", &version)
-	if err != nil || n != 1 {
-		return fmt.Errorf("%w: invalid version format", ErrPHCInvalidFormat)
-	}
-	if version != argon2.Version {
-		return fmt.Errorf("%w: unsupported version %d, expected %d", ErrPHCInvalidFormat, version, argon2.Version)
-	}
-
-	// Validate parameters
-	var memory, time uint32
-	var threads uint8
-	n, err = fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads)
-	if err != nil || n != 3 {
-		return fmt.Errorf("%w: failed to parse parameters", ErrPHCInvalidFormat)
-	}
-
-	// Validate parameter ranges
-	if time == 0 || memory == 0 || threads == 0 {
-		return fmt.Errorf("%w: parameters must be non-zero", ErrPHCInvalidFormat)
-	}
-	if memory > 4*1024*1024 { // 4GB limit
-		return fmt.Errorf("%w: memory parameter exceeds maximum (4GB)", ErrPHCInvalidFormat)
-	}
-	if time > 1000 { // Reasonable upper bound
-		return fmt.Errorf("%w: time parameter exceeds maximum (1000)", ErrPHCInvalidFormat)
-	}
-	if threads > 255 { // uint8 max, but practically much lower
-		return fmt.Errorf("%w: threads parameter exceeds maximum (255)", ErrPHCInvalidFormat)
-	}
-
-	// Validate salt encoding
-	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
-	if err != nil {
-		return fmt.Errorf("%w: %v", ErrPHCInvalidSalt, err)
-	}
-	if len(salt) < 8 { // Minimum safe salt length
-		return fmt.Errorf("%w: salt too short (%d bytes)", ErrPHCInvalidSalt, len(salt))
-	}
-	if len(salt) > MaxArgonSaltLen {
-		return fmt.Errorf("%w: salt too long (%d bytes)", ErrPHCInvalidSalt, len(salt))
-	}
-
-	// Validate hash encoding
-	hash, err := base64.RawStdEncoding.DecodeString(parts[5])
-	if err != nil {
-		return fmt.Errorf("%w: %v", ErrPHCInvalidHash, err)
-	}
-	if len(hash) < 16 { // Minimum hash length
-		return fmt.Errorf("%w: hash too short (%d bytes)", ErrPHCInvalidHash, len(hash))
-	}
-	if len(hash) > MaxArgonKeyLen {
-		return fmt.Errorf("%w: hash too long (%d bytes)", ErrPHCInvalidHash, len(hash))
-	}
-
-	return nil
+	_, err := parsePHC(phcHash)
+	return err
 }
 
 // parsed + verified PHC material, reused to avoid a second KDF pass
 type phcResult struct {
-	derived []byte // argon2.IDKey output; == SCRAM salted password when len == DefaultArgonKeyLen
-	salt    []byte
-	time    uint32
-	memory  uint32
-	threads uint8
+	derived      []byte
+	expectedHash []byte
+	salt         []byte
+	time         uint32
+	memory       uint32
+	threads      uint8
+}
+
+// verifyPHC validates format, bounds the password, runs the KDF once,
+// and constant-time compares against the encoded digest.
+func parsePHC(phcHash string) (*phcResult, error) {
+	if len(phcHash) > MaxPHCHashLen {
+		return nil, fmt.Errorf("%w: encoded hash exceeds %d bytes", ErrPHCInvalidFormat, MaxPHCHashLen)
+	}
+
+	parts := strings.Split(phcHash, "$")
+	if len(parts) != 6 {
+		return nil, fmt.Errorf("%w: expected 6 parts, got %d", ErrPHCInvalidFormat, len(parts))
+	}
+	if parts[0] != "" {
+		return nil, fmt.Errorf("%w: hash must start with $", ErrPHCInvalidFormat)
+	}
+	if parts[1] != "argon2id" {
+		return nil, fmt.Errorf("%w: unsupported algorithm %q, expected argon2id", ErrPHCInvalidFormat, parts[1])
+	}
+
+	var version int
+	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil || parts[2] != fmt.Sprintf("v=%d", version) {
+		return nil, fmt.Errorf("%w: invalid version format", ErrPHCInvalidFormat)
+	}
+	if version != argon2.Version {
+		return nil, fmt.Errorf("%w: unsupported version %d, expected %d", ErrPHCInvalidFormat, version, argon2.Version)
+	}
+
+	var memory, time uint32
+	var threads uint8
+	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads); err != nil || parts[3] != fmt.Sprintf("m=%d,t=%d,p=%d", memory, time, threads) {
+		return nil, fmt.Errorf("%w: failed to parse parameters", ErrPHCInvalidFormat)
+	}
+
+	if time == 0 || memory == 0 || threads == 0 {
+		return nil, fmt.Errorf("%w: parameters must be non-zero", ErrPHCInvalidFormat)
+	}
+	if memory > 4*1024*1024 {
+		return nil, fmt.Errorf("%w: memory parameter exceeds maximum (4GB)", ErrPHCInvalidFormat)
+	}
+	if time > 1000 {
+		return nil, fmt.Errorf("%w: time parameter exceeds maximum (1000)", ErrPHCInvalidFormat)
+	}
+	if threads > 255 {
+		return nil, fmt.Errorf("%w: threads parameter exceeds maximum (255)", ErrPHCInvalidFormat)
+	}
+
+	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrPHCInvalidSalt, err)
+	}
+	if len(salt) < 8 {
+		return nil, fmt.Errorf("%w: salt too short (%d bytes)", ErrPHCInvalidSalt, len(salt))
+	}
+	if len(salt) > MaxArgonSaltLen {
+		return nil, fmt.Errorf("%w: salt too long (%d bytes)", ErrPHCInvalidSalt, len(salt))
+	}
+
+	hash, err := base64.RawStdEncoding.DecodeString(parts[5])
+	if err != nil {
+		return nil, fmt.Errorf("%w: %v", ErrPHCInvalidHash, err)
+	}
+	if len(hash) < 16 {
+		return nil, fmt.Errorf("%w: hash too short (%d bytes)", ErrPHCInvalidHash, len(hash))
+	}
+	if len(hash) > MaxArgonKeyLen {
+		return nil, fmt.Errorf("%w: hash too long (%d bytes)", ErrPHCInvalidHash, len(hash))
+	}
+
+	return &phcResult{
+		expectedHash: hash,
+		salt:         salt,
+		time:         time,
+		memory:       memory,
+		threads:      threads,
+	}, nil
 }
 
 // verifyPHC validates format, bounds the password, runs the KDF once, and
 // constant-time compares against the encoded digest.
 func verifyPHC(password, phcHash string) (*phcResult, error) {
-	if err := ValidatePHCHashFormat(phcHash); err != nil {
-		return nil, err
-	}
 	if len(password) > MaxPasswordLen {
 		return nil, ErrPasswordTooLong
 	}
 
-	parts := strings.Split(phcHash, "$")
+	r, err := parsePHC(phcHash)
+	if err != nil {
+		return nil, err
+	}
 
-	r := &phcResult{}
-	// Parse is guaranteed well-formed by ValidatePHCHashFormat above.
-	fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &r.memory, &r.time, &r.threads)
+	// Bound the KDF before it runs; the record is untrusted input
+	if err := checkArgonCost(r.memory, r.time, r.threads); err != nil {
+		return nil, err
+	}
 
-	// Encodings validated above; errors are unreachable.
-	r.salt, _ = base64.RawStdEncoding.DecodeString(parts[4])
-	expected, _ := base64.RawStdEncoding.DecodeString(parts[5])
-
-	r.derived = argon2.IDKey([]byte(password), r.salt, r.time, r.memory, r.threads, uint32(len(expected)))
-	if !hmac.Equal(r.derived, expected) {
+	r.derived = argon2.IDKey([]byte(password), r.salt, r.time, r.memory, r.threads, uint32(len(r.expectedHash)))
+	if subtle.ConstantTimeCompare(r.derived, r.expectedHash) != 1 {
 		return nil, ErrInvalidCredentials
 	}
 	return r, nil
 }
+
+func checkArgonCost(memory, time uint32, threads uint8) error {
+	switch {
+	case memory > MaxVerifyArgonMemory:
+		return fmt.Errorf("%w: memory %d KiB exceeds %d", ErrPHCCostTooHigh, memory, MaxVerifyArgonMemory)
+	case time > MaxVerifyArgonTime:
+		return fmt.Errorf("%w: time %d exceeds %d", ErrPHCCostTooHigh, time, MaxVerifyArgonTime)
+	case threads > MaxVerifyArgonThreads:
+		return fmt.Errorf("%w: threads %d exceeds %d", ErrPHCCostTooHigh, threads, MaxVerifyArgonThreads)
+	}
+	return nil
+}
diff --git a/argon2_test.go b/argon2_test.go
index 3c0c00a..50392f3 100644
--- a/argon2_test.go
+++ b/argon2_test.go
@@ -1,192 +1,427 @@
 package auth
 
 import (
+	"bytes"
+	"crypto/sha256"
 	"encoding/base64"
+	"fmt"
 	"strings"
 	"sync"
 	"testing"
 
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
+	"golang.org/x/crypto/argon2"
 )
 
-func TestPasswordHashing(t *testing.T) {
-	password := "testPassword123"
+func TestHashPasswordEncoding(t *testing.T) {
+	const pw = "testPassword123"
+	hash, err := HashPassword(pw)
+	noErr(t, err, "HashPassword")
 
-	// Test hashing with default parameters
-	hash, err := HashPassword(password)
-	require.NoError(t, err, "Failed to hash password")
+	parts := strings.Split(hash, "$")
+	eq(t, len(parts), 6, "field count")
+	eq(t, parts[0], "", "leading field")
+	eq(t, parts[1], "argon2id", "algorithm")
+	eq(t, parts[2], fmt.Sprintf("v=%d", argon2.Version), "version")
+	eq(t, parts[3], fmt.Sprintf("m=%d,t=%d,p=%d", DefaultArgonMemory, DefaultArgonTime, DefaultArgonThreads), "parameters")
 
-	// Verify PHC format
-	assert.True(t, strings.HasPrefix(hash, "$argon2id$"),
-		"Hash should have argon2id prefix, got: %s", hash)
+	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
+	noErr(t, err, "salt decode")
+	eq(t, len(salt), DefaultArgonSaltLen, "salt length")
 
-	// Test verification with correct password
-	err = VerifyPassword(password, hash)
-	assert.NoError(t, err, "Failed to verify correct password")
+	digest, err := base64.RawStdEncoding.DecodeString(parts[5])
+	noErr(t, err, "digest decode")
+	eq(t, len(digest), DefaultArgonKeyLen, "digest length")
 
-	// Test verification with incorrect password
-	err = VerifyPassword("wrongPassword", hash)
-	assert.Error(t, err, "Verification should fail for incorrect password")
-	assert.Equal(t, ErrInvalidCredentials, err)
+	// the encoded digest must be reproducible from the encoded material
+	want := argon2.IDKey([]byte(pw), salt, DefaultArgonTime, DefaultArgonMemory, DefaultArgonThreads, DefaultArgonKeyLen)
+	eqBytes(t, digest, want, "digest")
 
-	// Test weak password
-	_, err = HashPassword("weak")
-	assert.Equal(t, ErrWeakPassword, err, "Should reject weak password")
+	isTrue(t, len(hash) <= MaxPHCHashLen, "default encoding within MaxPHCHashLen")
+	noErr(t, ValidatePHCHashFormat(hash), "self-validation")
+}
 
-	// Test with custom options
-	hash, err = HashPassword(password,
-		WithTime(5),
-		WithMemory(128*1024),
-		WithThreads(8))
-	require.NoError(t, err)
-
-	err = VerifyPassword(password, hash)
-	assert.NoError(t, err)
-
-	// Test malformed PHC hash
-	err = VerifyPassword(password, "$invalid$format")
-	assert.Error(t, err, "Should reject malformed hash")
-
-	// Test corrupted salt
-	corruptedHash := strings.Replace(hash, "$argon2id$", "$argon2id$", 1)
-	parts := strings.Split(corruptedHash, "$")
-	if len(parts) == 6 {
-		parts[4] = "invalid!base64"
-		corruptedHash = strings.Join(parts, "$")
-		err = VerifyPassword(password, corruptedHash)
-		assert.Error(t, err, "Should reject corrupted salt")
+func TestHashPasswordSaltUniqueness(t *testing.T) {
+	seen := make(map[string]struct{}, 64)
+	for range 64 {
+		h, err := HashPassword("testPassword123", cheapArgon...)
+		noErr(t, err, "HashPassword")
+		salt := strings.Split(h, "$")[4]
+		if _, dup := seen[salt]; dup {
+			t.Fatalf("duplicate salt: %s", salt)
+		}
+		seen[salt] = struct{}{}
 	}
 }
 
-func TestEmptyPasswordAfterValidation(t *testing.T) {
-	// Empty password should be rejected by length check
+func TestVerifyPassword(t *testing.T) {
+	const pw = "testPassword123"
+	hash, err := HashPassword(pw, cheapArgon...)
+	noErr(t, err, "HashPassword")
+
+	noErr(t, VerifyPassword(pw, hash), "correct password")
+	noErr(t, VerifyPassword(pw, hash), "repeat verification")
+
+	for _, wrong := range []string{"wrongPassword", "", pw + "\x00", pw + " ", strings.ToUpper(pw)} {
+		errIs(t, VerifyPassword(wrong, hash), ErrInvalidCredentials, fmt.Sprintf("password %q", wrong))
+	}
+
+	// verification has no minimum length: legacy hashes must stay verifiable
+	weak := phcFor("short", []byte("0123456789abcdef"), DefaultArgonKeyLen)
+	noErr(t, VerifyPassword("short", weak), "sub-minimum password verifies")
+}
+
+func TestPasswordLengthBounds(t *testing.T) {
 	_, err := HashPassword("")
-	assert.Equal(t, ErrWeakPassword, err)
+	errIs(t, err, ErrWeakPassword, "empty")
+	_, err = HashPassword("1234567")
+	errIs(t, err, ErrWeakPassword, "seven bytes")
 
-	// 8-character password should pass
-	hash, err := HashPassword("12345678")
-	require.NoError(t, err)
+	h8, err := HashPassword("12345678", cheapArgon...)
+	noErr(t, err, "eight bytes")
+	noErr(t, VerifyPassword("12345678", h8), "verify eight bytes")
 
-	err = VerifyPassword("12345678", hash)
-	assert.NoError(t, err)
+	// the minimum is measured in bytes, not runes
+	const multibyte = "ünïcödé"
+	eq(t, len([]rune(multibyte)), 7, "rune count")
+	eq(t, len(multibyte), 11, "byte count")
+	hm, err := HashPassword(multibyte, cheapArgon...)
+	noErr(t, err, "multibyte password")
+	noErr(t, VerifyPassword(multibyte, hm), "verify multibyte")
+
+	maxPw := strings.Repeat("a", MaxPasswordLen)
+	hMax, err := HashPassword(maxPw, cheapArgon...)
+	noErr(t, err, "maximum length")
+	noErr(t, VerifyPassword(maxPw, hMax), "verify maximum length")
+
+	_, err = HashPassword(maxPw+"a", cheapArgon...)
+	errIs(t, err, ErrPasswordTooLong, "over maximum")
+	// rejected before the KDF runs
+	errIs(t, VerifyPassword(maxPw+"a", hMax), ErrPasswordTooLong, "verify over maximum")
 }
 
-func TestConcurrentPasswordOperations(t *testing.T) {
-	password := "testPassword123"
-	hash, err := HashPassword(password)
-	require.NoError(t, err)
+func TestHashPasswordOptions(t *testing.T) {
+	const pw = "testPassword123"
 
-	// Test concurrent verification
-	var wg sync.WaitGroup
-	for i := 0; i < 10; i++ {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-			err := VerifyPassword(password, hash)
-			assert.NoError(t, err)
-		}()
+	h, err := HashPassword(pw, WithTime(2), WithMemory(16*1024), WithThreads(2))
+	noErr(t, err, "custom parameters")
+	eq(t, strings.Split(h, "$")[3], "m=16384,t=2,p=2", "encoded parameters")
+	noErr(t, VerifyPassword(pw, h), "verify custom parameters")
+
+	// zero values are discarded by the option guards
+	h, err = HashPassword(pw, WithMemory(testArgonMemory), WithTime(0), WithThreads(0))
+	noErr(t, err, "zero-valued options")
+	eq(t, strings.Split(h, "$")[3],
+		fmt.Sprintf("m=%d,t=%d,p=%d", testArgonMemory, DefaultArgonTime, DefaultArgonThreads),
+		"defaults retained")
+
+	// the last option wins
+	h, err = HashPassword(pw, WithMemory(64*1024), WithMemory(testArgonMemory), WithTime(1), WithThreads(1))
+	noErr(t, err, "repeated option")
+	eq(t, strings.Split(h, "$")[3], fmt.Sprintf("m=%d,t=1,p=1", testArgonMemory), "last option applied")
+}
+
+func TestVerifyPasswordNonStandardDigestLength(t *testing.T) {
+	// Records produced elsewhere may encode digests other than 32 bytes;
+	// verification must derive at the encoded length.
+	const pw = "testPassword123"
+	salt := []byte("0123456789abcdef0123")
+
+	for _, keyLen := range []uint32{16, 20, 32, MaxArgonKeyLen} {
+		ctx := fmt.Sprintf("keyLen=%d", keyLen)
+		h := phcFor(pw, salt, keyLen)
+		noErr(t, ValidatePHCHashFormat(h), "format "+ctx)
+		noErr(t, VerifyPassword(pw, h), "verify "+ctx)
+		errIs(t, VerifyPassword("wrongPassword", h), ErrInvalidCredentials, "wrong password "+ctx)
 	}
-	wg.Wait()
-}
-
-func TestPHCMigration(t *testing.T) {
-	password := "testPassword123"
-	username := "migrationUser"
-
-	// Generate PHC hash
-	phcHash, err := HashPassword(password)
-	require.NoError(t, err)
-
-	// Migrate to SCRAM credential
-	cred, err := MigrateFromPHC(username, password, phcHash)
-	require.NoError(t, err)
-	assert.Equal(t, username, cred.Username)
-	assert.NotNil(t, cred.StoredKey)
-	assert.NotNil(t, cred.ServerKey)
-
-	// Test with wrong password
-	_, err = MigrateFromPHC(username, "wrongPassword", phcHash)
-	assert.Equal(t, ErrInvalidCredentials, err)
-
-	// Test with invalid PHC format
-	_, err = MigrateFromPHC(username, password, "$invalid$format")
-	assert.Error(t, err)
 }
 
 func TestValidatePHCHashFormat(t *testing.T) {
-	// Generate valid hash for testing
-	validHash, err := HashPassword("testPassword123")
-	require.NoError(t, err)
+	ver := fmt.Sprintf("v=%d", argon2.Version)
+	const params = "m=65536,t=3,p=4"
+	okSalt, okDigest := rawB64(16), rawB64(32)
 
