diff --git a/README.md b/README.md
index c82d0e1..56e5174 100644
--- a/README.md
+++ b/README.md
@@ -1,38 +1,45 @@
 # Auth Package
 
-Pluggable authentication utilities for Go applications.
+Modular authentication utilities for Go applications.
 
 ## Features
 
-- **Password Hashing**: Argon2id with PHC format
-- **JWT**: HS256/RS256 token generation and validation
-- **SCRAM-SHA256**: Client/server implementation with Argon2id KDF
-- **HTTP Auth**: Basic/Bearer header parsing
+- **Password Hashing**: Standalone Argon2id hashing with PHC format.
+- **JWT**: HS256/RS256 token management via a simple facade over `golang-jwt`.
+- **SCRAM-SHA256**: Client/server implementation with Argon2id KDF.
+- **HTTP Auth**: Helpers for parsing Basic and Bearer authentication headers.
 
 ## Usage
+
 ```go
+// Argon2 Password Hashing
+hash, _ := auth.HashPassword("password123")
+err := auth.VerifyPassword("password123", hash)
+
 // JWT with HS256
-auth, _ := auth.NewAuthenticator([]byte("32-byte-secret-key..."))
-token, _ := auth.GenerateToken("user123", map[string]interface{}{"role": "admin"})
-userID, claims, _ := auth.ValidateToken(token)
+jwtMgr, _ := auth.NewJWT([]byte("a-very-secure-32-byte-secret-key"))
+token, _ := jwtMgr.GenerateToken("user123", map[string]any{"role": "admin"})
+userID, claims, _ := jwtMgr.ValidateToken(token)
 
 // SCRAM authentication
 server := auth.NewScramServer()
-cred, _ := auth.DeriveCredential("user", "password", salt, 1, 65536, 4)
+phcHash, _ := auth.HashPassword("password123")
+cred, _ := auth.MigrateFromPHC("user", "password123", phcHash)
 server.AddCredential(cred)
 ```
 
 ## Package Structure
 
-- `interfaces.go` - Core interfaces
-- `jwt.go` - JWT token operations
-- `argon2.go` - Password hashing
-- `scram.go` - SCRAM-SHA256 protocol
-- `token.go` - Token validation utilities
-- `http.go` - HTTP header parsing
-- `errors.go` - Error definitions
+- `doc.go` - Overview and package documentation
+- `argon2.go` - Standalone Argon2id password hashing
+- `jwt.go` - JWT manager (HS256/RS256) wrapping `golang-jwt`
+- `scram.go` - SCRAM-SHA256 client/server protocol
+- `http.go` - HTTP Basic/Bearer header parsing
+- `token.go` - Simple in-memory token validator
+- `error.go` - Centralized error definitions
 
 ## Testing
+
 ```bash
-go test -v ./auth
+go test -v ./
 ```
\ No newline at end of file
diff --git a/argon2.go b/argon2.go
index 533d5ce..5e0e9de 100644
--- a/argon2.go
+++ b/argon2.go
@@ -13,40 +13,85 @@ import (
 
 // Default Argon2id parameters
 const (
-	DefaultArgonTime    = 3         // iterations (reduce for faster but less secure auth)
+	DefaultArgonTime    = 3         // iterations
 	DefaultArgonMemory  = 64 * 1024 // 64 MB
 	DefaultArgonThreads = 4
 	DefaultArgonSaltLen = 16
 	DefaultArgonKeyLen  = 32
 )
 
-// HashPassword creates an Argon2id PHC-format hash
-func (a *Authenticator) HashPassword(password string) (string, error) {
+// argonParams holds configurable Argon2id parameters
+type argonParams struct {
+	time    uint32
+	memory  uint32
+	threads uint8
+	keyLen  uint32
+	saltLen uint32
+}
+
+// Option configures Argon2id hashing parameters
+type Option func(*argonParams)
+
+// WithTime sets Argon2 iterations
+func WithTime(t uint32) Option {
+	return func(p *argonParams) {
+		if t > 0 {
+			p.time = t
+		}
+	}
+}
+
+// WithMemory sets Argon2 memory in KiB
+func WithMemory(m uint32) Option {
+	return func(p *argonParams) {
+		if m > 0 {
+			p.memory = m
+		}
+	}
+}
+
+// WithThreads sets Argon2 parallelism
+func WithThreads(t uint8) Option {
+	return func(p *argonParams) {
+		if t > 0 {
+			p.threads = t
+		}
+	}
+}
+
+// HashPassword creates Argon2id PHC-format hash (standalone)
+func HashPassword(password string, opts ...Option) (string, error) {
 	if len(password) < 8 {
 		return "", ErrWeakPassword
 	}
 
-	// Generate salt
-	salt := make([]byte, DefaultArgonSaltLen)
+	params := &argonParams{
+		time:    DefaultArgonTime,
+		memory:  DefaultArgonMemory,
+		threads: DefaultArgonThreads,
+		keyLen:  DefaultArgonKeyLen,
+		saltLen: DefaultArgonSaltLen,
+	}
+
+	for _, opt := range opts {
+		opt(params)
+	}
+
+	salt := make([]byte, params.saltLen)
 	if _, err := rand.Read(salt); err != nil {
 		return "", fmt.Errorf("%w: %v", ErrSaltGenerationFailed, err)
 	}
 