-	// Test valid hash
-	err = ValidatePHCHashFormat(validHash)
-	assert.NoError(t, err, "Valid hash should pass validation")
+	build := func(alg, version, prm, salt, digest string) string {
+		return "$" + alg + "$" + version + "$" + prm + "$" + salt + "$" + digest
+	}
+	std := func(prm, salt, digest string) string { return build("argon2id", ver, prm, salt, digest) }
 
-	// Test malformed formats
-	testCases := []struct {
-		name    string
-		hash    string
-		wantErr error
+	generated, err := HashPassword("testPassword123", cheapArgon...)
+	noErr(t, err, "HashPassword")
+
+	// largest structurally valid record: proves MaxPHCHashLen never binds
+	maxRecord := std("m=4194304,t=1000,p=255", rawB64(MaxArgonSaltLen), rawB64(MaxArgonKeyLen))
+	isTrue(t, len(maxRecord) <= MaxPHCHashLen, "maximum record within length cap")
+
+	cases := []struct {
+		name string
+		hash string
+		want error
 	}{
+		{"generated", generated, nil},
+		{"minimal", std(params, okSalt, okDigest), nil},
+		{"maximum record", maxRecord, nil},
+
 		{"empty", "", ErrPHCInvalidFormat},
-		{"not PHC format", "plaintext", ErrPHCInvalidFormat},
-		{"wrong prefix", "argon2id$v=19$m=65536,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"wrong algorithm", "$bcrypt$v=19$m=65536,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"missing version", "$argon2id$$m=65536,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"wrong version", "$argon2id$v=1$m=65536,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"missing params", "$argon2id$v=19$$salt$hash", ErrPHCInvalidFormat},
-		{"invalid params format", "$argon2id$v=19$invalid$salt$hash", ErrPHCInvalidFormat},
-		{"zero time", "$argon2id$v=19$m=65536,t=0,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"zero memory", "$argon2id$v=19$m=0,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"zero threads", "$argon2id$v=19$m=65536,t=3,p=0$salt$hash", ErrPHCInvalidFormat},
-		{"excessive memory", "$argon2id$v=19$m=5000000,t=3,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"excessive time", "$argon2id$v=19$m=65536,t=2000,p=4$salt$hash", ErrPHCInvalidFormat},
-		{"invalid salt encoding", "$argon2id$v=19$m=65536,t=3,p=4$!!!invalid!!!$hash", ErrPHCInvalidSalt},
-		{"invalid hash encoding", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			base64.RawStdEncoding.EncodeToString([]byte("salt12345678")) + "$!!!invalid!!!", ErrPHCInvalidHash},
-		{"short salt", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			base64.RawStdEncoding.EncodeToString([]byte("short")) + "$" +
-			base64.RawStdEncoding.EncodeToString([]byte("hash1234567890123456")), ErrPHCInvalidSalt},
-		{"short hash", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			base64.RawStdEncoding.EncodeToString([]byte("salt12345678")) + "$" +
-			base64.RawStdEncoding.EncodeToString([]byte("short")), ErrPHCInvalidHash},
-		{"too few parts", "$argon2id$v=19$m=65536,t=3,p=4", ErrPHCInvalidFormat},
-		{"too many parts", "$argon2id$v=19$m=65536,t=3,p=4$salt$hash$extra", ErrPHCInvalidFormat},
-		{"oversized salt", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			base64.RawStdEncoding.EncodeToString(make([]byte, 128)) + "$" +
-			base64.RawStdEncoding.EncodeToString([]byte("hash1234567890123456")), ErrPHCInvalidSalt},
-		{"oversized hash", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			base64.RawStdEncoding.EncodeToString([]byte("salt12345678")) + "$" +
-			base64.RawStdEncoding.EncodeToString(make([]byte, 128)), ErrPHCInvalidHash},
-		{"oversized input", "$argon2id$v=19$m=65536,t=3,p=4$" +
-			strings.Repeat("A", 512) + "$hash", ErrPHCInvalidFormat},
+		{"plaintext", "plaintext", ErrPHCInvalidFormat},
+		{"missing leading separator", "argon2id$" + ver + "$" + params + "$" + okSalt + "$" + okDigest, ErrPHCInvalidFormat},
+		{"leading garbage", "x" + std(params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"leading space", " " + std(params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"too few fields", "$argon2id$" + ver + "$" + params, ErrPHCInvalidFormat},
+		{"too many fields", std(params, okSalt, okDigest) + "$extra", ErrPHCInvalidFormat},
+		{"over length cap", strings.Repeat("A", MaxPHCHashLen+1), ErrPHCInvalidFormat},
+
+		{"algorithm bcrypt", build("bcrypt", ver, params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"algorithm argon2i", build("argon2i", ver, params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"algorithm argon2d", build("argon2d", ver, params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"algorithm case", build("ARGON2ID", ver, params, okSalt, okDigest), ErrPHCInvalidFormat},
+
+		{"version empty", build("argon2id", "", params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"version malformed", build("argon2id", "version=19", params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"version leading zero", build("argon2id", "v=019", params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"version negative", build("argon2id", "v=-19", params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"version too low", build("argon2id", "v=18", params, okSalt, okDigest), ErrPHCInvalidFormat},
+		{"version too high", build("argon2id", "v=20", params, okSalt, okDigest), ErrPHCInvalidFormat},
+
+		{"parameters empty", std("", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"parameters reordered", std("t=3,m=65536,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"parameters spaced", std("m=65536, t=3, p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"parameters trailing", std("m=65536,t=3,p=4,x=1", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"parameters negative", std("m=-1,t=3,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"parameters overflow", std("m=99999999999999999999,t=3,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"zero time", std("m=65536,t=0,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"zero memory", std("m=0,t=3,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"zero threads", std("m=65536,t=3,p=0", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"memory at cap", std("m=4194304,t=3,p=4", okSalt, okDigest), nil},
+		{"memory over cap", std("m=4194305,t=3,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"time at cap", std("m=65536,t=1000,p=4", okSalt, okDigest), nil},
+		{"time over cap", std("m=65536,t=1001,p=4", okSalt, okDigest), ErrPHCInvalidFormat},
+		{"threads at cap", std("m=65536,t=3,p=255", okSalt, okDigest), nil},
+		{"threads overflow", std("m=65536,t=3,p=256", okSalt, okDigest), ErrPHCInvalidFormat},
+
+		{"salt not base64", std(params, "!!!invalid!!!", okDigest), ErrPHCInvalidSalt},
+		{"salt padded", std(params, base64.StdEncoding.EncodeToString(make([]byte, 10)), okDigest), ErrPHCInvalidSalt},
+		{"salt url alphabet", std(params, "abc-def_ghijklmn", okDigest), ErrPHCInvalidSalt},
+		{"salt empty", std(params, "", okDigest), ErrPHCInvalidSalt},
+		{"salt too short", std(params, rawB64(7), okDigest), ErrPHCInvalidSalt},
+		{"salt minimum", std(params, rawB64(8), okDigest), nil},
+		{"salt at cap", std(params, rawB64(MaxArgonSaltLen), okDigest), nil},
+		{"salt over cap", std(params, rawB64(MaxArgonSaltLen+1), okDigest), ErrPHCInvalidSalt},
+
+		{"digest not base64", std(params, okSalt, "!!!invalid!!!"), ErrPHCInvalidHash},
+		{"digest empty", std(params, okSalt, ""), ErrPHCInvalidHash},
+		{"digest too short", std(params, okSalt, rawB64(15)), ErrPHCInvalidHash},
+		{"digest minimum", std(params, okSalt, rawB64(16)), nil},
+		{"digest at cap", std(params, okSalt, rawB64(MaxArgonKeyLen)), nil},
+		{"digest over cap", std(params, okSalt, rawB64(MaxArgonKeyLen+1)), ErrPHCInvalidHash},
+
+		// encoding/base64 discards CR and LF, so PHC records are not
+		// byte-canonical: never compare them as strings.
+		{"digest with newline", std(params, okSalt, okDigest[:20]+"\n"+okDigest[20:]), nil},
 	}
 
-	for _, tc := range testCases {
+	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
 			err := ValidatePHCHashFormat(tc.hash)
-			assert.ErrorIs(t, err, tc.wantErr, "Test case: %s", tc.name)
+			if tc.want == nil {
+				noErr(t, err, tc.name)
+				return
+			}
+			errIs(t, err, tc.want, tc.name)
 		})
 	}
-
-	// Test that validation doesn't require password
-	err = ValidatePHCHashFormat(validHash)
-	assert.NoError(t, err, "Should validate format without password")
-
-	// Verify that a validated hash can still be used for verification
-	err = ValidatePHCHashFormat(validHash)
-	require.NoError(t, err)
-	err = VerifyPassword("testPassword123", validHash)
-	assert.NoError(t, err, "Validated hash should still work for password verification")
 }
 
-func TestVerifyPassword_MalformedParamsNoPanic(t *testing.T) {
+func TestVerifyPasswordMalformedNoPanic(t *testing.T) {
 	for _, h := range []string{
-		"$argon2id$v=19$m=65536,t=0,p=4$c2FsdHNhbHRzYWx0MTI$aGFzaGhhc2hoYXNoaGFzaA",
-		"$argon2id$v=19$m=65536,t=3,p=0$c2FsdHNhbHRzYWx0MTI$aGFzaGhhc2hoYXNoaGFzaA",
-		"$argon2id$v=19$garbage$c2FsdHNhbHRzYWx0MTI$aGFzaGhhc2hoYXNoaGFzaA",
+		"", "$", "$$$$$", "$argon2id$", strings.Repeat("$", 1000),
+		"$argon2id$v=19$m=65536,t=0,p=4$" + rawB64(16) + "$" + rawB64(32),
+		"$argon2id$v=19$m=65536,t=3,p=0$" + rawB64(16) + "$" + rawB64(32),
+		"$argon2id$v=19$garbage$" + rawB64(16) + "$" + rawB64(32),
+		"$argon2id$v=19$m=65536,t=3,p=4$$",
+		"$argon2id$v=19$m=99999999999999999999,t=3,p=4$" + rawB64(16) + "$" + rawB64(32),
 	} {
-		assert.Error(t, VerifyPassword("whatever", h))
+		hasErr(t, VerifyPassword("whatever", h), fmt.Sprintf("hash %q", h))
 	}
 }
+
+func TestMigrateFromPHC(t *testing.T) {
+	const user, pw = "migrationUser", "testPassword123"
+	phcHash, err := HashPassword(pw, cheapArgon...)
+	noErr(t, err, "HashPassword")
+
+	cred, err := MigrateFromPHC(user, pw, phcHash)
+	noErr(t, err, "MigrateFromPHC")
+	eq(t, cred.Username, user, "username")
+	eq(t, cred.ArgonTime, uint32(testArgonTime), "time")
+	eq(t, cred.ArgonMemory, uint32(testArgonMemory), "memory")
+	eq(t, cred.ArgonThreads, uint8(testArgonThreads), "threads")
+	eq(t, len(cred.StoredKey), sha256.Size, "stored key length")
+	eq(t, len(cred.ServerKey), sha256.Size, "server key length")
+
+	salt, err := base64.RawStdEncoding.DecodeString(strings.Split(phcHash, "$")[4])
+	noErr(t, err, "salt decode")
+	eqBytes(t, cred.Salt, salt, "salt carried over")
+
+	// migration must agree with direct derivation over the same material
+	direct, err := DeriveCredential(user, pw, cred.Salt, cred.ArgonTime, cred.ArgonMemory, cred.ArgonThreads)
+	noErr(t, err, "DeriveCredential")
+	eqBytes(t, cred.StoredKey, direct.StoredKey, "stored key")
+	eqBytes(t, cred.ServerKey, direct.ServerKey, "server key")
+
+	// keys are derived from the salted password, not copies of it
+	salted, err := base64.RawStdEncoding.DecodeString(strings.Split(phcHash, "$")[5])
+	noErr(t, err, "digest decode")
+	want := sha256.Sum256(computeHMAC(salted, []byte("Client Key")))
+	eqBytes(t, cred.StoredKey, want[:], "stored key derivation")
+	eqBytes(t, cred.ServerKey, computeHMAC(salted, []byte("Server Key")), "server key derivation")
+	if bytes.Equal(cred.StoredKey, salted) || bytes.Equal(cred.ServerKey, salted) {
+		t.Fatal("credential exposes the salted password")
+	}
+
+	_, err = MigrateFromPHC(user, "wrongPassword", phcHash)
+	errIs(t, err, ErrInvalidCredentials, "wrong password")
+	_, err = MigrateFromPHC(user, pw, "$invalid$format")
+	errIs(t, err, ErrPHCInvalidFormat, "malformed record")
+	_, err = MigrateFromPHC(user, strings.Repeat("a", MaxPasswordLen+1), phcHash)
+	errIs(t, err, ErrPasswordTooLong, "oversized password")
+}
+
+func TestMigrateFromPHCShortSalt(t *testing.T) {
+	// parsePHC accepts 8..64 byte salts; SCRAM requires >= 16. The 32-byte
+	// digest branch skips that check, the fallback branch enforces it.
+	// See the ‼️ note on MigrateFromPHC.
+	const pw = "testPassword123"
+	salt := []byte("12345678")
+
+	cred, err := MigrateFromPHC("u", pw, phcFor(pw, salt, DefaultArgonKeyLen))
+	noErr(t, err, "32-byte digest branch")
+	eq(t, len(cred.Salt), 8, "short salt retained")
+
+	// the credential it produced cannot survive an export/import cycle
+	_, err = ImportCredential(cred.Export())
+	errIs(t, err, ErrSCRAMSaltTooShort, "re-import")
+
+	_, err = MigrateFromPHC("u", pw, phcFor(pw, salt, 20))
+	errIs(t, err, ErrSCRAMSaltTooShort, "fallback branch")
+}
+
+func TestConcurrentPasswordOperations(t *testing.T) {
+	const pw = "testPassword123"
+	hash, err := HashPassword(pw, cheapArgon...)
+	noErr(t, err, "HashPassword")
+
+	const n = 16
+	errs := make(chan error, 2*n)
+	var wg sync.WaitGroup
+	for i := range n {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if err := VerifyPassword(pw, hash); err != nil {
+				errs <- err
+			}
+			local := fmt.Sprintf("password-%04d", i)
+			h, err := HashPassword(local, cheapArgon...)
+			if err != nil {
+				errs <- err
+				return
+			}
+			if err := VerifyPassword(local, h); err != nil {
+				errs <- err
+			}
+		}()
+	}
+	wg.Wait()
+	close(errs)
+	for err := range errs {
+		t.Errorf("concurrent operation: %v", err)
+	}
+}
+
+func FuzzParsePHC(f *testing.F) {
+	h, err := HashPassword("testPassword123", cheapArgon...)
+	if err != nil {
+		f.Fatal(err)
+	}
+	f.Add(h)
+	f.Add("")
+	f.Add("$argon2id$v=19$m=65536,t=3,p=4$" + rawB64(16) + "$" + rawB64(32))
+	f.Add("$argon2id$v=19$m=0,t=0,p=0$$")
+
+	f.Fuzz(func(t *testing.T, s string) {
+		r, err := parsePHC(s)
+		if err != nil {
+			if r != nil {
+				t.Fatalf("result returned alongside error: %v", err)
+			}
+			return
+		}
+		if r.time == 0 || r.memory == 0 || r.threads == 0 {
+			t.Fatalf("zero parameter accepted: m=%d t=%d p=%d", r.memory, r.time, r.threads)
+		}
+		if r.memory > 4*1024*1024 || r.time > 1000 {
+			t.Fatalf("cost bound exceeded: m=%d t=%d", r.memory, r.time)
+		}
+		if len(r.salt) < 8 || len(r.salt) > MaxArgonSaltLen {
+			t.Fatalf("salt length %d accepted", len(r.salt))
+		}
+		if len(r.expectedHash) < 16 || len(r.expectedHash) > MaxArgonKeyLen {
+			t.Fatalf("digest length %d accepted", len(r.expectedHash))
+		}
+	})
+}
+
+func FuzzVerifyPassword(f *testing.F) {
+	h, err := HashPassword("testPassword123", cheapArgon...)
+	if err != nil {
+		f.Fatal(err)
+	}
+	f.Add("testPassword123", h)
+	f.Add("", "")
+	f.Add("x", "$argon2id$v=19$m=8192,t=1,p=1$"+rawB64(16)+"$"+rawB64(32))
+
+	f.Fuzz(func(t *testing.T, password, hash string) {
+		// parsePHC admits m up to 4 GiB from untrusted input; clamp before
+		// letting the fuzzer choose the KDF cost.
+		if r, err := parsePHC(hash); err == nil &&
+			(r.memory > 64*1024 || r.time > 4 || r.threads > 8) {
+			return
+		}
+		_ = VerifyPassword(password, hash)
+	})
+}
+
+func BenchmarkHashPasswordDefault(b *testing.B) {
+	for b.Loop() {
+		if _, err := HashPassword("testPassword123"); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkVerifyPasswordDefault(b *testing.B) {
+	hash, err := HashPassword("testPassword123")
+	if err != nil {
+		b.Fatal(err)
+	}
+	for b.Loop() {
+		if err := VerifyPassword("testPassword123", hash); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func TestVerifyPasswordCostCeiling(t *testing.T) {
+	const pw = "testPassword123"
+	// well-formed per PHC, refused before the KDF runs
+	over := fmt.Sprintf("$argon2id$v=%d$m=%d,t=1,p=1$%s$%s",
+		argon2.Version, MaxVerifyArgonMemory+1, rawB64(16), rawB64(32))
+	noErr(t, ValidatePHCHashFormat(over), "format validation is unaffected")
+	errIs(t, VerifyPassword(pw, over), ErrPHCCostTooHigh, "memory over ceiling")
+
+	overTime := fmt.Sprintf("$argon2id$v=%d$m=8192,t=%d,p=1$%s$%s",
+		argon2.Version, MaxVerifyArgonTime+1, rawB64(16), rawB64(32))
+	errIs(t, VerifyPassword(pw, overTime), ErrPHCCostTooHigh, "time over ceiling")
+	_, err := MigrateFromPHC("u", pw, over)
+	errIs(t, err, ErrPHCCostTooHigh, "migration honors the ceiling")
+}
diff --git a/error.go b/error.go
index d416a8f..964aec2 100644
--- a/error.go
+++ b/error.go
@@ -42,6 +42,7 @@ var (
 	ErrPHCInvalidFormat = errors.New("phc: invalid format")
 	ErrPHCInvalidSalt   = errors.New("phc: invalid salt encoding")
 	ErrPHCInvalidHash   = errors.New("phc: invalid hash encoding")
+	ErrPHCCostTooHigh   = errors.New("phc: cost parameters exceed verification limit")
 )
 
 // SCRAM-specific errors
@@ -57,6 +58,7 @@ var (
 	ErrSCRAMZeroParams        = errors.New("scram: invalid Argon2 parameters")
 	ErrSCRAMSaltTooShort      = errors.New("scram: salt must be at least 16 bytes")
 	ErrSCRAMTooManyHandshakes = errors.New("scram: handshake capacity exceeded")
+	ErrSCRAMParamsTooLarge    = errors.New("scram: Argon2 parameters exceed limit")
 )
 
 // Credential import/export errors
diff --git a/go.mod b/go.mod
index 1b631d1..ec5a876 100644
--- a/go.mod
+++ b/go.mod
@@ -4,13 +4,7 @@ go 1.26.0
 
 require (
 	github.com/golang-jwt/jwt/v5 v5.3.1
-	github.com/stretchr/testify v1.11.1
 	golang.org/x/crypto v0.54.0
 )
 
-require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	golang.org/x/sys v0.47.0 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
-)
+require golang.org/x/sys v0.47.0 // indirect
diff --git a/go.sum b/go.sum
index 87ca3fd..6f26f6f 100644
--- a/go.sum
+++ b/go.sum
@@ -1,30 +1,6 @@
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/golang-jwt/jwt/v5 v5.3.0 h1:pv4AsKCKKZuqlgs5sUmn4x8UlGa0kEVt/puTpKx9vvo=
-github.com/golang-jwt/jwt/v5 v5.3.0/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
 github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
-github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
-golang.org/x/crypto v0.43.0 h1:dduJYIi3A3KOfdGOHX8AVZ/jGiyPa3IbBozJ5kNuE04=
-golang.org/x/crypto v0.43.0/go.mod h1:BFbav4mRNlXJL4wNeejLpWxB7wMbc79PdRGhWKncxR0=
-golang.org/x/crypto v0.52.0 h1:RMs7fP2rXdep0CftQlK8Uf+kibLm7qkCcradZWYz988=
-golang.org/x/crypto v0.52.0/go.mod h1:1QgfPxDqh0T2M/elOJtp9RvuR95kVjir0e6/BvEmGbc=
-golang.org/x/crypto v0.53.0 h1:QZ4Muo8THX6CizN2vPPd5fBGHyogrdK9fG4wLPFUsto=
-golang.org/x/crypto v0.53.0/go.mod h1:DNLU434OwVakk9PzuwV8w62mAJpRJL3vsgcfp4Qnsio=
 golang.org/x/crypto v0.54.0 h1:YLIA59K4fiNzHzjnZt2tUJQjQtUWfWbeHBqKtk3eScw=
 golang.org/x/crypto v0.54.0/go.mod h1:KWL8ny2AZdGR2cWmzeHrp2azQPGogOv+HeQaVEXC2dk=
-golang.org/x/sys v0.37.0 h1:fdNQudmxPjkdUTPnLn5mdQv7Zwvbvpaxqs831goi9kQ=
-golang.org/x/sys v0.37.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
-golang.org/x/sys v0.45.0 h1:dO4czNzziLiiXplLQgBCEpCvXQ3dnkn0SdaZSYdQ+FY=
-golang.org/x/sys v0.45.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-golang.org/x/sys v0.46.0 h1:noSf2Fq6F8DBgS+LysIkx7rIExoNHJsxOAtPp4rthXw=
-golang.org/x/sys v0.46.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
 golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
 golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
-gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/http.go b/http.go
index 474167b..dd1f6af 100644
--- a/http.go
+++ b/http.go
@@ -7,34 +7,30 @@ import (
 
 // ParseBasicAuth extracts username/password from Basic auth header
 func ParseBasicAuth(header string) (username, password string, err error) {
-	const prefix = "Basic "
-	if !strings.HasPrefix(header, prefix) {
+	encoded, ok := strings.CutPrefix(header, "Basic ")
+	if !ok {
 		return "", "", ErrAuthInvalidBasicFormat
 	}
 
-	encoded := strings.TrimPrefix(header, prefix)
 	decoded, err := base64.StdEncoding.DecodeString(encoded)
 	if err != nil {
 		return "", "", ErrAuthInvalidBasicEncoding
 	}
 
-	credentials := string(decoded)
-	idx := strings.IndexByte(credentials, ':')
-	if idx < 0 {
+	username, password, ok = strings.Cut(string(decoded), ":")
+	if !ok {
 		return "", "", ErrAuthInvalidBasicCreds
 	}
 
-	return credentials[:idx], credentials[idx+1:], nil
+	return username, password, nil
 }
 
 // ParseBearerToken extracts token from Bearer auth header
 func ParseBearerToken(header string) (token string, err error) {
-	const prefix = "Bearer "
-	if !strings.HasPrefix(header, prefix) {
+	token, ok := strings.CutPrefix(header, "Bearer ")
+	if !ok {
 		return "", ErrAuthInvalidBearerFormat
 	}
-
-	token = strings.TrimPrefix(header, prefix)
 	if token == "" {
 		return "", ErrAuthEmptyBearerToken
 	}
@@ -44,18 +40,8 @@ func ParseBearerToken(header string) (token string, err error) {
 
 // ExtractAuthType returns authentication type from header
 func ExtractAuthType(header string) string {
-	if strings.HasPrefix(header, "Basic ") {
-		return "Basic"
+	if authType, _, ok := strings.Cut(header, " "); ok {
+		return authType
 	}
-	if strings.HasPrefix(header, "Bearer ") {
-		return "Bearer"
-	}
-
-	// Extract first word as auth type
-	idx := strings.IndexByte(header, ' ')
-	if idx > 0 {
-		return header[:idx]
-	}
-	return ""
+	return "" // Matches original behavior if no space is found or string is empty
 }
-
diff --git a/http_test.go b/http_test.go
index 0dbb42e..5e5471c 100644
--- a/http_test.go
+++ b/http_test.go
@@ -2,53 +2,122 @@ package auth
 
 import (
 	"encoding/base64"
+	"fmt"
+	"strings"
 	"testing"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
-func TestHTTPAuthParsing(t *testing.T) {
-	// Test Basic Auth
-	basicHeader := "Basic " + base64.StdEncoding.EncodeToString([]byte("user:pass"))
-	username, password, err := ParseBasicAuth(basicHeader)
-	require.NoError(t, err)
-	assert.Equal(t, "user", username)
-	assert.Equal(t, "pass", password)
-
-	// Test Bearer Token
-	bearerHeader := "Bearer test-token-xyz"
-	token, err := ParseBearerToken(bearerHeader)
-	require.NoError(t, err)
-	assert.Equal(t, "test-token-xyz", token)
-
-	// Test ExtractAuthType
-	assert.Equal(t, "Basic", ExtractAuthType(basicHeader))
-	assert.Equal(t, "Bearer", ExtractAuthType(bearerHeader))
-	assert.Equal(t, "Custom", ExtractAuthType("Custom somedata"))
-	assert.Equal(t, "", ExtractAuthType("InvalidHeader"))
-
-	// Test invalid formats
-	_, _, err = ParseBasicAuth("Invalid header")
-	assert.Error(t, err)
-	assert.Equal(t, ErrAuthInvalidBasicFormat, err)
-
-	_, err = ParseBearerToken("Invalid header")
-	assert.Error(t, err)
-	assert.Equal(t, ErrAuthInvalidBearerFormat, err)
-
-	// Test malformed Basic auth
-	_, _, err = ParseBasicAuth("Basic not-base64!")
-	assert.Error(t, err)
-	assert.Equal(t, ErrAuthInvalidBasicEncoding, err)
-
-	_, _, err = ParseBasicAuth("Basic " + base64.StdEncoding.EncodeToString([]byte("no-colon")))
-	assert.Error(t, err)
-	assert.Equal(t, ErrAuthInvalidBasicCreds, err)
-
-	// Test empty Bearer token
-	_, err = ParseBearerToken("Bearer ")
-	assert.Error(t, err)
-	assert.Equal(t, ErrAuthEmptyBearerToken, err)
+func basicHeader(payload string) string {
+	return "Basic " + base64.StdEncoding.EncodeToString([]byte(payload))
 }
 
+func TestParseBasicAuth(t *testing.T) {
+	cases := []struct {
+		name, header, username, password string
+		want                             error
+	}{
+		{name: "standard", header: basicHeader("user:pass"), username: "user", password: "pass"},
+		{name: "empty password", header: basicHeader("user:"), username: "user"},
+		{name: "empty username", header: basicHeader(":pass"), password: "pass"},
+		{name: "colon in password", header: basicHeader("user:pa:ss"), username: "user", password: "pa:ss"},
+		{name: "unicode", header: basicHeader("üser:pässwörd"), username: "üser", password: "pässwörd"},
+		{name: "separator only", header: basicHeader(":")},
+
+		{name: "missing scheme", header: "Invalid header", want: ErrAuthInvalidBasicFormat},
+		{name: "no space", header: "Basic", want: ErrAuthInvalidBasicFormat},
+		{name: "empty header", header: "", want: ErrAuthInvalidBasicFormat},
+		// ☢ RFC 7235 auth-scheme is case-insensitive; matching here is not
+		{name: "lowercase scheme", header: "basic dXNlcjpwYXNz", want: ErrAuthInvalidBasicFormat},
+		{name: "bad base64", header: "Basic not-base64!", want: ErrAuthInvalidBasicEncoding},
+		{name: "no colon", header: basicHeader("no-colon"), want: ErrAuthInvalidBasicCreds},
+		{name: "empty payload", header: "Basic ", want: ErrAuthInvalidBasicCreds},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			username, password, err := ParseBasicAuth(tc.header)
+			if tc.want != nil {
+				errIs(t, err, tc.want, tc.name)
+				eq(t, username, "", "username on failure")
+				eq(t, password, "", "password on failure")
+				return
+			}
+			noErr(t, err, tc.name)
+			eq(t, username, tc.username, "username")
+			eq(t, password, tc.password, "password")
+		})
+	}
+}
+
+func TestParseBearerToken(t *testing.T) {
+	cases := []struct {
+		name, header, token string
+		want                error
+	}{
+		{name: "standard", header: "Bearer test-token-xyz", token: "test-token-xyz"},
+		{name: "embedded space", header: "Bearer a b", token: "a b"},
+		{name: "trailing space", header: "Bearer x ", token: "x "},
+
+		{name: "empty token", header: "Bearer ", want: ErrAuthEmptyBearerToken},
+		{name: "missing scheme", header: "Invalid header", want: ErrAuthInvalidBearerFormat},
+		{name: "lowercase scheme", header: "bearer x", want: ErrAuthInvalidBearerFormat},
+		{name: "no space", header: "Bearer", want: ErrAuthInvalidBearerFormat},
+		{name: "empty header", header: "", want: ErrAuthInvalidBearerFormat},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			token, err := ParseBearerToken(tc.header)
+			if tc.want != nil {
+				errIs(t, err, tc.want, tc.name)
+				eq(t, token, "", "token on failure")
+				return
+			}
+			noErr(t, err, tc.name)
+			eq(t, token, tc.token, "token")
+		})
+	}
+}
+
+func TestExtractAuthType(t *testing.T) {
+	cases := map[string]string{
+		"Basic dXNlcjpwYXNz": "Basic",
+		"Bearer token":       "Bearer",
+		"Custom somedata":    "Custom",
+		"Basic ":             "Basic",
+		"InvalidHeader":      "",
+		"":                   "",
+		" Bearer token":      "", // cut at the first space yields an empty scheme
+	}
+	for header, want := range cases {
+		eq(t, ExtractAuthType(header), want, fmt.Sprintf("header %q", header))
+	}
+}
+
+func FuzzParseBasicAuth(f *testing.F) {
+	f.Add(basicHeader("user:pass"))
+	f.Add("Basic ")
+	f.Add("")
+	f.Add("Basic !!!")
+
+	f.Fuzz(func(t *testing.T, header string) {
+		username, password, err := ParseBasicAuth(header)
+		if err != nil {
+			if username != "" || password != "" {
+				t.Fatal("values returned alongside error")
+			}
+			return
+		}
+		encoded, ok := strings.CutPrefix(header, "Basic ")
+		if !ok {
+			t.Fatal("accepted a header without the Basic prefix")
+		}
+		decoded, decErr := base64.StdEncoding.DecodeString(encoded)
+		if decErr != nil {
+			t.Fatal("accepted an undecodable payload")
+		}
+		if string(decoded) != username+":"+password {
+			t.Fatalf("lossy split: %q vs %q", decoded, username+":"+password)
+		}
+	})
+}
diff --git a/jwt.go b/jwt.go
index 6c342c7..dd48357 100644
--- a/jwt.go
+++ b/jwt.go
@@ -111,6 +111,16 @@ func NewJWTRSA(privateKey *rsa.PrivateKey, opts ...JWTOption) (*JWT, error) {
 	return j, nil
 }
 
+// NewJWTRSAFromPEM creates a JWT manager for RS256 from raw PEM-encoded private key data.
+func NewJWTRSAFromPEM(privateKeyPEM []byte, opts ...JWTOption) (*JWT, error) {
+	privateKey, err := parseRSAPrivateKey(privateKeyPEM)
+	if err != nil {
+		return nil, err
+	}
+	// Call the original constructor with the now-parsed key
+	return NewJWTRSA(privateKey, opts...)
+}
+
 // NewJWTVerifier creates JWT manager for verification only (RS256)
 func NewJWTVerifier(publicKey *rsa.PublicKey, opts ...JWTOption) (*JWT, error) {
 	if publicKey == nil {
@@ -132,6 +142,16 @@ func NewJWTVerifier(publicKey *rsa.PublicKey, opts ...JWTOption) (*JWT, error) {
 	return j, nil
 }
 
+// NewJWTVerifierFromPEM creates a JWT manager for verification from raw PEM-encoded public key data.
+func NewJWTVerifierFromPEM(publicKeyPEM []byte, opts ...JWTOption) (*JWT, error) {
+	publicKey, err := parseRSAPublicKey(publicKeyPEM)
+	if err != nil {
+		return nil, err
+	}
+	// Call the original constructor with the now-parsed key
+	return NewJWTVerifier(publicKey, opts...)
+}
+
 // GenerateToken creates signed JWT with claims
 func (j *JWT) GenerateToken(userID string, claims map[string]any) (string, error) {
 	if userID == "" {
@@ -191,27 +211,26 @@ func (j *JWT) ValidateToken(tokenString string) (string, map[string]any, error)
 func mapJWTError(err error) error {
 	switch {
 	case errors.Is(err, jwt.ErrTokenMalformed):
-		return fmt.Errorf("%w : %w", ErrTokenMalformed, err)
+		return fmt.Errorf("%w: %w", ErrTokenMalformed, err)
 	case errors.Is(err, jwt.ErrTokenUnverifiable):
-		return fmt.Errorf("%w : %w", ErrTokenMalformed, err)
+		return fmt.Errorf("%w: %w", ErrTokenMalformed, err)
 	case errors.Is(err, jwt.ErrTokenSignatureInvalid):
-		return fmt.Errorf("%w : %w", ErrTokenInvalidSignature, err)
+		return fmt.Errorf("%w: %w", ErrTokenInvalidSignature, err)
 	case errors.Is(err, jwt.ErrTokenExpired):
-		return fmt.Errorf("%w : %w", ErrTokenExpired, err)
+		return fmt.Errorf("%w: %w", ErrTokenExpired, err)
 	case errors.Is(err, jwt.ErrTokenNotValidYet):
-		return fmt.Errorf("%w : %w", ErrTokenNotYetValid, err)
+		return fmt.Errorf("%w: %w", ErrTokenNotYetValid, err)
 	case errors.Is(err, jwt.ErrTokenInvalidAudience):
-		return fmt.Errorf("%w : %w", ErrTokenMissingClaim, err)
+		return fmt.Errorf("%w: %w", ErrTokenMissingClaim, err)
 	case errors.Is(err, jwt.ErrTokenInvalidIssuer):
-		return fmt.Errorf("%w : %w", ErrTokenMissingClaim, err)
+		return fmt.Errorf("%w: %w", ErrTokenMissingClaim, err)
+	case errors.Is(err, jwt.ErrTokenRequiredClaimMissing):
+		return fmt.Errorf("%w: %w", ErrTokenMissingClaim, err)
 	default:
-		// Alg rejection (WithValidMethods) surfaces as ErrTokenSignatureInvalid.
-		return fmt.Errorf("%w : %w", ErrTokenMalformed, err)
+		return fmt.Errorf("%w: %w", ErrTokenMalformed, err)
 	}
 }
 
-// Standalone helper functions for one-off operations
-
 // GenerateHS256Token creates HS256 JWT without manager instance
 func GenerateHS256Token(secret []byte, userID string, claims map[string]any, lifetime time.Duration) (string, error) {
 	if len(secret) < 32 {
@@ -263,28 +282,6 @@ func ValidateHS256Token(secret []byte, tokenString string) (string, map[string]a
 	return claims.Subject, claims.Extra, nil
 }
 