-	// Derive key using Argon2id
-	hash := argon2.IDKey([]byte(password), salt, a.argonTime, a.argonMemory, a.argonThreads, DefaultArgonKeyLen)
+	hash := argon2.IDKey([]byte(password), salt, params.time, params.memory, params.threads, params.keyLen)
 
-	// Construct PHC format
 	saltB64 := base64.RawStdEncoding.EncodeToString(salt)
 	hashB64 := base64.RawStdEncoding.EncodeToString(hash)
-	phcHash := fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
-		argon2.Version, a.argonMemory, a.argonTime, a.argonThreads, saltB64, hashB64)
-
-	return phcHash, nil
+	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		argon2.Version, params.memory, params.time, params.threads, saltB64, hashB64), nil
 }
 
-// VerifyPassword checks password against PHC-format hash
-func (a *Authenticator) VerifyPassword(password, phcHash string) error {
-	// Parse PHC format
+// VerifyPassword checks password against PHC-format hash (standalone)
+func VerifyPassword(password, phcHash string) error {
 	parts := strings.Split(phcHash, "$")
 	if len(parts) != 6 || parts[1] != "argon2id" {
 		return ErrPHCInvalidFormat
@@ -66,10 +111,8 @@ func (a *Authenticator) VerifyPassword(password, phcHash string) error {
 		return fmt.Errorf("%w: %v", ErrPHCInvalidHash, err)
 	}
 
-	// Compute hash with same parameters
 	computedHash := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(expectedHash)))
 
-	// Constant-time comparison
 	if subtle.ConstantTimeCompare(computedHash, expectedHash) != 1 {
 		return ErrInvalidCredentials
 	}
@@ -77,9 +120,8 @@ func (a *Authenticator) VerifyPassword(password, phcHash string) error {
 	return nil
 }
 
-// MigrateFromPHC converts existing Argon2 PHC hash to SCRAM credential
+// MigrateFromPHC converts PHC hash to SCRAM credential
 func MigrateFromPHC(username, password, phcHash string) (*Credential, error) {
-	// Parse PHC format
 	parts := strings.Split(phcHash, "$")
 	if len(parts) != 6 || parts[1] != "argon2id" {
 		return nil, ErrPHCInvalidFormat
@@ -94,17 +136,10 @@ func MigrateFromPHC(username, password, phcHash string) (*Credential, error) {
 		return nil, ErrPHCInvalidSalt
 	}
 
-	expectedHash, err := base64.RawStdEncoding.DecodeString(parts[5])
-	if err != nil {
-		return nil, ErrPHCInvalidHash
+	// Use standalone function for verification
+	if err := VerifyPassword(password, phcHash); err != nil {
+		return nil, err
 	}
 
-	// Verify password against hash
-	computedHash := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(expectedHash)))
-	if subtle.ConstantTimeCompare(computedHash, expectedHash) != 1 {
-		return nil, ErrInvalidCredentials
-	}
-
-	// Derive SCRAM credential with same parameters
 	return DeriveCredential(username, password, salt, time, memory, threads)
 }
\ No newline at end of file
diff --git a/argon2_test.go b/argon2_test.go
index f58f066..af78978 100644
--- a/argon2_test.go
+++ b/argon2_test.go
@@ -11,13 +11,10 @@ import (
 )
 
 func TestPasswordHashing(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err, "Failed to create authenticator")
-
 	password := "testPassword123"
 
-	// Test hashing
-	hash, err := auth.HashPassword(password)
+	// Test hashing with default parameters
+	hash, err := HashPassword(password)
 	require.NoError(t, err, "Failed to hash password")
 
 	// Verify PHC format
@@ -25,20 +22,30 @@ func TestPasswordHashing(t *testing.T) {
 		"Hash should have argon2id prefix, got: %s", hash)
 
 	// Test verification with correct password
-	err = auth.VerifyPassword(password, hash)
+	err = VerifyPassword(password, hash)
 	assert.NoError(t, err, "Failed to verify correct password")
 
 	// Test verification with incorrect password
-	err = auth.VerifyPassword("wrongPassword", hash)
+	err = VerifyPassword("wrongPassword", hash)
 	assert.Error(t, err, "Verification should fail for incorrect password")
 	assert.Equal(t, ErrInvalidCredentials, err)
 
 	// Test weak password
-	_, err = auth.HashPassword("weak")
+	_, err = HashPassword("weak")
 	assert.Equal(t, ErrWeakPassword, err, "Should reject weak password")
 
+	// 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 = auth.VerifyPassword(password, "$invalid$format")
+	err = VerifyPassword(password, "$invalid$format")
 	assert.Error(t, err, "Should reject malformed hash")
 