-// RSA Utilities
-
-// NewJWTRSAFromPEM creates a JWT manager for RS256 from raw PEM-encoded private key data.
-func NewJWTRSAFromPEM(privateKeyPEM []byte, opts ...JWTOption) (*JWT, error) {
-	privateKey, err := parseRSAPrivateKey(privateKeyPEM)
-	if err != nil {
-		return nil, err
-	}
-	// Call the original constructor with the now-parsed key
-	return NewJWTRSA(privateKey, opts...)
-}
-
-// NewJWTVerifierFromPEM creates a JWT manager for verification from raw PEM-encoded public key data.
-func NewJWTVerifierFromPEM(publicKeyPEM []byte, opts ...JWTOption) (*JWT, error) {
-	publicKey, err := parseRSAPublicKey(publicKeyPEM)
-	if err != nil {
-		return nil, err
-	}
-	// Call the original constructor with the now-parsed key
-	return NewJWTVerifier(publicKey, opts...)
-}
-
 // parseRSAPrivateKey parses a PEM-encoded RSA private key.
 func parseRSAPrivateKey(pemBytes []byte) (*rsa.PrivateKey, error) {
 	block, _ := pem.Decode(pemBytes)
diff --git a/jwt_test.go b/jwt_test.go
index 3aebc96..befaf2a 100644
--- a/jwt_test.go
+++ b/jwt_test.go
@@ -1,259 +1,606 @@
 package auth
 
 import (
+	"bytes"
+	"crypto/ed25519"
+	"crypto/hmac"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
 	"encoding/pem"
+	"errors"
+	"fmt"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
 	"github.com/golang-jwt/jwt/v5"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
-func TestJWTHS256(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
-	jwtMgr, err := NewJWT(secret)
-	require.NoError(t, err)
+var testSecret = []byte("test-secret-key-must-be-32-bytes")
 
-	userID := "user123"
-	claims := map[string]any{
+func genRSAKey() *rsa.PrivateKey {
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic(err)
+	}
+	return key
+}
+
+// RSA key generation is the dominant cost in this file; amortize it.
+var (
+	testRSAKey    = sync.OnceValue(genRSAKey)
+	testRSAKeyAlt = sync.OnceValue(genRSAKey)
+)
+
+func defaultHeader() map[string]any { return map[string]any{"alg": "HS256", "typ": "JWT"} }
+
+// signHS256 assembles a token from raw maps, bypassing the package so that
+// malformed and hostile tokens can be constructed.
+func signHS256(t *testing.T, secret []byte, header, claims map[string]any) string {
+	t.Helper()
+	h, err := json.Marshal(header)
+	noErr(t, err, "marshal header")
+	c, err := json.Marshal(claims)
+	noErr(t, err, "marshal claims")
+
+	signing := base64.RawURLEncoding.EncodeToString(h) + "." + base64.RawURLEncoding.EncodeToString(c)
+	mac := hmac.New(sha256.New, secret)
+	mac.Write([]byte(signing))
+	return signing + "." + base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+}
+
+func unsignedToken(t *testing.T, header, claims map[string]any) string {
+	t.Helper()
+	h, err := json.Marshal(header)
+	noErr(t, err, "marshal header")
+	c, err := json.Marshal(claims)
+	noErr(t, err, "marshal claims")
+	return base64.RawURLEncoding.EncodeToString(h) + "." + base64.RawURLEncoding.EncodeToString(c) + "."
+}
+
+func decodeSegment(t *testing.T, segment string) map[string]any {
+	t.Helper()
+	raw, err := base64.RawURLEncoding.DecodeString(segment)
+	noErr(t, err, "segment decode")
+	var m map[string]any
+	noErr(t, json.Unmarshal(raw, &m), "segment unmarshal")
+	return m
+}
+
+func jwtParts(t *testing.T, token string) (header, payload map[string]any) {
+	t.Helper()
+	parts := strings.Split(token, ".")
+	eq(t, len(parts), 3, "token segments")
+	return decodeSegment(t, parts[0]), decodeSegment(t, parts[1])
+}
+
+func str(t *testing.T, m map[string]any, key string) string {
+	t.Helper()
+	v, ok := m[key].(string)
+	if !ok {
+		t.Fatalf("claim %q: %v is not a string", key, m[key])
+	}
+	return v
+}
+
+func num(t *testing.T, m map[string]any, key string) float64 {
+	t.Helper()
+	v, ok := m[key].(float64)
+	if !ok {
+		t.Fatalf("claim %q: %v is not a number", key, m[key])
+	}
+	return v
+}
+
+func TestJWTHS256RoundTrip(t *testing.T) {
+	manager, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
+
+	token, err := manager.GenerateToken("user123", map[string]any{
 		"email": "test@example.com",
 		"role":  "admin",
+	})
+	noErr(t, err, "GenerateToken")
+
+	header, payload := jwtParts(t, token)
+	eq(t, str(t, header, "alg"), "HS256", "alg")
+	eq(t, str(t, header, "typ"), "JWT", "typ")
+	eq(t, str(t, payload, "sub"), "user123", "subject")
+
+	exp, iat, nbf := num(t, payload, "exp"), num(t, payload, "iat"), num(t, payload, "nbf")
+	eq(t, int64(exp-iat), int64(DefaultTokenLifetime/time.Second), "default lifetime")
+	eq(t, nbf, iat, "nbf equals iat")
+	if _, present := payload["iss"]; present {
+		t.Fatal("issuer emitted without configuration")
 	}
 
-	// Generate token
-	token, err := jwtMgr.GenerateToken(userID, claims)
-	require.NoError(t, err)
-	assert.NotEmpty(t, token)
+	userID, claims, err := manager.ValidateToken(token)
+	noErr(t, err, "ValidateToken")
+	eq(t, userID, "user123", "user id")
+	eq(t, len(claims), 2, "claim count")
+	eq(t, str(t, claims, "email"), "test@example.com", "email claim")
+	eq(t, str(t, claims, "role"), "admin", "role claim")
 
-	// Validate token
-	extractedUserID, extractedClaims, err := jwtMgr.ValidateToken(token)
-	require.NoError(t, err)
+	// nil claims must not emit an extra object
+	bare, err := manager.GenerateToken("user123", nil)
+	noErr(t, err, "GenerateToken without claims")
+	_, barePayload := jwtParts(t, bare)
+	if _, present := barePayload["extra"]; present {
+		t.Fatal("empty extra claim emitted")
+	}
+	_, claims, err = manager.ValidateToken(bare)
+	noErr(t, err, "ValidateToken without claims")
+	eq(t, len(claims), 0, "no extra claims")
+}
 
-	assert.Equal(t, userID, extractedUserID)
-	assert.Equal(t, "test@example.com", extractedClaims["email"])
-	assert.Equal(t, "admin", extractedClaims["role"])
+func TestJWTSecretLength(t *testing.T) {
+	_, err := NewJWT(nil)
+	errIs(t, err, ErrSecretTooShort, "nil secret")
+	_, err = NewJWT(make([]byte, 31))
+	errIs(t, err, ErrSecretTooShort, "31 bytes")
+	_, err = NewJWT(make([]byte, 32))
+	noErr(t, err, "32 bytes")
+
+	_, err = GenerateHS256Token(make([]byte, 31), "u", nil, time.Hour)
+	errIs(t, err, ErrSecretTooShort, "standalone generate")
+	_, _, err = ValidateHS256Token(make([]byte, 31), "irrelevant")
+	errIs(t, err, ErrSecretTooShort, "standalone validate")
+}
+
+func TestJWTEmptyUserID(t *testing.T) {
+	manager, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
+	_, err = manager.GenerateToken("", map[string]any{"role": "admin"})
+	errIs(t, err, ErrTokenEmptyUserID, "empty user id")
+	_, err = GenerateHS256Token(testSecret, "", nil, time.Hour)
+	errIs(t, err, ErrTokenEmptyUserID, "standalone empty user id")
 }
 
 func TestJWTRS256(t *testing.T) {
-	// Generate RSA key pair
-	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
-	require.NoError(t, err)
+	key := testRSAKey()
 
-	// Test with private key (can sign and verify)
-	jwtMgr, err := NewJWTRSA(privateKey)
-	require.NoError(t, err)
+	signer, err := NewJWTRSA(key)
+	noErr(t, err, "NewJWTRSA")
+	token, err := signer.GenerateToken("user456", map[string]any{"scope": "read:all"})
+	noErr(t, err, "GenerateToken")
 
-	userID := "user456"
-	claims := map[string]any{
-		"scope": "read:all",
-	}
+	header, _ := jwtParts(t, token)
+	eq(t, str(t, header, "alg"), "RS256", "alg")
 
-	// Generate token
-	token, err := jwtMgr.GenerateToken(userID, claims)
-	require.NoError(t, err)
-	assert.NotEmpty(t, token)
+	userID, claims, err := signer.ValidateToken(token)
+	noErr(t, err, "self validation")
+	eq(t, userID, "user456", "user id")
+	eq(t, str(t, claims, "scope"), "read:all", "scope claim")
 
-	// Validate with same manager
-	extractedUserID, extractedClaims, err := jwtMgr.ValidateToken(token)
-	require.NoError(t, err)
-	assert.Equal(t, userID, extractedUserID)
-	assert.Equal(t, "read:all", extractedClaims["scope"])
+	verifier, err := NewJWTVerifier(&key.PublicKey)
+	noErr(t, err, "NewJWTVerifier")
+	userID, _, err = verifier.ValidateToken(token)
+	noErr(t, err, "verifier validation")
+	eq(t, userID, "user456", "user id from verifier")
 
-	// Test with verifier only (public key)
-	verifier, err := NewJWTVerifier(&privateKey.PublicKey)
-	require.NoError(t, err)
+	_, err = verifier.GenerateToken("user456", nil)
+	errIs(t, err, ErrTokenNoPrivateKey, "verifier must not sign")
 
-	// Should validate token
-	extractedUserID, _, err = verifier.ValidateToken(token)
-	require.NoError(t, err)
-	assert.Equal(t, userID, extractedUserID)
+	// an unrelated key must not verify
+	foreign, err := NewJWTVerifier(&testRSAKeyAlt().PublicKey)
+	noErr(t, err, "NewJWTVerifier foreign")
+	_, _, err = foreign.ValidateToken(token)
+	errIs(t, err, ErrTokenInvalidSignature, "foreign public key")
 
-	// Should not generate token
-	_, err = verifier.GenerateToken(userID, claims)
-	assert.Equal(t, ErrTokenNoPrivateKey, err)
+	_, err = NewJWTRSA(nil)
+	errIs(t, err, ErrTokenNoPrivateKey, "nil private key")
+	_, err = NewJWTVerifier(nil)
+	errIs(t, err, ErrTokenNoPublicKey, "nil public key")
 }
 
-func TestJWTOptions(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
+func TestJWTAlgorithmEnforcement(t *testing.T) {
+	key := testRSAKey()
+	hs, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
+	rs, err := NewJWTRSA(key)
+	noErr(t, err, "NewJWTRSA")
 
-	// Test custom lifetime
-	jwtMgr, err := NewJWT(secret,
-		WithTokenLifetime(1*time.Hour),
+	hsToken, err := hs.GenerateToken("u", nil)
+	noErr(t, err, "HS256 token")
+	rsToken, err := rs.GenerateToken("u", nil)
+	noErr(t, err, "RS256 token")
+
+	_, _, err = hs.ValidateToken(rsToken)
+	errIs(t, err, ErrTokenInvalidSignature, "RS256 token to HS256 manager")
+	_, _, err = rs.ValidateToken(hsToken)
+	errIs(t, err, ErrTokenInvalidSignature, "HS256 token to RS256 manager")
+
+	// ☢ algorithm confusion: HS256 token keyed with the RSA public key
+	pubBytes, err := x509.MarshalPKIXPublicKey(&key.PublicKey)
+	noErr(t, err, "marshal public key")
+	pubPEM := pem.EncodeToMemory(&pem.Block{Type: "PUBLIC KEY", Bytes: pubBytes})
+	forged := signHS256(t, pubPEM, defaultHeader(), map[string]any{
+		"sub": "attacker", "exp": time.Now().Add(time.Hour).Unix(),
+	})
+	_, _, err = rs.ValidateToken(forged)
+	errIs(t, err, ErrTokenInvalidSignature, "algorithm confusion")
+
+	// alg: none
+	none := unsignedToken(t, map[string]any{"alg": "none", "typ": "JWT"}, map[string]any{
+		"sub": "attacker", "exp": time.Now().Add(time.Hour).Unix(),
+	})
+	_, _, err = hs.ValidateToken(none)
+	errIs(t, err, ErrTokenInvalidSignature, "alg none against HS256")
+	_, _, err = rs.ValidateToken(none)
+	errIs(t, err, ErrTokenInvalidSignature, "alg none against RS256")
+
+	// an unregistered algorithm is unverifiable, mapped to malformed
+	unknown := signHS256(t, testSecret, map[string]any{"alg": "HS999", "typ": "JWT"}, map[string]any{
+		"sub": "attacker", "exp": time.Now().Add(time.Hour).Unix(),
+	})
+	_, _, err = hs.ValidateToken(unknown)
+	errIs(t, err, ErrTokenMalformed, "unknown algorithm")
+}
+
+func TestJWTTampering(t *testing.T) {
+	manager, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
+	token, err := manager.GenerateToken("user1", map[string]any{"role": "user"})
+	noErr(t, err, "GenerateToken")
+	parts := strings.Split(token, ".")
+
+	escalated, err := json.Marshal(map[string]any{
+		"sub": "user1", "exp": time.Now().Add(time.Hour).Unix(),
+		"extra": map[string]any{"role": "admin"},
+	})
+	noErr(t, err, "marshal forged claims")
+
+	// length-preserving corruption, so the segment still decodes and
+	// the failure is attributable to the MAC rather than the encoding
+	corrupt := []byte(parts[2])
+	if corrupt[0] == 'A' {
+		corrupt[0] = 'B'
+	} else {
+		corrupt[0] = 'A'
+	}
+
+	cases := []struct {
+		name  string
+		token string
+		want  error
+	}{
+		{"payload rewrite", parts[0] + "." + base64.RawURLEncoding.EncodeToString(escalated) + "." + parts[2], ErrTokenInvalidSignature},
+		{"corrupt signature", parts[0] + "." + parts[1] + "." + string(corrupt), ErrTokenInvalidSignature},
+		{"truncated signature", parts[0] + "." + parts[1] + "." + parts[2][:len(parts[2])-2], ErrTokenMalformed},
+		{"replaced signature", parts[0] + "." + parts[1] + ".invalidsignature", ErrTokenInvalidSignature},
+		{"empty", "", ErrTokenMalformed},
+		{"two segments", parts[0] + "." + parts[1], ErrTokenMalformed},
+		{"four segments", token + ".extra", ErrTokenMalformed},
+		{"separators only", "..", ErrTokenMalformed},
+		{"payload not base64", parts[0] + ".!!!." + parts[2], ErrTokenMalformed},
+		{"header not base64", "!!!." + parts[1] + "." + parts[2], ErrTokenMalformed},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			userID, claims, err := manager.ValidateToken(tc.token)
+			errIs(t, err, tc.want, tc.name)
+			eq(t, userID, "", "user id on failure")
+			if claims != nil {
+				t.Fatalf("%s: claims returned on failure", tc.name)
+			}
+		})
+	}
+}
+
+func TestJWTExpiryAndLeeway(t *testing.T) {
+	now := time.Now()
+	strict, err := NewJWT(testSecret, WithLeeway(0))
+	noErr(t, err, "NewJWT strict")
+	lenient, err := NewJWT(testSecret, WithLeeway(5*time.Minute))
+	noErr(t, err, "NewJWT lenient")
+
+	expired := signHS256(t, testSecret, defaultHeader(), map[string]any{
+		"sub": "u", "iat": now.Add(-2 * time.Hour).Unix(), "exp": now.Add(-time.Minute).Unix(),
+	})
+	_, _, err = strict.ValidateToken(expired)
+	errIs(t, err, ErrTokenExpired, "expired token")
+	_, _, err = lenient.ValidateToken(expired)
+	noErr(t, err, "expiry inside leeway")
+
+	notYet := signHS256(t, testSecret, defaultHeader(), map[string]any{
+		"sub": "u", "nbf": now.Add(2 * time.Second).Unix(), "exp": now.Add(time.Hour).Unix(),
+	})
+	_, _, err = strict.ValidateToken(notYet)
+	errIs(t, err, ErrTokenNotYetValid, "nbf in the future")
+	_, _, err = lenient.ValidateToken(notYet)
+	noErr(t, err, "nbf inside leeway")
+
+	// exp is mandatory. ‼️ mapJWTError has no case for
+	// jwt.ErrTokenRequiredClaimMissing, so this surfaces as malformed.
+	noExp := signHS256(t, testSecret, defaultHeader(), map[string]any{"sub": "u", "iat": now.Unix()})
+	_, _, err = strict.ValidateToken(noExp)
+	errIs(t, err, ErrTokenMissingClaim, "missing exp")
+
+	// generated lifetimes are honored without waiting for them
+	short, err := NewJWT(testSecret, WithTokenLifetime(time.Second))
+	noErr(t, err, "NewJWT short lifetime")
+	token, err := short.GenerateToken("u", nil)
+	noErr(t, err, "GenerateToken")
+	_, payload := jwtParts(t, token)
+	eq(t, int64(num(t, payload, "exp")-num(t, payload, "iat")), int64(1), "encoded lifetime")
+	_, _, err = short.ValidateToken(token)
+	noErr(t, err, "valid immediately")
+}
+
+func TestJWTIssuerAudience(t *testing.T) {
+	manager, err := NewJWT(testSecret,
+		WithTokenLifetime(time.Hour),
 		WithIssuer("test-issuer"),
 		WithAudience([]string{"api.example.com"}),
 	)
-	require.NoError(t, err)
+	noErr(t, err, "NewJWT")
 
-	token, err := jwtMgr.GenerateToken("user1", nil)
-	require.NoError(t, err)
+	token, err := manager.GenerateToken("user1", nil)
+	noErr(t, err, "GenerateToken")
 