 	// Test corrupted salt
@@ -47,33 +54,27 @@ func TestPasswordHashing(t *testing.T) {
 	if len(parts) == 6 {
 		parts[4] = "invalid!base64"
 		corruptedHash = strings.Join(parts, "$")
-		err = auth.VerifyPassword(password, corruptedHash)
+		err = VerifyPassword(password, corruptedHash)
 		assert.Error(t, err, "Should reject corrupted salt")
 	}
 }
 
 func TestEmptyPasswordAfterValidation(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err)
-
 	// Empty password should be rejected by length check
-	_, err = auth.HashPassword("")
+	_, err := HashPassword("")
 	assert.Equal(t, ErrWeakPassword, err)
 
 	// 8-character password should pass
-	hash, err := auth.HashPassword("12345678")
+	hash, err := HashPassword("12345678")
 	require.NoError(t, err)
 
-	err = auth.VerifyPassword("12345678", hash)
+	err = VerifyPassword("12345678", hash)
 	assert.NoError(t, err)
 }
 
 func TestConcurrentPasswordOperations(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err)
-
 	password := "testPassword123"
-	hash, err := auth.HashPassword(password)
+	hash, err := HashPassword(password)
 	require.NoError(t, err)
 
 	// Test concurrent verification
@@ -82,7 +83,7 @@ func TestConcurrentPasswordOperations(t *testing.T) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			err := auth.VerifyPassword(password, hash)
+			err := VerifyPassword(password, hash)
 			assert.NoError(t, err)
 		}()
 	}
@@ -90,14 +91,11 @@ func TestConcurrentPasswordOperations(t *testing.T) {
 }
 
 func TestPHCMigration(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err)
-
 	password := "testPassword123"
 	username := "migrationUser"
 
 	// Generate PHC hash
-	phcHash, err := auth.HashPassword(password)
+	phcHash, err := HashPassword(password)
 	require.NoError(t, err)
 