-	// Parse token to check claims
-	parsed, _ := jwt.Parse(token, func(token *jwt.Token) (any, error) {
-		return secret, nil
+	_, payload := jwtParts(t, token)
+	eq(t, str(t, payload, "iss"), "test-issuer", "issuer")
+	audience, ok := payload["aud"].([]any)
+	isTrue(t, ok, "audience encoded as an array")
+	eq(t, len(audience), 1, "audience length")
+	eq(t, audience[0], any("api.example.com"), "audience value")
+	eq(t, int64(num(t, payload, "exp")-num(t, payload, "iat")), int64(3600), "custom lifetime")
+
+	_, _, err = manager.ValidateToken(token)
+	noErr(t, err, "self-issued token")
+
+	other, err := NewJWT(testSecret, WithTokenLifetime(time.Hour), WithIssuer("other-issuer"))
+	noErr(t, err, "NewJWT other issuer")
+	otherToken, err := other.GenerateToken("user1", nil)
+	noErr(t, err, "GenerateToken other issuer")
+	_, _, err = manager.ValidateToken(otherToken)
+	errIs(t, err, ErrTokenMissingClaim, "issuer mismatch")
+
+	missingAudience := signHS256(t, testSecret, defaultHeader(), map[string]any{
+		"sub": "u", "iss": "test-issuer", "exp": time.Now().Add(time.Hour).Unix(),
 	})
+	_, _, err = manager.ValidateToken(missingAudience)
+	errIs(t, err, ErrTokenMissingClaim, "absent audience is rejected when expected")
 
-	claims := parsed.Claims.(jwt.MapClaims)
-
-	// Check issuer
-	assert.Equal(t, "test-issuer", claims["iss"])
-
-	// Check audience
-	aud := claims["aud"].([]any)
-	assert.Contains(t, aud, "api.example.com")
-
-	// Check expiration is ~1 hour
-	exp := int64(claims["exp"].(float64))
-	iat := int64(claims["iat"].(float64))
-	assert.InDelta(t, 3600, exp-iat, 10)
-}
-
-func TestJWTErrors(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
-	jwtMgr, err := NewJWT(secret)
-	require.NoError(t, err)
-
-	// Empty user ID
-	_, err = jwtMgr.GenerateToken("", nil)
-	assert.Equal(t, ErrTokenEmptyUserID, err)
-
-	// Invalid token format
-	_, _, err = jwtMgr.ValidateToken("invalid.token")
-	assert.ErrorIs(t, err, ErrTokenMalformed)
-
-	// Tampered signature
-	token, _ := jwtMgr.GenerateToken("user1", nil)
-	parts := strings.Split(token, ".")
-	tampered := parts[0] + "." + parts[1] + ".invalidsignature"
-	_, _, err = jwtMgr.ValidateToken(tampered)
-	assert.ErrorIs(t, err, ErrTokenInvalidSignature)
-
-	// Wrong algorithm
-	rsaKey, _ := rsa.GenerateKey(rand.Reader, 2048)
-	rsaMgr, _ := NewJWTRSA(rsaKey)
-	rsaToken, _ := rsaMgr.GenerateToken("user1", nil)
-
-	_, _, err = jwtMgr.ValidateToken(rsaToken)
-	assert.ErrorIs(t, err, ErrTokenInvalidSignature)
-}
-
-func TestJWTExpiration(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
-
-	// Create token with 1 second lifetime
-	jwtMgr, err := NewJWT(secret, WithTokenLifetime(1*time.Second), WithLeeway(0))
-	require.NoError(t, err)
-
-	token, err := jwtMgr.GenerateToken("user1", nil)
-	require.NoError(t, err)
-
-	// Should be valid immediately
-	_, _, err = jwtMgr.ValidateToken(token)
-	assert.NoError(t, err)
-
-	// Wait for expiration
-	time.Sleep(2 * time.Second)
-
-	// Should be expired
-	_, _, err = jwtMgr.ValidateToken(token)
-	assert.ErrorIs(t, err, ErrTokenExpired)
-}
-
-func TestLeeway(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
-
-	// Create manager with no leeway
-	jwtMgr, err := NewJWT(secret, WithLeeway(0))
-	require.NoError(t, err)
-
-	// Manually create a token with NotBefore in future
-	now := time.Now()
-	token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwt.MapClaims{
-		"sub": "user1",
-		"nbf": now.Add(2 * time.Second).Unix(),
-		"exp": now.Add(1 * time.Hour).Unix(),
+	superset := signHS256(t, testSecret, defaultHeader(), map[string]any{
+		"sub": "u", "iss": "test-issuer", "exp": time.Now().Add(time.Hour).Unix(),
+		"aud": []string{"other.example.com", "api.example.com"},
 	})
-	tokenString, err := token.SignedString(secret)
-	require.NoError(t, err)
+	_, _, err = manager.ValidateToken(superset)
+	noErr(t, err, "expected audience among others")
 
-	// Should fail immediately (not valid yet)
-	_, _, err = jwtMgr.ValidateToken(tokenString)
-	assert.ErrorIs(t, err, ErrTokenNotYetValid)
-
-	// Create manager with leeway
-	jwtMgrWithLeeway, err := NewJWT(secret, WithLeeway(5*time.Second))
-	require.NoError(t, err)
-
-	// Should pass with leeway
-	_, _, err = jwtMgrWithLeeway.ValidateToken(tokenString)
-	assert.NoError(t, err)
+	// an unconstrained manager imposes neither claim
+	plain, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT plain")
+	_, _, err = plain.ValidateToken(token)
+	noErr(t, err, "unconstrained validation")
 }
 
-func TestStandaloneFunctions(t *testing.T) {
-	secret := []byte("test-secret-key-must-be-32-bytes")
-	userID := "standalone-user"
-	claims := map[string]any{"test": "value"}
+func TestJWTUnenforcedClaims(t *testing.T) {
+	// Documented gaps: sub is not required and iat is not verified.
+	// Callers must reject an empty user id themselves.
+	manager, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
 
-	// Generate token
-	token, err := GenerateHS256Token(secret, userID, claims, 1*time.Hour)
-	require.NoError(t, err)
-
-	// Validate token
-	extractedUserID, extractedClaims, err := ValidateHS256Token(secret, token)
-	require.NoError(t, err)
-
-	assert.Equal(t, userID, extractedUserID)
-	assert.Equal(t, "value", extractedClaims["test"])
-
-	// Test with short secret
-	_, err = GenerateHS256Token([]byte("short"), userID, claims, 1*time.Hour)
-	assert.Equal(t, ErrSecretTooShort, err)
+	token := signHS256(t, testSecret, defaultHeader(), map[string]any{
+		"exp": time.Now().Add(time.Hour).Unix(),
+		"iat": time.Now().Add(24 * time.Hour).Unix(),
+	})
+	userID, claims, err := manager.ValidateToken(token)
+	noErr(t, err, "token without subject")
+	eq(t, userID, "", "empty subject accepted")
+	eq(t, len(claims), 0, "no extra claims")
 }
 
-func TestJWTRSAFromPEM(t *testing.T) {
-	// 1. Generate a new RSA key pair for this test
-	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
-	require.NoError(t, err)
+func TestJWTOptionGuards(t *testing.T) {
+	manager, err := NewJWT(testSecret,
+		WithTokenLifetime(0), WithTokenLifetime(-time.Hour), WithLeeway(-time.Second))
+	noErr(t, err, "NewJWT")
+	eq(t, manager.tokenLifetime, DefaultTokenLifetime, "lifetime unchanged")
+	eq(t, manager.leeway, DefaultLeeway, "leeway unchanged")
 
-	// 2. Encode the private key to PEM format
-	privateKeyPEM := pem.EncodeToMemory(&pem.Block{
-		Type:  "RSA PRIVATE KEY",
-		Bytes: x509.MarshalPKCS1PrivateKey(privateKey),
-	})
+	manager, err = NewJWT(testSecret, WithLeeway(0))
+	noErr(t, err, "NewJWT zero leeway")
+	eq(t, manager.leeway, time.Duration(0), "zero leeway applied")
 
-	// 3. Encode the public key to PEM format
-	publicKeyBytes, err := x509.MarshalPKIXPublicKey(&privateKey.PublicKey)
-	require.NoError(t, err)
-	publicKeyPEM := pem.EncodeToMemory(&pem.Block{
-		Type:  "PUBLIC KEY",
-		Bytes: publicKeyBytes,
-	})
+	// options apply to every constructor
+	verifier, err := NewJWTVerifier(&testRSAKey().PublicKey, WithIssuer("iss"), WithLeeway(time.Minute))
+	noErr(t, err, "NewJWTVerifier")
+	eq(t, verifier.issuer, "iss", "issuer")
+	eq(t, verifier.leeway, time.Minute, "leeway")
+}
 
-	// 4. Test the PEM constructor for the signer
-	jwtMgr, err := NewJWTRSAFromPEM(privateKeyPEM)
-	require.NoError(t, err)
+func TestJWTStandaloneFunctions(t *testing.T) {
+	token, err := GenerateHS256Token(testSecret, "standalone-user",
+		map[string]any{"test": "value", "count": 42}, time.Hour)
+	noErr(t, err, "GenerateHS256Token")
 
-	token, err := jwtMgr.GenerateToken("user-from-pem", nil)
-	require.NoError(t, err)
-	assert.NotEmpty(t, token)
+	userID, claims, err := ValidateHS256Token(testSecret, token)
+	noErr(t, err, "ValidateHS256Token")
+	eq(t, userID, "standalone-user", "user id")
+	eq(t, str(t, claims, "test"), "value", "string claim")
+	eq(t, claims["count"], any(float64(42)), "numeric claim after JSON round trip")
 
-	// 5. Test the PEM constructor for the verifier
-	verifier, err := NewJWTVerifierFromPEM(publicKeyPEM)
-	require.NoError(t, err)
+	_, _, err = ValidateHS256Token(bytes.Repeat([]byte("x"), 32), token)
+	errIs(t, err, ErrTokenInvalidSignature, "wrong secret")
 
-	userID, _, err := verifier.ValidateToken(token)
-	require.NoError(t, err)
-	assert.Equal(t, "user-from-pem", userID)
+	expired, err := GenerateHS256Token(testSecret, "u", nil, -time.Hour)
+	noErr(t, err, "expired token")
+	_, _, err = ValidateHS256Token(testSecret, expired)
+	errIs(t, err, ErrTokenExpired, "expired beyond default leeway")
+
+	// standalone validation checks neither issuer nor audience
+	scoped, err := NewJWT(testSecret, WithIssuer("iss"), WithAudience([]string{"aud"}))
+	noErr(t, err, "NewJWT scoped")
+	scopedToken, err := scoped.GenerateToken("u", nil)
+	noErr(t, err, "GenerateToken scoped")
+	_, _, err = ValidateHS256Token(testSecret, scopedToken)
+	noErr(t, err, "issuer and audience are not enforced standalone")
+
+	// tokens are interchangeable with the manager form
+	managed, err := NewJWT(testSecret)
+	noErr(t, err, "NewJWT")
+	_, _, err = managed.ValidateToken(token)
+	noErr(t, err, "standalone token accepted by manager")
+}
+
+func TestJWTPEM(t *testing.T) {
+	key := testRSAKey()
+
+	pkcs1 := pem.EncodeToMemory(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(key)})
+	pkcs8Bytes, err := x509.MarshalPKCS8PrivateKey(key)
+	noErr(t, err, "marshal pkcs8")
+	pkcs8 := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: pkcs8Bytes})
+	pkixBytes, err := x509.MarshalPKIXPublicKey(&key.PublicKey)
+	noErr(t, err, "marshal pkix")
+	pkix := pem.EncodeToMemory(&pem.Block{Type: "PUBLIC KEY", Bytes: pkixBytes})
+
+	for name, blob := range map[string][]byte{"pkcs1": pkcs1, "pkcs8": pkcs8} {
+		t.Run(name, func(t *testing.T) {
+			signer, err := NewJWTRSAFromPEM(blob, WithTokenLifetime(time.Hour))
+			noErr(t, err, "NewJWTRSAFromPEM")
+			eq(t, signer.tokenLifetime, time.Hour, "options forwarded")
+
+			token, err := signer.GenerateToken("user-from-pem", nil)
+			noErr(t, err, "GenerateToken")
+
+			verifier, err := NewJWTVerifierFromPEM(pkix)
+			noErr(t, err, "NewJWTVerifierFromPEM")
+			userID, _, err := verifier.ValidateToken(token)
+			noErr(t, err, "ValidateToken")
+			eq(t, userID, "user-from-pem", "user id")
+		})
+	}
 
-	// 6. Test failure cases with invalid data
 	_, err = NewJWTRSAFromPEM([]byte("invalid pem data"))
-	assert.ErrorIs(t, err, ErrRSAInvalidPEM)
-
+	errIs(t, err, ErrRSAInvalidPEM, "private: not pem")
+	_, err = NewJWTRSAFromPEM(nil)
+	errIs(t, err, ErrRSAInvalidPEM, "private: empty")
 	_, err = NewJWTVerifierFromPEM([]byte("invalid pem data"))
-	assert.ErrorIs(t, err, ErrRSAInvalidPEM)
+	errIs(t, err, ErrRSAInvalidPEM, "public: not pem")
+
+	_, err = NewJWTRSAFromPEM(pkix)
+	errIs(t, err, ErrRSAInvalidPrivateKey, "public key supplied as private")
+	_, err = NewJWTVerifierFromPEM(pkcs8)
+	errIs(t, err, ErrRSAInvalidPublicKey, "private key supplied as public")
+
+	// well-formed keys of the wrong algorithm
+	edPub, edPriv, err := ed25519.GenerateKey(rand.Reader)
+	noErr(t, err, "ed25519 keygen")
+	edPubBytes, err := x509.MarshalPKIXPublicKey(edPub)
+	noErr(t, err, "marshal ed25519 public")
+	_, err = NewJWTVerifierFromPEM(pem.EncodeToMemory(&pem.Block{Type: "PUBLIC KEY", Bytes: edPubBytes}))
+	errIs(t, err, ErrRSANotPublicKey, "non-rsa public key")
+
+	edPrivBytes, err := x509.MarshalPKCS8PrivateKey(edPriv)
+	noErr(t, err, "marshal ed25519 private")
+	_, err = NewJWTRSAFromPEM(pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: edPrivBytes}))
+	errIs(t, err, ErrRSAInvalidPrivateKey, "non-rsa private key")
 }
 
+func TestMapJWTError(t *testing.T) {
+	cases := []struct {
+		in   error
+		want error
+	}{
+		{jwt.ErrTokenMalformed, ErrTokenMalformed},
+		{jwt.ErrTokenUnverifiable, ErrTokenMalformed},
+		{jwt.ErrTokenSignatureInvalid, ErrTokenInvalidSignature},
+		{jwt.ErrTokenExpired, ErrTokenExpired},
+		{jwt.ErrTokenNotValidYet, ErrTokenNotYetValid},
+		{jwt.ErrTokenInvalidAudience, ErrTokenMissingClaim},
+		{jwt.ErrTokenInvalidIssuer, ErrTokenMissingClaim},
+		{jwt.ErrTokenRequiredClaimMissing, ErrTokenMissingClaim},
+		{errors.New("unclassified"), ErrTokenMalformed},
+	}
+	for _, tc := range cases {
+		got := mapJWTError(tc.in)
+		errIs(t, got, tc.want, "mapped sentinel")
+		errIs(t, got, tc.in, "original error preserved")
+	}
+}
+
+func TestJWTConcurrency(t *testing.T) {
+	manager, err := NewJWT(testSecret, WithTokenLifetime(time.Hour), WithIssuer("iss"))
+	noErr(t, err, "NewJWT")
+	shared, err := manager.GenerateToken("user1", map[string]any{"role": "admin"})
+	noErr(t, err, "GenerateToken")
+
+	const n = 32
+	errs := make(chan error, n)
+	var wg sync.WaitGroup
+	for i := range n {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, _, err := manager.ValidateToken(shared); err != nil {
+				errs <- err
+				return
+			}
+			token, err := manager.GenerateToken(fmt.Sprintf("user-%d", i), map[string]any{"n": i})
+			if err != nil {
+				errs <- err
+				return
+			}
+			if _, _, err := manager.ValidateToken(token); err != nil {
+				errs <- err
+			}
+		}()
+	}
+	wg.Wait()
+	close(errs)
+	for err := range errs {
+		t.Errorf("concurrent operation: %v", err)
+	}
+}
+
+func FuzzValidateHS256Token(f *testing.F) {
+	token, err := GenerateHS256Token(testSecret, "seed", map[string]any{"a": 1}, time.Hour)
+	if err != nil {
+		f.Fatal(err)
+	}
+	f.Add(token)
+	f.Add("")
+	f.Add("a.b.c")
+	f.Add(strings.Repeat(".", 16))
+
+	f.Fuzz(func(t *testing.T, s string) {
+		_, _, err := ValidateHS256Token(testSecret, s)
+		if err != nil {
+			return
+		}
+		// acceptance implies a well-formed, correctly signed token
+		parts := strings.Split(s, ".")
+		if len(parts) != 3 {
+			t.Fatalf("accepted token with %d segments", len(parts))
+		}
+		mac := hmac.New(sha256.New, testSecret)
+		mac.Write([]byte(parts[0] + "." + parts[1]))
+		sig, decErr := base64.RawURLEncoding.DecodeString(parts[2])
+		if decErr != nil || !hmac.Equal(sig, mac.Sum(nil)) {
+			t.Fatal("accepted token with an invalid signature")
+		}
+	})
+}
+
+func BenchmarkJWTHS256(b *testing.B) {
+	manager, err := NewJWT(testSecret)
+	if err != nil {
+		b.Fatal(err)
+	}
+	claims := map[string]any{"role": "admin"}
+	for b.Loop() {
+		token, err := manager.GenerateToken("user1", claims)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if _, _, err := manager.ValidateToken(token); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
diff --git a/scram.go b/scram.go
index 1303ab1..7e57905 100644
--- a/scram.go
+++ b/scram.go
@@ -154,12 +154,18 @@ func ImportCredential(data map[string]any) (*Credential, error) {
 	if argonTime == 0 || argonMemory == 0 || argonThreads == 0 {
 		return nil, ErrSCRAMZeroParams
 	}
+	if err := checkArgonCost(argonMemory, argonTime, argonThreads); err != nil {
+		return nil, ErrSCRAMParamsTooLarge
+	}
 	if len(salt) < 16 {
 		return nil, ErrSCRAMSaltTooShort
 	}
-	if len(storedKey) != sha256.Size || len(serverKey) != sha256.Size {
+	if len(storedKey) != sha256.Size {
 		return nil, ErrCredInvalidStoredKey
 	}
+	if len(serverKey) != sha256.Size {
+		return nil, ErrCredInvalidServerKey
+	}
 
 	return &Credential{
 		Username:     username,
@@ -405,7 +411,8 @@ func (s *ScramServer) ProcessClientFinalMessage(fullNonce, clientProof string) (
 	if len(clientProofBytes) != len(clientSignature) {
 		return ServerFinalMessage{}, ErrSCRAMInvalidProofLen
 	}
-	clientKey := xorBytes(clientProofBytes, clientSignature)
+	clientKey := make([]byte, len(clientProofBytes))
+	subtle.XORBytes(clientKey, clientProofBytes, clientSignature)
 
 	// Verify by computing StoredKey
 	computedStoredKey := sha256.Sum256(clientKey)
@@ -491,6 +498,11 @@ func (c *ScramClient) ProcessServerFirstMessage(msg ServerFirstMessage) (ClientF
 	if msg.ArgonTime == 0 || msg.ArgonMemory == 0 || msg.ArgonThreads == 0 {
 		return ClientFinalRequest{}, ErrSCRAMZeroParams
 	}
+	// The peer chooses these values. Unbounded, they are a remote OOM
+	// against every client that talks to a hostile or compromised server.
+	if err := checkArgonCost(msg.ArgonMemory, msg.ArgonTime, msg.ArgonThreads); err != nil {
+		return ClientFinalRequest{}, ErrSCRAMParamsTooLarge
+	}
 
 	// Derive keys using Argon2id
 	saltedPassword := argon2.IDKey([]byte(c.Password), salt, msg.ArgonTime, msg.ArgonMemory, msg.ArgonThreads, 32)
@@ -506,7 +518,8 @@ func (c *ScramClient) ProcessServerFirstMessage(msg ServerFirstMessage) (ClientF
 
 	// Compute client proof
 	clientSignature := computeHMAC(storedKey[:], []byte(c.authMessage))
-	clientProof := xorBytes(clientKey, clientSignature)
+	clientProof := make([]byte, len(clientKey))
+	subtle.XORBytes(clientProof, clientKey, clientSignature)
 
 	// Store server key for verification
 	c.serverKey = serverKey
@@ -589,14 +602,3 @@ func computeHMAC(key, message []byte) []byte {
 	mac.Write(message)
 	return mac.Sum(nil)
 }
-
-func xorBytes(a, b []byte) []byte {
-	if len(a) != len(b) {
-		panic("xor length mismatch")
-	}
-	result := make([]byte, len(a))
-	for i := range a {
-		result[i] = a[i] ^ b[i]
-	}
-	return result
-}
diff --git a/scram_test.go b/scram_test.go
index e379e23..01a5721 100644
--- a/scram_test.go
+++ b/scram_test.go
@@ -1,334 +1,700 @@
 package auth
 
 import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
 	"fmt"
+	"math"
+	"strings"
 	"sync"
 	"testing"
 	"time"
 
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
+	"golang.org/x/crypto/argon2"
 )
 
-// setupScramTest is a helper to initialize a server and user credential for testing.
-// It performs the full Argon2 -> SCRAM migration workflow.
-func setupScramTest(t *testing.T) (server *ScramServer, username, password string, cred *Credential) {
-	username = "testuser"
-	password = "SecurePassword123"
-
-	// 1. Start with an Argon2 PHC hash, as a real application would.
-	phcHash, err := HashPassword(password)
-	require.NoError(t, err, "Setup failed: could not hash password")
-
-	// 2. Migrate the PHC hash to a SCRAM credential.
-	cred, err = MigrateFromPHC(username, password, phcHash)
-	require.NoError(t, err, "Setup failed: could not migrate from PHC hash")
-
-	// 3. Create a server and add the new credential.
-	server = NewScramServer()
-	t.Cleanup(server.Stop)
-	server.AddCredential(cred)
-
-	return server, username, password, cred
+func newTestServer(t *testing.T) *ScramServer {
+	t.Helper()
+	s := NewScramServer()
+	t.Cleanup(s.Stop)
+	return s
 }
 
-// TestScram_FullRoundtrip_Success simulates a complete, successful authentication handshake.
-func TestScram_FullRoundtrip_Success(t *testing.T) {
-	server, username, password, _ := setupScramTest(t)
-	client := NewScramClient(username, password)
-
-	// --- Step 1: Client sends its first message ---
-	clientFirst, err := client.StartAuthentication()
-	require.NoError(t, err)
-
-	// --- Step 2: Server receives client's message and responds ---
-	serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-	require.NoError(t, err, "Server failed to process client's first message")
-
-	// --- Step 3: Client receives server's message, computes proof ---
-	clientFinal, err := client.ProcessServerFirstMessage(serverFirst)
-	require.NoError(t, err, "Client failed to process server's first message")
-
-	// --- Step 4: Server receives client's proof and verifies it ---
-	serverFinal, err := server.ProcessClientFinalMessage(clientFinal.FullNonce, clientFinal.ClientProof)
-	require.NoError(t, err, "Server failed to verify client's final proof")
-	assert.NotEmpty(t, serverFinal.ServerSignature, "Server signature should not be empty")
-
-	// --- Step 5: Client verifies server's signature (mutual authentication) ---
-	err = client.VerifyServerFinalMessage(serverFinal)
-	assert.NoError(t, err, "Client failed to verify server's final signature")
-
-	t.Log("SCRAM full roundtrip successful")
+func testCredential(t *testing.T, username, password string) *Credential {
+	t.Helper()
+	phcHash, err := HashPassword(password, cheapArgon...)
+	noErr(t, err, "HashPassword")
+	cred, err := MigrateFromPHC(username, password, phcHash)
+	noErr(t, err, "MigrateFromPHC")
+	return cred
 }
 
-// TestScram_FullRoundtrip_WrongPassword ensures authentication fails with an incorrect password.
-func TestScram_FullRoundtrip_WrongPassword(t *testing.T) {
-	server, username, _, _ := setupScramTest(t)
-	defer server.Stop()
-	// Create a client with the WRONG password
-	client := NewScramClient(username, "WrongPassword!!!")
-
-	// Steps 1-3 will appear to succeed, as the client doesn't know the password is wrong yet.
-	clientFirst, err := client.StartAuthentication()
-	require.NoError(t, err)
-
-	serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-	require.NoError(t, err)
-
-	clientFinal, err := client.ProcessServerFirstMessage(serverFirst)
-	require.NoError(t, err)
-
-	// --- Step 4: Server verification should fail here ---
-	_, err = server.ProcessClientFinalMessage(clientFinal.FullNonce, clientFinal.ClientProof)
-	assert.ErrorIs(t, err, ErrInvalidCredentials, "Server should reject proof from wrong password")
-
-	t.Log("SCRAM correctly failed for wrong password")
+func setupScram(t *testing.T) (*ScramServer, string, string, *Credential) {
+	t.Helper()
+	const username, password = "testuser", "SecurePassword123"
+	cred := testCredential(t, username, password)
+	s := newTestServer(t)
+	s.AddCredential(cred)
+	return s, username, password, cred
 }
 
-// TestScram_FullRoundtrip_UserNotFound tests for user enumeration protection.
-// The server must be indistinguishable from the wrong-password path: no error at
-// first message, stable decoy salt across probes, ErrInvalidCredentials at proof.
-func TestScram_FullRoundtrip_UserNotFound(t *testing.T) {
-	server, _, _, _ := setupScramTest(t)
-	defer server.Stop()
-	client := NewScramClient("unknown_user", "any_password")
-
-	clientFirst, err := client.StartAuthentication()
-	require.NoError(t, err)
-
-	// unknown user must not error here
-	serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-	require.NoError(t, err, "unknown user must not be signalled at first message")
-	assert.NotEmpty(t, serverFirst.FullNonce)
-	assert.NotEmpty(t, serverFirst.Salt)
-
-	// decoy salt must be stable across repeated probes
-	second, err := server.ProcessClientFirstMessage(clientFirst.Username, "probe-nonce-2")
-	require.NoError(t, err)
-	assert.Equal(t, serverFirst.Salt, second.Salt, "decoy salt must be deterministic")
-	assert.Equal(t, serverFirst.ArgonTime, second.ArgonTime)
-	assert.Equal(t, serverFirst.ArgonMemory, second.ArgonMemory)
-	assert.Equal(t, serverFirst.ArgonThreads, second.ArgonThreads)
-
-	clientFinal, err := client.ProcessServerFirstMessage(serverFirst)
-	require.NoError(t, err)
-
-	// failure surfaces only here, identical to wrong-password path
-	_, err = server.ProcessClientFinalMessage(clientFinal.FullNonce, clientFinal.ClientProof)
-	assert.ErrorIs(t, err, ErrInvalidCredentials, "unknown user must fail like wrong password")
+func handshakeCount(s *ScramServer) int {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return len(s.handshakes)
 }
 
-// TestScram_InvalidNonce simulates a replay attack or message mismatch.
-func TestScram_InvalidNonce(t *testing.T) {
-	server, username, password, _ := setupScramTest(t)
-	client := NewScramClient(username, password)
-
-	// Perform the first part of the handshake
-	clientFirst, _ := client.StartAuthentication()
-	serverFirst, _ := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-	clientFinal, _ := client.ProcessServerFirstMessage(serverFirst)
-
-	// Attempt to finalize with a completely different nonce
-	_, err := server.ProcessClientFinalMessage("this-is-a-bad-nonce", clientFinal.ClientProof)
-	assert.ErrorIs(t, err, ErrSCRAMInvalidNonce, "Server should reject a final message with an unknown nonce")
+// startHandshake drives a fresh client to the point where a proof is pending.
+func startHandshake(t *testing.T, s *ScramServer, username, password string) ClientFinalRequest {
+	t.Helper()
+	c := NewScramClient(username, password)
+	first, err := c.StartAuthentication()
+	noErr(t, err, "StartAuthentication")
+	serverFirst, err := s.ProcessClientFirstMessage(first.Username, first.ClientNonce)
+	noErr(t, err, "ProcessClientFirstMessage")
+	final, err := c.ProcessServerFirstMessage(serverFirst)
+	noErr(t, err, "ProcessServerFirstMessage")
+	return final
 }
 
-// TestScram_CredentialImportExport verifies that credentials can be serialized and deserialized correctly.
-func TestScram_CredentialImportExport(t *testing.T) {
-	_, _, _, originalCred := setupScramTest(t)
-
-	// Export the credential to a map
-	exportedData := originalCred.Export()
-	require.NotNil(t, exportedData)
-
-	// Assert that required fields exist and are strings (as they are base64 encoded)
-	assert.IsType(t, "", exportedData["salt"])
-	assert.IsType(t, "", exportedData["stored_key"])
-	assert.IsType(t, "", exportedData["server_key"])
-
-	// Import the credential back from the map
-	importedCred, err := ImportCredential(exportedData)
-	require.NoError(t, err)
-	require.NotNil(t, importedCred)
-
-	// Verify that the imported credential is identical to the original
-	assert.Equal(t, originalCred.Username, importedCred.Username)
-	assert.Equal(t, originalCred.Salt, importedCred.Salt)
-	assert.Equal(t, originalCred.ArgonTime, importedCred.ArgonTime)
-	assert.Equal(t, originalCred.ArgonMemory, importedCred.ArgonMemory)
-	assert.Equal(t, originalCred.ArgonThreads, importedCred.ArgonThreads)
-	assert.Equal(t, originalCred.StoredKey, importedCred.StoredKey)
-	assert.Equal(t, originalCred.ServerKey, importedCred.ServerKey)
-
-	t.Log("SCRAM credential import/export successful")
-}
-
-// TestScramServerCleanup verifies automatic cleanup of expired handshakes
-func TestScramServerCleanup(t *testing.T) {
-	// Create server with short cleanup interval for testing
-	server := NewScramServer()
-	defer server.Stop()
-
-	// Add a test credential
-	cred := &Credential{
-		Username:     "testuser",
-		Salt:         []byte("salt1234567890123456"),
-		ArgonTime:    1,
-		ArgonMemory:  64,
-		ArgonThreads: 1,
-		StoredKey:    []byte("stored_key_placeholder"),
-		ServerKey:    []byte("server_key_placeholder"),
+func runHandshake(s *ScramServer, c *ScramClient) error {
+	first, err := c.StartAuthentication()
+	if err != nil {
+		return err
 	}
-	server.AddCredential(cred)
+	serverFirst, err := s.ProcessClientFirstMessage(first.Username, first.ClientNonce)
+	if err != nil {
+		return err
+	}
+	final, err := c.ProcessServerFirstMessage(serverFirst)
+	if err != nil {
+		return err
+	}
+	serverFinal, err := s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	if err != nil {
+		return err
+	}
+	return c.VerifyServerFinalMessage(serverFinal)
+}
 
-	// Start multiple handshakes
-	var nonces []string
-	for i := 0; i < 5; i++ {
-		clientNonce := fmt.Sprintf("client-nonce-%d", i)
-		msg, err := server.ProcessClientFirstMessage("testuser", clientNonce)
-		require.NoError(t, err)
-		nonces = append(nonces, msg.FullNonce)
+func TestScramRoundtrip(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+	noErr(t, runHandshake(s, NewScramClient(user, pw)), "handshake")
+	eq(t, handshakeCount(s), 0, "handshake retained after success")
+}
+
+func TestScramWrongPassword(t *testing.T) {
+	s, user, _, _ := setupScram(t)
+	errIs(t, runHandshake(s, NewScramClient(user, "WrongPassword!!!")), ErrInvalidCredentials, "wrong password")
+	eq(t, handshakeCount(s), 0, "handshake retained after failure")
+}
+
+func TestScramUnknownUser(t *testing.T) {
+	s, _, _, cred := setupScram(t)
+	c := NewScramClient("unknown_user", "any_password")
+
+	first, err := c.StartAuthentication()
+	noErr(t, err, "StartAuthentication")
+
+	serverFirst, err := s.ProcessClientFirstMessage(first.Username, first.ClientNonce)
+	noErr(t, err, "unknown user must not be signalled at the first message")
+
+	// the decoy must mirror the registered parameter shape
+	eq(t, serverFirst.ArgonTime, cred.ArgonTime, "decoy time")
+	eq(t, serverFirst.ArgonMemory, cred.ArgonMemory, "decoy memory")
+	eq(t, serverFirst.ArgonThreads, cred.ArgonThreads, "decoy threads")
+	decoySalt, err := base64.StdEncoding.DecodeString(serverFirst.Salt)
+	noErr(t, err, "decoy salt decode")
+	eq(t, len(decoySalt), len(cred.Salt), "decoy salt length")
+
+	// stable across probes, distinct per username
+	second, err := s.ProcessClientFirstMessage("unknown_user", "probe-2")
+	noErr(t, err, "second probe")
+	eq(t, second.Salt, serverFirst.Salt, "decoy salt must be deterministic")
+	third, err := s.ProcessClientFirstMessage("other_unknown", "probe-3")
+	noErr(t, err, "third probe")
+	if third.Salt == serverFirst.Salt {
+		t.Fatal("decoy salt is not username-bound")
 	}
 
-	// Verify all handshakes exist
-	server.mu.RLock()
-	assert.Len(t, server.handshakes, 5)
-	server.mu.RUnlock()
+	// failure surfaces only at the proof step, as for a wrong password
+	final, err := c.ProcessServerFirstMessage(serverFirst)
+	noErr(t, err, "ProcessServerFirstMessage")
+	_, err = s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	errIs(t, err, ErrInvalidCredentials, "unknown user")
+}
 