 	// Migrate to SCRAM credential
diff --git a/auth.go b/auth.go
deleted file mode 100644
index 3a80ab3..0000000
--- a/auth.go
+++ /dev/null
@@ -1,71 +0,0 @@
-// FILE: auth/auth.go
-package auth
-
-import (
-	"crypto/rsa"
-	"fmt"
-)
-
-// Authenticator provides password hashing and JWT operations
-type Authenticator struct {
-	algorithm    string
-	jwtSecret    []byte          // For HS256
-	privateKey   *rsa.PrivateKey // For RS256
-	publicKey    *rsa.PublicKey  // For RS256
-	argonTime    uint32
-	argonMemory  uint32
-	argonThreads uint8
-}
-
-// NewAuthenticator creates a new authenticator with specified algorithm
-func NewAuthenticator(key any, algorithm ...string) (*Authenticator, error) {
-	alg := "HS256"
-	if len(algorithm) > 0 && algorithm[0] != "" {
-		alg = algorithm[0]
-	}
-
-	auth := &Authenticator{
-		algorithm:    alg,
-		argonTime:    DefaultArgonTime,
-		argonMemory:  DefaultArgonMemory,
-		argonThreads: DefaultArgonThreads,
-	}
-
-	switch alg {
-	case "HS256":
-		secret, ok := key.([]byte)
-		if !ok {
-			return nil, ErrInvalidKeyType
-		}
-		if len(secret) < 32 {
-			return nil, ErrSecretTooShort
-		}
-		auth.jwtSecret = secret
-
-	case "RS256":
-		switch k := key.(type) {
-		case *rsa.PrivateKey:
-			auth.privateKey = k
-			auth.publicKey = &k.PublicKey
-		case *rsa.PublicKey:
-			auth.publicKey = k
-		case []byte:
-			// Try parsing as PEM
-			if privKey, err := parseRSAPrivateKey(k); err == nil {
-				auth.privateKey = privKey
-				auth.publicKey = &privKey.PublicKey
-			} else if pubKey, err := parseRSAPublicKey(k); err == nil {
-				auth.publicKey = pubKey
-			} else {
-				return nil, fmt.Errorf("failed to parse RSA key: %w", err)
-			}
-		default:
-			return nil, ErrInvalidKeyType
-		}
-
-	default:
-		return nil, ErrInvalidAlgorithm
-	}
-
-	return auth, nil
-}
\ No newline at end of file
diff --git a/auth_test.go b/auth_test.go
deleted file mode 100644
index 5176f37..0000000
--- a/auth_test.go
+++ /dev/null
@@ -1,68 +0,0 @@
-// FILE: auth/auth_test.go
-package auth
-
-import (
-	"crypto/rand"
-	"crypto/rsa"
-	"testing"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-)
-
-func TestNewAuthenticator(t *testing.T) {
-	// Test HS256 creation
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err, "Failed to create HS256 authenticator")
-	assert.Equal(t, "HS256", auth.algorithm)
-
-	// Test with short secret
-	_, err = NewAuthenticator([]byte("short"))
-	assert.Equal(t, ErrSecretTooShort, err)
-
-	// Test RS256 with private key
-	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
-	require.NoError(t, err)
-
-	authRS, err := NewAuthenticator(privateKey, "RS256")
-	require.NoError(t, err)
-	assert.Equal(t, "RS256", authRS.algorithm)
-	assert.NotNil(t, authRS.privateKey)
-	assert.NotNil(t, authRS.publicKey)
-
-	// Test RS256 with public key only
-	authPub, err := NewAuthenticator(&privateKey.PublicKey, "RS256")
-	require.NoError(t, err)
-	assert.Equal(t, "RS256", authPub.algorithm)
-	assert.Nil(t, authPub.privateKey)
-	assert.NotNil(t, authPub.publicKey)
-
-	// Test invalid algorithm
-	_, err = NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"), "INVALID")
-	assert.Equal(t, ErrInvalidAlgorithm, err)
-
-	// Test invalid key type for HS256
-	_, err = NewAuthenticator(privateKey, "HS256")
-	assert.Equal(t, ErrInvalidKeyType, err)
-}
-
-func TestInterfaceCompliance(t *testing.T) {
-	// Verify Authenticator implements AuthenticatorInterface
-	auth, _ := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-
-	var _ AuthenticatorInterface = auth
-
-	// Test interface methods work
-	hash, err := auth.HashPassword("testpass123")
-	require.NoError(t, err)
-
-	err = auth.VerifyPassword("testpass123", hash)
-	assert.NoError(t, err)
-
-	token, err := auth.GenerateToken("user1", nil)
-	require.NoError(t, err)
-
-	userID, _, err := auth.ValidateToken(token)
-	require.NoError(t, err)
-	assert.Equal(t, "user1", userID)
-}
\ No newline at end of file
diff --git a/doc.go b/doc.go
new file mode 100644
index 0000000..ad50a02
--- /dev/null
+++ b/doc.go
@@ -0,0 +1,53 @@
+// FILE: auth/doc.go
+package auth
+
+/*
+Package auth provides modular authentication components:
+
+# Argon2 Password Hashing
+
+Standalone password hashing using Argon2id:
+
+	hash, err := auth.HashPassword("password123")
+	err = auth.VerifyPassword("password123", hash)
+
+	// With custom parameters
+	hash, err := auth.HashPassword("password123",
+		auth.WithTime(5),
+		auth.WithMemory(128*1024))
+
+# JWT Token Management
+
+JSON Web Token generation and validation:
+
+	// HS256 (symmetric)
+	jwtMgr, _ := auth.NewJWT(secret)
+	token, _ := jwtMgr.GenerateToken("user1", claims)
+	userID, claims, _ := jwtMgr.ValidateToken(token)
+
+	// RS256 (asymmetric)
+	jwtMgr, _ := auth.NewJWTRSA(privateKey)
+
+	// One-off operations
+	token, _ := auth.GenerateHS256Token(secret, "user1", claims, 1*time.Hour)
+
+# SCRAM-SHA256 Authentication
+
+Server and client implementation for SCRAM:
+
+	// Server
+	server := auth.NewScramServer()
+	server.AddCredential(credential)
+
+	// Client
+	client := auth.NewScramClient(username, password)
+
+# HTTP Authentication Parsing
+
+Utility functions for HTTP headers:
+
+	username, password, _ := auth.ParseBasicAuth(header)
+	token, _ := auth.ParseBearerToken(header)
+
+Each module can be used independently without initializing other components.
+*/
\ No newline at end of file
diff --git a/error.go b/error.go
index 824a494..b4ca76c 100644
--- a/error.go
+++ b/error.go
@@ -10,8 +10,6 @@ import (
 var (
 	ErrInvalidCredentials = errors.New("invalid credentials")
 	ErrWeakPassword       = errors.New("password must be at least 8 characters")
-	ErrInvalidAlgorithm   = errors.New("invalid algorithm")
-	ErrInvalidKeyType     = errors.New("invalid key type for algorithm")
 )
 
 // JWT-specific errors
@@ -22,9 +20,6 @@ var (
 	ErrTokenInvalidSignature  = errors.New("token: invalid signature")
 	ErrTokenAlgorithmMismatch = errors.New("token: algorithm mismatch")
 	ErrTokenMissingClaim      = errors.New("token: missing required claim")
-	ErrTokenInvalidHeader     = errors.New("token: invalid header encoding")
-	ErrTokenInvalidClaims     = errors.New("token: invalid claims encoding")
-	ErrTokenInvalidJSON       = errors.New("token: malformed JSON")
 	ErrTokenEmptyUserID       = errors.New("token: empty user ID")
 	ErrTokenNoPrivateKey      = errors.New("token: private key required for signing")
 	ErrTokenNoPublicKey       = errors.New("token: public key required for verification")
diff --git a/go.mod b/go.mod
index b6154ca..a4a8ef3 100644
--- a/go.mod
+++ b/go.mod
@@ -1,8 +1,9 @@
-module auth
+module github.com/lixenwraith/auth
 
 go 1.25.3
 
 require (
+	github.com/golang-jwt/jwt/v5 v5.3.0
 	github.com/stretchr/testify v1.11.1
 	golang.org/x/crypto v0.43.0
 )
diff --git a/go.sum b/go.sum
index 4a5adfe..d143a61 100644
--- a/go.sum
+++ b/go.sum
@@ -1,5 +1,7 @@
 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/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=
diff --git a/interface.go b/interface.go
deleted file mode 100644
index e84d390..0000000
--- a/interface.go
+++ /dev/null
@@ -1,17 +0,0 @@
-// FILE: auth/interface.go
-package auth
-
-// AuthenticatorInterface defines the authentication operations
-type AuthenticatorInterface interface {
-	HashPassword(password string) (hash string, err error)
-	VerifyPassword(password, hash string) (err error)
-	GenerateToken(userID string, claims map[string]any) (token string, err error)
-	ValidateToken(token string) (userID string, claims map[string]any, err error)
-}
-
-// TokenValidator validates bearer tokens
-type TokenValidator interface {
-	ValidateToken(token string) (valid bool)
-	AddToken(token string)
-	RemoveToken(token string)
-}
\ No newline at end of file
diff --git a/jwt.go b/jwt.go
index e5d4f53..ab10899 100644
--- a/jwt.go
+++ b/jwt.go
@@ -2,179 +2,289 @@
 package auth
 
 import (
-	"crypto"
-	"crypto/hmac"
-	"crypto/rand"
 	"crypto/rsa"
-	"crypto/sha256"
-	"crypto/subtle"
 	"crypto/x509"
-	"encoding/base64"
-	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"fmt"
-	"strings"
 	"time"
+
+	"github.com/golang-jwt/jwt/v5"
 )
 
-// GenerateToken creates a JWT token with user claims
-func (a *Authenticator) GenerateToken(userID string, claims map[string]any) (string, error) {
+// JWT configuration defaults
+const (
+	DefaultTokenLifetime = 24 * time.Hour
+	DefaultLeeway        = 5 * time.Minute
+)
+
+// customClaims extends RegisteredClaims with arbitrary user data
+type customClaims struct {
+	jwt.RegisteredClaims
+	Extra map[string]any `json:"extra,omitempty"`
+}
+
+// JWT manages token generation and validation
+type JWT struct {
+	algorithm     jwt.SigningMethod
+	signKey       any // []byte for HMAC, *rsa.PrivateKey for RSA
+	verifyKey     any // []byte for HMAC, *rsa.PublicKey for RSA
+	tokenLifetime time.Duration
+	leeway        time.Duration
+	issuer        string
+	audience      []string
+}
+
+// JWTOption configures JWT behavior
+type JWTOption func(*JWT)
+
+// WithTokenLifetime sets token expiration duration
+func WithTokenLifetime(d time.Duration) JWTOption {
+	return func(j *JWT) {
+		if d > 0 {
+			j.tokenLifetime = d
+		}
+	}
+}
+
+// WithLeeway sets clock skew tolerance
+func WithLeeway(d time.Duration) JWTOption {
+	return func(j *JWT) {
+		if d >= 0 {
+			j.leeway = d
+		}
+	}
+}
+
+// WithIssuer sets token issuer claim
+func WithIssuer(iss string) JWTOption {
+	return func(j *JWT) {
+		j.issuer = iss
+	}
+}
+
+// WithAudience sets token audience claim
+func WithAudience(aud []string) JWTOption {
+	return func(j *JWT) {
+		j.audience = aud
+	}
+}
+
+// NewJWT creates JWT manager for HS256 (symmetric)
+func NewJWT(secret []byte, opts ...JWTOption) (*JWT, error) {
+	if len(secret) < 32 {
+		return nil, ErrSecretTooShort
+	}
+
+	j := &JWT{
+		algorithm:     jwt.SigningMethodHS256,
+		signKey:       secret,
+		verifyKey:     secret,
+		tokenLifetime: DefaultTokenLifetime,
+		leeway:        DefaultLeeway,
+	}
+
+	for _, opt := range opts {
+		opt(j)
+	}
+
+	return j, nil
+}
+
+// NewJWTRSA creates JWT manager for RS256 (asymmetric)
+func NewJWTRSA(privateKey *rsa.PrivateKey, opts ...JWTOption) (*JWT, error) {
+	if privateKey == nil {
+		return nil, ErrTokenNoPrivateKey
+	}
+
+	j := &JWT{
+		algorithm:     jwt.SigningMethodRS256,
+		signKey:       privateKey,
+		verifyKey:     &privateKey.PublicKey,
+		tokenLifetime: DefaultTokenLifetime,
+		leeway:        DefaultLeeway,
+	}
+
+	for _, opt := range opts {
+		opt(j)
+	}
+
+	return j, nil
+}
+
+// NewJWTVerifier creates JWT manager for verification only (RS256)
+func NewJWTVerifier(publicKey *rsa.PublicKey, opts ...JWTOption) (*JWT, error) {
+	if publicKey == nil {
+		return nil, ErrTokenNoPublicKey
+	}
+
+	j := &JWT{
+		algorithm:     jwt.SigningMethodRS256,
+		signKey:       nil, // Cannot sign
+		verifyKey:     publicKey,
+		tokenLifetime: DefaultTokenLifetime,
+		leeway:        DefaultLeeway,
+	}
+
+	for _, opt := range opts {
+		opt(j)
+	}
+
+	return j, nil
+}
+
+// GenerateToken creates signed JWT with claims
+func (j *JWT) GenerateToken(userID string, claims map[string]any) (string, error) {
 	if userID == "" {
 		return "", ErrTokenEmptyUserID
 	}
 