-	// Manually set old timestamp for first 3 handshakes
-	server.mu.Lock()
-	oldTime := time.Now().Add(-2 * ScramHandshakeTimeout)
-	count := 0
-	for nonce := range server.handshakes {
-		if count < 3 {
-			server.handshakes[nonce].CreatedAt = oldTime
-			count++
+func TestScramDecoySaltIsolation(t *testing.T) {
+	// decoyKey is per-instance: a shared decoy salt would be a global oracle
+	// for account existence across a cluster.
+	a, b := newTestServer(t), newTestServer(t)
+	first, err := a.ProcessClientFirstMessage("ghost", "n1")
+	noErr(t, err, "server a")
+	second, err := b.ProcessClientFirstMessage("ghost", "n2")
+	noErr(t, err, "server b")
+	if first.Salt == second.Salt {
+		t.Fatal("decoy salt is identical across server instances")
+	}
+
+	// with no credential registered the template is empty and defaults apply
+	raw, err := base64.StdEncoding.DecodeString(first.Salt)
+	noErr(t, err, "decode")
+	eq(t, len(raw), DefaultArgonSaltLen, "fallback salt length")
+	eq(t, first.ArgonTime, uint32(DefaultArgonTime), "fallback time")
+	eq(t, first.ArgonMemory, uint32(DefaultArgonMemory), "fallback memory")
+	eq(t, first.ArgonThreads, uint8(DefaultArgonThreads), "fallback threads")
+}
+
+func TestScramDecoySaltMultiBlock(t *testing.T) {
+	s := newTestServer(t)
+	cred := testCredential(t, "u", "SecurePassword123")
+	cred.Salt = make([]byte, 48) // exceeds one HMAC-SHA256 block
+	s.AddCredential(cred)
+
+	msg, err := s.ProcessClientFirstMessage("ghost", "n")
+	noErr(t, err, "first message")
+	raw, err := base64.StdEncoding.DecodeString(msg.Salt)
+	noErr(t, err, "decode")
+	eq(t, len(raw), 48, "decoy salt length")
+}
+
+func TestScramReplayAndUnknownNonce(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+	final := startHandshake(t, s, user, pw)
+
+	_, err := s.ProcessClientFinalMessage("this-is-a-bad-nonce", final.ClientProof)
+	errIs(t, err, ErrSCRAMInvalidNonce, "unknown nonce")
+
+	_, err = s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	noErr(t, err, "first proof")
+
+	_, err = s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	errIs(t, err, ErrSCRAMInvalidNonce, "replayed proof")
+}
+
+func TestScramProofBinding(t *testing.T) {
+	// A proof commits to its own auth message; moving it to another live
+	// handshake for the same user must fail.
+	s, user, pw, _ := setupScram(t)
+	a := startHandshake(t, s, user, pw)
+	b := startHandshake(t, s, user, pw)
+
+	_, err := s.ProcessClientFinalMessage(a.FullNonce, b.ClientProof)
+	errIs(t, err, ErrInvalidCredentials, "cross-handshake proof")
+
+	// the rejected attempt consumed handshake a but left b intact
+	_, err = s.ProcessClientFinalMessage(a.FullNonce, a.ClientProof)
+	errIs(t, err, ErrSCRAMInvalidNonce, "handshake a consumed")
+	_, err = s.ProcessClientFinalMessage(b.FullNonce, b.ClientProof)
+	noErr(t, err, "handshake b unaffected")
+}
+
+func TestScramProofEncoding(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+
+	cases := []struct {
+		name  string
+		proof string
+		want  error
+	}{
+		{"not base64", "!!!not base64!!!", ErrSCRAMInvalidProof},
+		{"empty", "", ErrSCRAMInvalidProofLen},
+		{"short", base64.StdEncoding.EncodeToString(make([]byte, 16)), ErrSCRAMInvalidProofLen},
+		{"long", base64.StdEncoding.EncodeToString(make([]byte, sha256.Size+1)), ErrSCRAMInvalidProofLen},
+		{"zeroed", base64.StdEncoding.EncodeToString(make([]byte, sha256.Size)), ErrInvalidCredentials},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			final := startHandshake(t, s, user, pw)
+			_, err := s.ProcessClientFinalMessage(final.FullNonce, tc.proof)
+			errIs(t, err, tc.want, tc.name)
+		})
+	}
+}
+
+func TestScramVerifyInProgress(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+	final := startHandshake(t, s, user, pw)
+
+	s.mu.RLock()
+	state := s.handshakes[final.FullNonce]
+	s.mu.RUnlock()
+	if state == nil {
+		t.Fatal("handshake not registered")
+	}
+
+	state.verifying.Store(1)
+	_, err := s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	errIs(t, err, ErrSCRAMVerifyInProgress, "concurrent verification")
+
+	// the rejected attempt must not consume the handshake
+	state.verifying.Store(0)
+	_, err = s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	noErr(t, err, "retry after release")
+}
+
+func TestScramTimeouts(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+	final := startHandshake(t, s, user, pw)
+
+	s.mu.Lock()
+	s.handshakes[final.FullNonce].CreatedAt = time.Now().Add(-2 * ScramHandshakeTimeout)
+	s.mu.Unlock()
+
+	_, err := s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	errIs(t, err, ErrSCRAMTimeout, "server-side timeout")
+	_, err = s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	errIs(t, err, ErrSCRAMInvalidNonce, "expired handshake consumed")
+
+	// client-side clock
+	c := NewScramClient(user, pw)
+	_, err = c.StartAuthentication()
+	noErr(t, err, "StartAuthentication")
+	c.startTime = time.Now().Add(-2 * ScramHandshakeTimeout)
+
+	_, err = c.ProcessServerFirstMessage(ServerFirstMessage{
+		FullNonce:   "n",
+		Salt:        base64.StdEncoding.EncodeToString(make([]byte, 16)),
+		ArgonTime:   testArgonTime,
+		ArgonMemory: testArgonMemory, ArgonThreads: testArgonThreads,
+	})
+	errIs(t, err, ErrSCRAMTimeout, "client timeout on server-first")
+
+	c.authMessage = "seeded"
+	c.serverKey = make([]byte, sha256.Size)
+	errIs(t, c.VerifyServerFinalMessage(ServerFinalMessage{}), ErrSCRAMTimeout, "client timeout on server-final")
+}
+
+func TestScramCleanup(t *testing.T) {
+	s, user, _, _ := setupScram(t)
+	for i := range 5 {
+		_, err := s.ProcessClientFirstMessage(user, fmt.Sprintf("client-nonce-%d", i))
+		noErr(t, err, "first message")
+	}
+	eq(t, handshakeCount(s), 5, "registered handshakes")
+
+	s.mu.Lock()
+	aged := 0
+	for _, state := range s.handshakes {
+		if aged == 3 {
+			break
+		}
+		state.CreatedAt = time.Now().Add(-2 * ScramHandshakeTimeout)
+		aged++
+	}
+	s.mu.Unlock()
+
+	s.cleanupExpiredHandshakes()
+	eq(t, handshakeCount(s), 2, "after sweep")
+
+	// a handshake under verification survives the sweep regardless of age
+	s.mu.Lock()
+	for _, state := range s.handshakes {
+		state.CreatedAt = time.Now().Add(-2 * ScramHandshakeTimeout)
+		state.verifying.Store(1)
+	}
+	s.mu.Unlock()
+
+	s.cleanupExpiredHandshakes()
+	eq(t, handshakeCount(s), 2, "verifying handshakes must not be evicted")
+}
+
+func TestScramHandshakeCap(t *testing.T) {
+	s, user, _, _ := setupScram(t)
+	for i := range ScramMaxHandshakes {
+		_, err := s.ProcessClientFirstMessage(user, fmt.Sprintf("n-%d", i))
+		noErr(t, err, "first message")
+	}
+	eq(t, handshakeCount(s), ScramMaxHandshakes, "at capacity")
+
+	_, err := s.ProcessClientFirstMessage(user, "overflow")
+	errIs(t, err, ErrSCRAMTooManyHandshakes, "known user at capacity")
+
+	// the cap precedes credential lookup, so it is not an enumeration oracle
+	_, err = s.ProcessClientFirstMessage("ghost", "overflow")
+	errIs(t, err, ErrSCRAMTooManyHandshakes, "unknown user at capacity")
+
+	s.mu.Lock()
+	for _, state := range s.handshakes {
+		state.CreatedAt = time.Now().Add(-2 * ScramHandshakeTimeout)
+	}
+	s.mu.Unlock()
+
+	_, err = s.ProcessClientFirstMessage(user, "after-sweep")
+	noErr(t, err, "capacity reclaimed by the opportunistic sweep")
+	eq(t, handshakeCount(s), 1, "all expired slots reclaimed")
+}
+
+func TestScramNonceUniqueness(t *testing.T) {
+	s, user, _, _ := setupScram(t)
+	seen := make(map[string]struct{}, 256)
+	for range 256 {
+		// a fixed client nonce must not produce a fixed full nonce
+		msg, err := s.ProcessClientFirstMessage(user, "fixed-client-nonce")
+		noErr(t, err, "first message")
+		if _, dup := seen[msg.FullNonce]; dup {
+			t.Fatalf("duplicate full nonce: %s", msg.FullNonce)
+		}
+		seen[msg.FullNonce] = struct{}{}
+	}
+}
+
+func TestScramStopIdempotent(t *testing.T) {
+	s := NewScramServer()
+	s.Stop()
+	s.Stop() // stopOnce must absorb the second close
+}
+
+func TestScramClientState(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+	c := NewScramClient(user, pw)
+
+	errIs(t, c.VerifyServerFinalMessage(ServerFinalMessage{}), ErrSCRAMInvalidState, "unstarted client")
+
+	first, err := c.StartAuthentication()
+	noErr(t, err, "StartAuthentication")
+	serverFirst, err := s.ProcessClientFirstMessage(first.Username, first.ClientNonce)
+	noErr(t, err, "ProcessClientFirstMessage")
+	final, err := c.ProcessServerFirstMessage(serverFirst)
+	noErr(t, err, "ProcessServerFirstMessage")
+	serverFinal, err := s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof)
+	noErr(t, err, "ProcessClientFinalMessage")
+
+	tampered := serverFinal
+	tampered.ServerSignature = base64.StdEncoding.EncodeToString(make([]byte, sha256.Size))
+	errIs(t, c.VerifyServerFinalMessage(tampered), ErrSCRAMServerAuthFailed, "forged signature")
+	tampered.ServerSignature = "!!!"
+	errIs(t, c.VerifyServerFinalMessage(tampered), ErrSCRAMServerAuthFailed, "malformed signature")
+	noErr(t, c.VerifyServerFinalMessage(serverFinal), "valid signature")
+
+	c.Reset()
+	errIs(t, c.VerifyServerFinalMessage(serverFinal), ErrSCRAMInvalidState, "after reset")
+	next, err := c.StartAuthentication()
+	noErr(t, err, "restart")
+	if next.ClientNonce == first.ClientNonce {
+		t.Fatal("client nonce reused after Reset")
+	}
+}
+
+func TestScramClientRejectsBadServerFirst(t *testing.T) {
+	c := NewScramClient("u", "SecurePassword123")
+	_, err := c.StartAuthentication()
+	noErr(t, err, "StartAuthentication")
+
+	_, err = c.ProcessServerFirstMessage(ServerFirstMessage{
+		FullNonce: "n", Salt: "!!!",
+		ArgonTime: testArgonTime, ArgonMemory: testArgonMemory, ArgonThreads: testArgonThreads,
+	})
+	errIs(t, err, ErrSCRAMInvalidSalt, "salt encoding")
+
+	// ☢ no upper bound is applied to server-supplied cost parameters
+	good := base64.StdEncoding.EncodeToString(make([]byte, 16))
+	for _, msg := range []ServerFirstMessage{
+		{FullNonce: "n", Salt: good, ArgonTime: 0, ArgonMemory: testArgonMemory, ArgonThreads: 1},
+		{FullNonce: "n", Salt: good, ArgonTime: 1, ArgonMemory: 0, ArgonThreads: 1},
+		{FullNonce: "n", Salt: good, ArgonTime: 1, ArgonMemory: testArgonMemory, ArgonThreads: 0},
+	} {
+		_, err = c.ProcessServerFirstMessage(msg)
+		errIs(t, err, ErrSCRAMZeroParams, "zero parameter")
+	}
+
+	// A hostile server cannot dictate an unbounded KDF
+	_, err = c.ProcessServerFirstMessage(ServerFirstMessage{
+		FullNonce: "n", Salt: good,
+		ArgonTime: 1, ArgonMemory: MaxVerifyArgonMemory + 1, ArgonThreads: 1,
+	})
+	errIs(t, err, ErrSCRAMParamsTooLarge, "memory over ceiling")
+}
+
+func TestScramClientOversizedPassword(t *testing.T) {
+	c := NewScramClient("u", strings.Repeat("a", MaxPasswordLen+1))
+	_, err := c.StartAuthentication()
+	errIs(t, err, ErrPasswordTooLong, "oversized password rejected before the KDF")
+}
+
+func TestServerFirstMessageMarshal(t *testing.T) {
+	// the auth message binds this exact encoding; changes break every client
+	msg := ServerFirstMessage{
+		FullNonce: "abc", Salt: "c2FsdA==",
+		ArgonTime: 3, ArgonMemory: 65536, ArgonThreads: 4,
+	}
+	eq(t, msg.Marshal(), "r=abc,s=c2FsdA==,t=3,m=65536,p=4", "marshal")
+}
+
+func TestScramMigratedNonStandardDigest(t *testing.T) {
+	// MigrateFromPHC falls back to DeriveCredential for digests other than 32
+	// bytes; the resulting credential must still complete a handshake.
+	const user, pw = "legacy", "SecurePassword123"
+	cred, err := MigrateFromPHC(user, pw, phcFor(pw, []byte("0123456789abcdef"), 20))
+	noErr(t, err, "MigrateFromPHC")
+	eq(t, len(cred.StoredKey), sha256.Size, "stored key length")
+
+	s := newTestServer(t)
+	s.AddCredential(cred)
+	noErr(t, runHandshake(s, NewScramClient(user, pw)), "handshake with migrated credential")
+}
+
+func TestDeriveCredential(t *testing.T) {
+	const pw = "SecurePassword123"
+	salt := make([]byte, 16)
+	for i := range salt {
+		salt[i] = byte(i)
+	}
+
+	first, err := DeriveCredential("u", pw, salt, testArgonTime, testArgonMemory, testArgonThreads)
+	noErr(t, err, "DeriveCredential")
+	second, err := DeriveCredential("u", pw, salt, testArgonTime, testArgonMemory, testArgonThreads)
+	noErr(t, err, "DeriveCredential repeat")
+	eqBytes(t, first.StoredKey, second.StoredKey, "deterministic stored key")
+	eqBytes(t, first.ServerKey, second.ServerKey, "deterministic server key")
+
+	salted := argon2.IDKey([]byte(pw), salt, testArgonTime, testArgonMemory, testArgonThreads, DefaultArgonKeyLen)
+	want := sha256.Sum256(computeHMAC(salted, []byte("Client Key")))
+	eqBytes(t, first.StoredKey, want[:], "stored key derivation")
+	eqBytes(t, first.ServerKey, computeHMAC(salted, []byte("Server Key")), "server key derivation")
+	if bytes.Equal(first.StoredKey, salted) || bytes.Equal(first.ServerKey, salted) {
+		t.Fatal("credential exposes the salted password")
+	}
+
+	// a different password must not collide
+	other, err := DeriveCredential("u", pw+"x", salt, testArgonTime, testArgonMemory, testArgonThreads)
+	noErr(t, err, "DeriveCredential other password")
+	if bytes.Equal(first.StoredKey, other.StoredKey) {
+		t.Fatal("stored key is independent of the password")
+	}
+
+	_, err = DeriveCredential("u", pw, make([]byte, 15), testArgonTime, testArgonMemory, testArgonThreads)
+	errIs(t, err, ErrSCRAMSaltTooShort, "short salt")
+
+	for _, p := range []struct {
+		time, memory uint32
+		threads      uint8
+	}{{0, testArgonMemory, 1}, {1, 0, 1}, {1, testArgonMemory, 0}} {
+		_, err = DeriveCredential("u", pw, salt, p.time, p.memory, p.threads)
+		errIs(t, err, ErrSCRAMZeroParams, "zero parameter")
+	}
+
+	_, err = DeriveCredential("u", strings.Repeat("a", MaxPasswordLen+1), salt,
+		testArgonTime, testArgonMemory, testArgonThreads)
+	errIs(t, err, ErrPasswordTooLong, "oversized password")
+}
+
+func TestCredentialExportImportRoundTrip(t *testing.T) {
+	cred := testCredential(t, "roundtrip", "SecurePassword123")
+
+	imported, err := ImportCredential(cred.Export())
+	noErr(t, err, "ImportCredential")
+	eq(t, imported.Username, cred.Username, "username")
+	eqBytes(t, imported.Salt, cred.Salt, "salt")
+	eq(t, imported.ArgonTime, cred.ArgonTime, "time")
+	eq(t, imported.ArgonMemory, cred.ArgonMemory, "memory")
+	eq(t, imported.ArgonThreads, cred.ArgonThreads, "threads")
+	eqBytes(t, imported.StoredKey, cred.StoredKey, "stored key")
+	eqBytes(t, imported.ServerKey, cred.ServerKey, "server key")
+
+	// JSON transport converts every number to float64
+	raw, err := json.Marshal(cred.Export())
+	noErr(t, err, "marshal")
+	var decoded map[string]any
+	noErr(t, json.Unmarshal(raw, &decoded), "unmarshal")
+	viaJSON, err := ImportCredential(decoded)
+	noErr(t, err, "import via JSON")
+	eq(t, viaJSON.ArgonMemory, cred.ArgonMemory, "memory via JSON")
+	eqBytes(t, viaJSON.StoredKey, cred.StoredKey, "stored key via JSON")
+
+	// int-typed input, as produced by YAML and TOML decoders
+	m := cred.Export()
+	m["argon_time"] = int(cred.ArgonTime)
+	m["argon_memory"] = int(cred.ArgonMemory)
+	m["argon_threads"] = int(cred.ArgonThreads)
+	viaInt, err := ImportCredential(m)
+	noErr(t, err, "import from int-typed map")
+	eq(t, viaInt.ArgonThreads, cred.ArgonThreads, "threads via int")
+
+	// an imported credential must still authenticate
+	s := newTestServer(t)
+	s.AddCredential(imported)
+	noErr(t, runHandshake(s, NewScramClient("roundtrip", "SecurePassword123")), "handshake after import")
+}
+
+func TestImportCredentialErrors(t *testing.T) {
+	cred := testCredential(t, "u", "SecurePassword123")
+	with := func(mutate func(map[string]any)) map[string]any {
+		m := cred.Export()
+		mutate(m)
+		return m
+	}
+	b64 := func(n int) string { return base64.StdEncoding.EncodeToString(make([]byte, n)) }
+
+	cases := []struct {
+		name string
+		data map[string]any
+		want error
+	}{
+		{"empty map", map[string]any{}, ErrCredMissingUsername},
+		{"username wrong type", with(func(m map[string]any) { m["username"] = 42 }), ErrCredMissingUsername},
+
+		{"salt missing", with(func(m map[string]any) { delete(m, "salt") }), ErrCredMissingSalt},
+		{"salt not base64", with(func(m map[string]any) { m["salt"] = "!!!" }), ErrCredInvalidSalt},
+		{"salt too short", with(func(m map[string]any) { m["salt"] = b64(15) }), ErrSCRAMSaltTooShort},
+
+		{"time missing", with(func(m map[string]any) { delete(m, "argon_time") }), ErrCredMissingTime},
+		{"time wrong type", with(func(m map[string]any) { m["argon_time"] = "3" }), ErrCredInvalidType},
+		{"time fractional", with(func(m map[string]any) { m["argon_time"] = 3.5 }), ErrCredInvalidType},
+		{"time negative float", with(func(m map[string]any) { m["argon_time"] = float64(-1) }), ErrCredInvalidType},
+		{"time float overflow", with(func(m map[string]any) { m["argon_time"] = float64(math.MaxUint32 + 1) }), ErrCredInvalidType},
+		{"time negative int", with(func(m map[string]any) { m["argon_time"] = -1 }), ErrCredInvalidType},
+		{"time zero", with(func(m map[string]any) { m["argon_time"] = uint32(0) }), ErrSCRAMZeroParams},
+
+		{"memory missing", with(func(m map[string]any) { delete(m, "argon_memory") }), ErrCredMissingMemory},
+		{"memory zero", with(func(m map[string]any) { m["argon_memory"] = uint32(0) }), ErrSCRAMZeroParams},
+
+		{"threads missing", with(func(m map[string]any) { delete(m, "argon_threads") }), ErrCredMissingThreads},
+		{"threads wrong type", with(func(m map[string]any) { m["argon_threads"] = "4" }), ErrCredInvalidType},
+		{"threads overflow", with(func(m map[string]any) { m["argon_threads"] = float64(256) }), ErrCredInvalidType},
+		{"threads negative", with(func(m map[string]any) { m["argon_threads"] = -1 }), ErrCredInvalidType},
+		{"threads zero", with(func(m map[string]any) { m["argon_threads"] = uint8(0) }), ErrSCRAMZeroParams},
+
+		{"stored key missing", with(func(m map[string]any) { delete(m, "stored_key") }), ErrCredMissingStoredKey},
+		{"stored key not base64", with(func(m map[string]any) { m["stored_key"] = "!!!" }), ErrCredInvalidStoredKey},
+		{"stored key short", with(func(m map[string]any) { m["stored_key"] = b64(sha256.Size - 1) }), ErrCredInvalidStoredKey},
+
+		{"server key missing", with(func(m map[string]any) { delete(m, "server_key") }), ErrCredMissingServerKey},
+		{"server key not base64", with(func(m map[string]any) { m["server_key"] = "!!!" }), ErrCredInvalidServerKey},
+		{"server key short", with(func(m map[string]any) { m["server_key"] = b64(sha256.Size - 1) }), ErrCredInvalidServerKey},
+
+		{"stored key long", with(func(m map[string]any) { m["stored_key"] = b64(sha256.Size + 1) }), ErrCredInvalidStoredKey},
+		{"server key long", with(func(m map[string]any) { m["server_key"] = b64(sha256.Size + 1) }), ErrCredInvalidServerKey},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := ImportCredential(tc.data)
+			errIs(t, err, tc.want, tc.name)
+			if got != nil {
+				t.Fatalf("%s: credential returned alongside error", tc.name)
+			}
+		})
+	}
+}
+
+func TestScramConcurrentSameUser(t *testing.T) {
+	s, user, pw, _ := setupScram(t)
+
+	const n = 12
+	errs := make(chan error, n)
+	var wg sync.WaitGroup
+	for range n {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			errs <- runHandshake(s, NewScramClient(user, pw))
+		}()
+	}
+	wg.Wait()
+	close(errs)
+
+	for err := range errs {
+		if err != nil {
+			t.Errorf("concurrent handshake: %v", err)
 		}
 	}
-	server.mu.Unlock()
-
-	// Trigger cleanup manually
-	server.cleanupExpiredHandshakes()
-
-	// Verify only 2 handshakes remain
-	server.mu.RLock()
-	assert.Len(t, server.handshakes, 2, "Expired handshakes should be cleaned up")
-	server.mu.RUnlock()
+	eq(t, handshakeCount(s), 0, "handshakes leaked")
 }
 