-	if a.algorithm == "RS256" && a.privateKey == nil {
+	if j.signKey == nil {
 		return "", ErrTokenNoPrivateKey
 	}
 
-	// Build JWT claims
 	now := time.Now()
-	jwtClaims := map[string]any{
-		"sub": userID,
-		"iat": now.Unix(),
-		"exp": now.Add(7 * 24 * time.Hour).Unix(), // 7 days expiry
+	registeredClaims := jwt.RegisteredClaims{
+		Subject:   userID,
+		Issuer:    j.issuer,
+		Audience:  j.audience,
+		IssuedAt:  jwt.NewNumericDate(now),
+		ExpiresAt: jwt.NewNumericDate(now.Add(j.tokenLifetime)),
+		NotBefore: jwt.NewNumericDate(now),
 	}
 
-	// Reserved claims that cannot be overridden
-	reservedClaims := map[string]bool{
-		"sub": true, "iat": true, "exp": true, "nbf": true,
-		"iss": true, "aud": true, "jti": true, "typ": true,
-		"alg": true,
-	}
+	token := jwt.NewWithClaims(j.algorithm, customClaims{
+		RegisteredClaims: registeredClaims,
+		Extra:            claims,
+	})
 
-	// Merge custom claims
-	for k, v := range claims {
-		if !reservedClaims[k] {
-			jwtClaims[k] = v
-		}
-	}
-
-	// Create JWT header
-	header := map[string]any{
-		"alg": a.algorithm,
-		"typ": "JWT",
-	}
-
-	// Encode header and payload
-	headerJSON, _ := json.Marshal(header)
-	claimsJSON, _ := json.Marshal(jwtClaims)
-
-	headerB64 := base64.RawURLEncoding.EncodeToString(headerJSON)
-	claimsB64 := base64.RawURLEncoding.EncodeToString(claimsJSON)
-
-	// Create signature
-	signingInput := headerB64 + "." + claimsB64
-	var signature string
-
-	switch a.algorithm {
-	case "HS256":
-		h := hmac.New(sha256.New, a.jwtSecret)
-		h.Write([]byte(signingInput))
-		signature = base64.RawURLEncoding.EncodeToString(h.Sum(nil))
-
-	case "RS256":
-		hashed := sha256.Sum256([]byte(signingInput))
-		sig, err := rsa.SignPKCS1v15(rand.Reader, a.privateKey, crypto.SHA256, hashed[:])
-		if err != nil {
-			return "", fmt.Errorf("failed to sign token: %w", err)
-		}
-		signature = base64.RawURLEncoding.EncodeToString(sig)
-	}
-
-	// Combine to form JWT
-	token := signingInput + "." + signature
-
-	return token, nil
+	return token.SignedString(j.signKey)
 }
 
-// ValidateToken verifies JWT and returns userID and claims
-func (a *Authenticator) ValidateToken(token string) (string, map[string]any, error) {
-	// Split token
-	parts := strings.Split(token, ".")
-	if len(parts) != 3 {
+// ValidateToken verifies JWT and extracts claims
+func (j *JWT) ValidateToken(tokenString string) (string, map[string]any, error) {
+	parser := jwt.NewParser(
+		jwt.WithLeeway(j.leeway),
+		jwt.WithAudience(j.audience...),
+		jwt.WithIssuer(j.issuer),
+		jwt.WithValidMethods([]string{j.algorithm.Alg()}),
+		jwt.WithExpirationRequired(),
+	)
+
+	token, err := parser.ParseWithClaims(tokenString, &customClaims{}, func(token *jwt.Token) (any, error) {
+		// Algorithm already validated by WithValidMethods
+		return j.verifyKey, nil
+	})
+
+	if err != nil {
+		return "", nil, mapJWTError(err)
+	}
+
+	claims, ok := token.Claims.(*customClaims)
+	if !ok || !token.Valid {
 		return "", nil, ErrTokenMalformed
 	}
 