-// TestScramConcurrentSameUser verifies multiple concurrent authentications for same user
-func TestScramConcurrentSameUser(t *testing.T) {
-	server, username, password, _ := setupScramTest(t)
-	defer server.Stop()
-
-	// Number of concurrent authentication attempts
-	numAttempts := 10
-	results := make(chan error, numAttempts)
+func TestScramConcurrentMixedTraffic(t *testing.T) {
+	s := newTestServer(t)
+	creds := make([]*Credential, 4)
+	for i := range creds {
+		creds[i] = testCredential(t, fmt.Sprintf("user-%d", i), "SecurePassword123")
+	}
 
 	var wg sync.WaitGroup
-	for i := 0; i < numAttempts; i++ {
+	for _, cred := range creds {
 		wg.Add(1)
-		go func(attempt int) {
+		go func() {
 			defer wg.Done()
-
-			// Each goroutine performs full authentication
-			client := NewScramClient(username, password)
-
-			// Step 1: Client first
-			clientFirst, err := client.StartAuthentication()
-			if err != nil {
-				results <- err
-				return
-			}
-
-			// Step 2: Server first
-			serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-			if err != nil {
-				results <- err
-				return
-			}
-
-			// Step 3: Client final
-			clientFinal, err := client.ProcessServerFirstMessage(serverFirst)
-			if err != nil {
-				results <- err
-				return
-			}
-
-			// Step 4: Server final
-			serverFinal, err := server.ProcessClientFinalMessage(clientFinal.FullNonce, clientFinal.ClientProof)
-			if err != nil {
-				results <- err
-				return
-			}
-
-			// Step 5: Client verify
-			err = client.VerifyServerFinalMessage(serverFinal)
-			results <- err
-		}(i)
+			s.AddCredential(cred)
+		}()
+	}
+	for i := range 16 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			// registration races against lookup; both paths take s.mu
+			_, _ = s.ProcessClientFirstMessage(fmt.Sprintf("user-%d", i%4), fmt.Sprintf("n-%d", i))
+			_, _ = s.ProcessClientFirstMessage(fmt.Sprintf("ghost-%d", i), fmt.Sprintf("g-%d", i))
+		}()
 	}
-
 	wg.Wait()
-	close(results)
+}
 
-	// Verify all attempts succeeded
-	successCount := 0
-	for err := range results {
-		if err == nil {
-			successCount++
-		} else {
-			t.Logf("Auth attempt failed: %v", err)
+func FuzzImportCredential(f *testing.F) {
+	cred, err := DeriveCredential("u", "SecurePassword123", make([]byte, 16),
+		testArgonTime, testArgonMemory, testArgonThreads)
+	if err != nil {
+		f.Fatal(err)
+	}
+	seed, err := json.Marshal(cred.Export())
+	if err != nil {
+		f.Fatal(err)
+	}
+	f.Add(seed)
+	f.Add([]byte(`{}`))
+	f.Add([]byte(`{"username":"u","salt":"","argon_time":1e309}`))
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		var m map[string]any
+		if err := json.Unmarshal(data, &m); err != nil || m == nil {
+			return
+		}
+		got, err := ImportCredential(m)
+		if err != nil {
+			if got != nil {
+				t.Fatal("credential returned alongside error")
+			}
+			return
+		}
+		if len(got.Salt) < 16 {
+			t.Fatalf("accepted salt of %d bytes", len(got.Salt))
+		}
+		if len(got.StoredKey) != sha256.Size || len(got.ServerKey) != sha256.Size {
+			t.Fatalf("accepted keys of %d/%d bytes", len(got.StoredKey), len(got.ServerKey))
+		}
+		if got.ArgonTime == 0 || got.ArgonMemory == 0 || got.ArgonThreads == 0 {
+			t.Fatalf("accepted zero parameters: %+v", got)
+		}
+	})
+}
+
+func BenchmarkScramHandshake(b *testing.B) {
+	const user, pw = "bench", "SecurePassword123"
+	cred, err := DeriveCredential(user, pw, make([]byte, 16), testArgonTime, testArgonMemory, testArgonThreads)
+	if err != nil {
+		b.Fatal(err)
+	}
+	s := NewScramServer()
+	defer s.Stop()
+	s.AddCredential(cred)
+
+	for b.Loop() {
+		c := NewScramClient(user, pw)
+		first, err := c.StartAuthentication()
+		if err != nil {
+			b.Fatal(err)
+		}
+		serverFirst, err := s.ProcessClientFirstMessage(first.Username, first.ClientNonce)
+		if err != nil {
+			b.Fatal(err)
+		}
+		final, err := c.ProcessServerFirstMessage(serverFirst)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if _, err := s.ProcessClientFinalMessage(final.FullNonce, final.ClientProof); err != nil {
+			b.Fatal(err)
 		}
 	}
-
-	assert.Equal(t, numAttempts, successCount,
-		"All concurrent authentication attempts should succeed")
-
-	// Verify no handshakes remain after completion
-	server.mu.RLock()
-	assert.Empty(t, server.handshakes, "All handshakes should be cleaned up after completion")
-	server.mu.RUnlock()
-}
-
-// TestScramExplicitTimeout verifies timeout enforcement
-func TestScramExplicitTimeout(t *testing.T) {
-	// Save original timeout and set shorter one for testing
-	originalTimeout := ScramHandshakeTimeout
-	// Note: Can't modify const at runtime, so we test with delay instead
-
-	server, username, password, _ := setupScramTest(t)
-	defer server.Stop()
-
-	client := NewScramClient(username, password)
-
-	// Start authentication
-	clientFirst, err := client.StartAuthentication()
-	require.NoError(t, err)
-
-	serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
-	require.NoError(t, err)
-
-	// Manually expire the handshake
-	server.mu.Lock()
-	for nonce := range server.handshakes {
-		server.handshakes[nonce].CreatedAt = time.Now().Add(-2 * ScramHandshakeTimeout)
-	}
-	server.mu.Unlock()
-
-	// Client processes server message (should work, client tracks own timeout)
-	clientFinal, err := client.ProcessServerFirstMessage(serverFirst)
-	require.NoError(t, err)
-
-	// Server should reject due to timeout
-	_, err = server.ProcessClientFinalMessage(clientFinal.FullNonce, clientFinal.ClientProof)
-	assert.ErrorIs(t, err, ErrSCRAMTimeout, "Server should reject expired handshake")
-
-	// Test client-side timeout
-	client2 := NewScramClient(username, password)
-	client2.startTime = time.Now().Add(-2 * ScramHandshakeTimeout)
-
-	_, err = client2.ProcessServerFirstMessage(serverFirst)
-	assert.ErrorIs(t, err, ErrSCRAMTimeout, "Client should reject after timeout")
-
-	_ = originalTimeout // Suppress unused variable warning
 }
diff --git a/testhelpers_test.go b/testhelpers_test.go
new file mode 100644
index 0000000..ad1162f
--- /dev/null
+++ b/testhelpers_test.go
@@ -0,0 +1,85 @@
+package auth
+
+import (
+	"bytes"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"testing"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// Assertion helpers. Failures are fatal: most tests below are sequential
+// protocol exchanges where continuing past a failure only produces noise.
+
+func noErr(t *testing.T, err error, ctx string) {
+	t.Helper()
+	if err != nil {
+		t.Fatalf("%s: unexpected error: %v", ctx, err)
+	}
+}
+
+func hasErr(t *testing.T, err error, ctx string) {
+	t.Helper()
+	if err == nil {
+		t.Fatalf("%s: expected error, got nil", ctx)
+	}
+}
+
+func errIs(t *testing.T, err, target error, ctx string) {
+	t.Helper()
+	if !errors.Is(err, target) {
+		t.Fatalf("%s: got %v, want %v", ctx, err, target)
+	}
+}
+
+func eq[T comparable](t *testing.T, got, want T, ctx string) {
+	t.Helper()
+	if got != want {
+		t.Fatalf("%s: got %v, want %v", ctx, got, want)
+	}
+}
+
+func eqBytes(t *testing.T, got, want []byte, ctx string) {
+	t.Helper()
+	if !bytes.Equal(got, want) {
+		t.Fatalf("%s: got %x, want %x", ctx, got, want)
+	}
+}
+
+func isTrue(t *testing.T, v bool, ctx string) {
+	t.Helper()
+	if !v {
+		t.Fatalf("%s: got false, want true", ctx)
+	}
+}
+
+// Test KDF cost. Parameter handling is verified independently of parameter
+// magnitude, so every test that does not measure defaults uses these.
+const (
+	testArgonTime    = 1
+	testArgonMemory  = 8 * 1024
+	testArgonThreads = 1
+)
+
+var cheapArgon = []Option{
+	WithTime(testArgonTime),
+	WithMemory(testArgonMemory),
+	WithThreads(testArgonThreads),
+}
+
+// rawB64 encodes n zero bytes in the PHC alphabet.
+func rawB64(n int) string {
+	return base64.RawStdEncoding.EncodeToString(make([]byte, n))
+}
+
+// phcFor builds a PHC record with an arbitrary salt and digest length, which
+// HashPassword cannot produce.
+func phcFor(password string, salt []byte, keyLen uint32) string {
+	digest := argon2.IDKey([]byte(password), salt, testArgonTime, testArgonMemory, testArgonThreads, keyLen)
+	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		argon2.Version, testArgonMemory, testArgonTime, testArgonThreads,
+		base64.RawStdEncoding.EncodeToString(salt),
+		base64.RawStdEncoding.EncodeToString(digest))
+}
diff --git a/token_test.go b/token_test.go
index 2ace02c..63696f1 100644
--- a/token_test.go
+++ b/token_test.go
@@ -2,80 +2,97 @@ package auth
 
 import (
 	"fmt"
+	"strings"
 	"sync"
 	"testing"
-
-	"github.com/stretchr/testify/assert"
 )
 
 func TestSimpleTokenValidator(t *testing.T) {
-	validator := NewSimpleTokenValidator()
+	v := NewSimpleTokenValidator()
+	const first, second = "test-token-123", "test-token-456"
 
-	token1 := "test-token-123"
-	token2 := "test-token-456"
+	isTrue(t, !v.ValidateToken(first), "empty validator rejects")
 
-	// Add tokens
-	validator.AddToken(token1)
-	validator.AddToken(token2)
+	v.AddToken(first)
+	v.AddToken(second)
+	isTrue(t, v.ValidateToken(first), "first token")
+	isTrue(t, v.ValidateToken(second), "second token")
+	isTrue(t, !v.ValidateToken("invalid-token"), "unknown token")
 
-	// Validate existing tokens
-	assert.True(t, validator.ValidateToken(token1))
-	assert.True(t, validator.ValidateToken(token2))
+	// matching is exact
+	isTrue(t, !v.ValidateToken(first+"x"), "suffix")
+	isTrue(t, !v.ValidateToken(first[:len(first)-1]), "prefix")
+	isTrue(t, !v.ValidateToken(strings.ToUpper(first)), "case")
+	isTrue(t, !v.ValidateToken(" "+first), "leading space")
 
-	// Invalid token
-	assert.False(t, validator.ValidateToken("invalid-token"))
+	v.RemoveToken(first)
+	isTrue(t, !v.ValidateToken(first), "removed token")
+	isTrue(t, v.ValidateToken(second), "surviving token")
 
-	// Remove token
-	validator.RemoveToken(token1)
-	assert.False(t, validator.ValidateToken(token1))
-	assert.True(t, validator.ValidateToken(token2))
+	// removing an absent token is a no-op
+	v.RemoveToken("never-added")
+	eq(t, len(v.tokens), 1, "entry count after no-op removal")
+
+	// repeated adds are idempotent
+	v.AddToken(second)
+	v.AddToken(second)
+	eq(t, len(v.tokens), 1, "entry count after duplicate adds")
+
+	// the empty token is storable and matches only itself
+	v.AddToken("")
+	isTrue(t, v.ValidateToken(""), "empty token accepted once added")
+	v.RemoveToken("")
+	isTrue(t, !v.ValidateToken(""), "empty token removed")
 }
 
-func TestConcurrentTokenValidator(t *testing.T) {
-	validator := NewSimpleTokenValidator()
+func TestSimpleTokenValidatorKeying(t *testing.T) {
+	v := NewSimpleTokenValidator()
+	tokens := []string{
+		"", "a", "a\x00b", "a\x00c", "🔑", strings.Repeat("a", 1<<16),
+	}
+	for _, tok := range tokens {
+		v.AddToken(tok)
+	}
+	eq(t, len(v.tokens), len(tokens), "distinct entries")
 
-	// Add tokens concurrently
+	for i, tok := range tokens {
+		isTrue(t, v.ValidateToken(tok), fmt.Sprintf("token %d", i))
+	}
+	for i, tok := range tokens {
+		v.RemoveToken(tok)
+		isTrue(t, !v.ValidateToken(tok), fmt.Sprintf("token %d removed", i))
+	}
+	eq(t, len(v.tokens), 0, "empty after removal")
+}
+
+func TestSimpleTokenValidatorConcurrent(t *testing.T) {
+	v := NewSimpleTokenValidator()
+	const n = 256
+
+	// pre-populate half the space so readers see hits and misses
+	for i := n / 2; i < n; i++ {
+		v.AddToken(fmt.Sprintf("token-%d", i))
+	}
+
+	// each goroutine owns one token, so the final state is deterministic
 	var wg sync.WaitGroup
-	for i := 0; i < 100; i++ {
+	for i := range n {
 		wg.Add(1)
-		go func(idx int) {
+		go func() {
 			defer wg.Done()
-			token := fmt.Sprintf("token-%d", idx)
-			validator.AddToken(token)
-		}(i)
+			token := fmt.Sprintf("token-%d", i)
+			v.AddToken(token)
+			v.ValidateToken(token)
+			v.ValidateToken(fmt.Sprintf("absent-%d", i))
+			if i%2 == 0 {
+				v.RemoveToken(token)
+			}
+		}()
 	}
 	wg.Wait()
 
-	// Validate concurrently
-	for i := 0; i < 100; i++ {
-		wg.Add(1)
-		go func(idx int) {
-			defer wg.Done()
-			token := fmt.Sprintf("token-%d", idx)
-			assert.True(t, validator.ValidateToken(token))
-		}(i)
-	}
-	wg.Wait()
-
-	// Remove concurrently
-	for i := 0; i < 50; i++ {
-		wg.Add(1)
-		go func(idx int) {
-			defer wg.Done()
-			token := fmt.Sprintf("token-%d", idx)
-			validator.RemoveToken(token)
-		}(i)
-	}
-	wg.Wait()
-
-	// Verify removal
-	for i := 0; i < 50; i++ {
-		token := fmt.Sprintf("token-%d", i)
-		assert.False(t, validator.ValidateToken(token))
-	}
-	for i := 50; i < 100; i++ {
-		token := fmt.Sprintf("token-%d", i)
-		assert.True(t, validator.ValidateToken(token))
+	for i := range n {
+		eq(t, v.ValidateToken(fmt.Sprintf("token-%d", i)), i%2 != 0, fmt.Sprintf("token %d", i))
 	}
+	eq(t, len(v.tokens), n/2, "surviving entries")
 }
-