-	// Decode header to check algorithm
-	headerJSON, err := base64.RawURLEncoding.DecodeString(parts[0])
-	if err != nil {
-		return "", nil, ErrTokenInvalidHeader
-	}
-
-	var header map[string]any
-	if err = json.Unmarshal(headerJSON, &header); err != nil {
-		return "", nil, ErrTokenInvalidJSON
-	}
-
-	// Verify algorithm matches
-	if alg, ok := header["alg"].(string); !ok || alg != a.algorithm {
-		return "", nil, ErrTokenAlgorithmMismatch
-	}
-
-	// Verify signature
-	signingInput := parts[0] + "." + parts[1]
-
-	switch a.algorithm {
-	case "HS256":
-		h := hmac.New(sha256.New, a.jwtSecret)
-		h.Write([]byte(signingInput))
-		expectedSig := base64.RawURLEncoding.EncodeToString(h.Sum(nil))
-
-		if subtle.ConstantTimeCompare([]byte(parts[2]), []byte(expectedSig)) != 1 {
-			return "", nil, ErrTokenInvalidSignature
-		}
-
-	case "RS256":
-		if a.publicKey == nil {
-			return "", nil, ErrTokenNoPublicKey
-		}
-
-		sig, err := base64.RawURLEncoding.DecodeString(parts[2])
-		if err != nil {
-			return "", nil, ErrTokenInvalidSignature
-		}
-
-		hashed := sha256.Sum256([]byte(signingInput))
-		if err := rsa.VerifyPKCS1v15(a.publicKey, crypto.SHA256, hashed[:], sig); err != nil {
-			return "", nil, ErrTokenInvalidSignature
-		}
-	}
-
-	// Decode claims
-	claimsJSON, err := base64.RawURLEncoding.DecodeString(parts[1])
-	if err != nil {
-		return "", nil, ErrTokenInvalidClaims
-	}
-
-	var claims map[string]any
-	if err := json.Unmarshal(claimsJSON, &claims); err != nil {
-		return "", nil, ErrTokenInvalidJSON
-	}
-
-	// Check expiration
-	if exp, ok := claims["exp"].(float64); ok {
-		if time.Now().Unix() > int64(exp) {
-			return "", nil, ErrTokenExpired
-		}
-	}
-
-	// Check not before
-	if nbf, ok := claims["nbf"].(float64); ok {
-		if time.Now().Unix() < int64(nbf) {
-			return "", nil, ErrTokenNotYetValid
-		}
-	}
-
-	// Extract userID
-	userID, ok := claims["sub"].(string)
-	if !ok {
-		return "", nil, ErrTokenMissingClaim
-	}
-
-	return userID, claims, nil
+	return claims.Subject, claims.Extra, nil
 }
 
-// parseRSAPrivateKey parses PEM encoded RSA private key
+// mapJWTError translates jwt library errors to auth package errors
+func mapJWTError(err error) error {
+	switch {
+	case errors.Is(err, jwt.ErrTokenMalformed):
+		return fmt.Errorf("%w : %w", ErrTokenMalformed, err)
+	case errors.Is(err, jwt.ErrTokenUnverifiable):
+		return fmt.Errorf("%w : %w", ErrTokenMalformed, err)
+	case errors.Is(err, jwt.ErrTokenSignatureInvalid):
+		return fmt.Errorf("%w : %w", ErrTokenInvalidSignature, err)
+	case errors.Is(err, jwt.ErrTokenExpired):
+		return fmt.Errorf("%w : %w", ErrTokenExpired, err)
+	case errors.Is(err, jwt.ErrTokenNotValidYet):
+		return fmt.Errorf("%w : %w", ErrTokenNotYetValid, err)
+	case errors.Is(err, jwt.ErrTokenInvalidAudience):
+		return fmt.Errorf("%w : %w", ErrTokenMissingClaim, err)
+	case errors.Is(err, jwt.ErrTokenInvalidIssuer):
+		return fmt.Errorf("%w : %w", ErrTokenMissingClaim, err)
+	default:
+		// Check for algorithm mismatch in error message
+		if errors.Is(err, jwt.ErrTokenSignatureInvalid) {
+			return fmt.Errorf("%w : %w", ErrTokenAlgorithmMismatch, 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 {
+		return "", ErrSecretTooShort
+	}
+
+	now := time.Now()
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, customClaims{
+		RegisteredClaims: jwt.RegisteredClaims{
+			Subject:   userID,
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(lifetime)),
+		},
+		Extra: claims,
+	})
+
+	return token.SignedString(secret)
+}
+
+// ValidateHS256Token verifies HS256 JWT without manager instance
+func ValidateHS256Token(secret []byte, tokenString string) (string, map[string]any, error) {
+	if len(secret) < 32 {
+		return "", nil, ErrSecretTooShort
+	}
+
+	parser := jwt.NewParser(
+		jwt.WithValidMethods([]string{"HS256"}),
+		jwt.WithLeeway(DefaultLeeway),
+	)
+
+	token, err := parser.ParseWithClaims(tokenString, &customClaims{}, func(token *jwt.Token) (any, error) {
+		return secret, nil
+	})
+
+	if err != nil {
+		return "", nil, mapJWTError(err)
+	}
+
+	claims, ok := token.Claims.(*customClaims)
+	if !ok || !token.Valid {
+		return "", nil, ErrTokenMalformed
+	}
+
+	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)
 	if block == nil {
@@ -187,7 +297,7 @@ func parseRSAPrivateKey(pemBytes []byte) (*rsa.PrivateKey, error) {
 	return key, nil
 }
 
-// parseRSAPublicKey parses PEM encoded RSA public key
+// parseRSAPublicKey parses a PEM-encoded RSA public key.
 func parseRSAPublicKey(pemBytes []byte) (*rsa.PublicKey, error) {
 	block, _ := pem.Decode(pemBytes)
 	if block == nil {
diff --git a/jwt_test.go b/jwt_test.go
index 56ec30d..c65f28a 100644
--- a/jwt_test.go
+++ b/jwt_test.go
@@ -4,19 +4,20 @@ package auth
 import (
 	"crypto/rand"
 	"crypto/rsa"
-	"encoding/base64"
-	"errors"
-	"fmt"
+	"crypto/x509"
+	"encoding/pem"
 	"strings"
 	"testing"
 	"time"
 
+	"github.com/golang-jwt/jwt/v5"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
 func TestJWTHS256(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
+	secret := []byte("test-secret-key-must-be-32-bytes")
+	jwtMgr, err := NewJWT(secret)
 	require.NoError(t, err)
 
 	userID := "user123"
@@ -26,54 +27,17 @@ func TestJWTHS256(t *testing.T) {
 	}
 
 	// Generate token
-	token, err := auth.GenerateToken(userID, claims)
-	require.NoError(t, err, "Failed to generate token")
+	token, err := jwtMgr.GenerateToken(userID, claims)
+	require.NoError(t, err)
 	assert.NotEmpty(t, token)
 
 	// Validate token
-	extractedUserID, extractedClaims, err := auth.ValidateToken(token)
-	require.NoError(t, err, "Failed to validate token")
+	extractedUserID, extractedClaims, err := jwtMgr.ValidateToken(token)
+	require.NoError(t, err)
 
 	assert.Equal(t, userID, extractedUserID)
 	assert.Equal(t, "test@example.com", extractedClaims["email"])
 	assert.Equal(t, "admin", extractedClaims["role"])
-
-	// Test invalid token
-	_, _, err = auth.ValidateToken("invalid.token.here")
-	assert.Error(t, err)
-	assert.True(t, errors.Is(err, ErrTokenInvalidJSON))
-
-	// Test tampered token
-	parts := strings.Split(token, ".")
-	require.Len(t, parts, 3, "JWT should have 3 parts")
-
-	tampered := parts[0] + "." + parts[1] + ".invalidsignature"
-	_, _, err = auth.ValidateToken(tampered)
-	assert.Error(t, err)
-	assert.True(t, errors.Is(err, ErrTokenInvalidSignature))
-
-	// Test reserved claims cannot be overridden
-	overrideClaims := map[string]any{
-		"sub": "override",
-		"iat": 12345,
-		"exp": 67890,
-		"nbf": 11111,
-		"iss": "attacker",
-		"aud": "victim",
-		"jti": "fake",
-	}
-
-	token, err = auth.GenerateToken(userID, overrideClaims)
-	require.NoError(t, err)
-
-	extractedUserID, extractedClaims, err = auth.ValidateToken(token)
-	require.NoError(t, err)
-
-	assert.Equal(t, userID, extractedUserID, "UserID should not be overridden")
-	assert.NotEqual(t, 12345, extractedClaims["iat"], "iat should not be overridden")
-	assert.NotEqual(t, 67890, extractedClaims["exp"], "exp should not be overridden")
-	assert.NotContains(t, extractedClaims, "nbf", "nbf should not be added from user claims")
-	assert.NotContains(t, extractedClaims, "iss", "iss should not be added from user claims")
 }
 
 func TestJWTRS256(t *testing.T) {
@@ -82,95 +46,214 @@ func TestJWTRS256(t *testing.T) {
 	require.NoError(t, err)
 
 	// Test with private key (can sign and verify)
-	authPriv, err := NewAuthenticator(privateKey, "RS256")
+	jwtMgr, err := NewJWTRSA(privateKey)
 	require.NoError(t, err)
 
 	userID := "user456"
 	claims := map[string]any{
-		"email": "rs256@example.com",
 		"scope": "read:all",
 	}
 
 	// Generate token
-	token, err := authPriv.GenerateToken(userID, claims)
+	token, err := jwtMgr.GenerateToken(userID, claims)
 	require.NoError(t, err)
 	assert.NotEmpty(t, token)
 
-	// Validate token with private key auth (has public key too)
-	extractedUserID, extractedClaims, err := authPriv.ValidateToken(token)
+	// Validate with same manager
+	extractedUserID, extractedClaims, err := jwtMgr.ValidateToken(token)
 	require.NoError(t, err)
-
 	assert.Equal(t, userID, extractedUserID)
-	assert.Equal(t, "rs256@example.com", extractedClaims["email"])
 	assert.Equal(t, "read:all", extractedClaims["scope"])
 
-	// Test with public key only (can only verify)
-	authPub, err := NewAuthenticator(&privateKey.PublicKey, "RS256")
+	// Test with verifier only (public key)
+	verifier, err := NewJWTVerifier(&privateKey.PublicKey)
 	require.NoError(t, err)
 
-	// Should be able to validate token
-	extractedUserID, extractedClaims, err = authPub.ValidateToken(token)
+	// Should validate token
+	extractedUserID, _, err = verifier.ValidateToken(token)
 	require.NoError(t, err)
 	assert.Equal(t, userID, extractedUserID)
 
-	// Should not be able to generate token
-	_, err = authPub.GenerateToken(userID, claims)
-	assert.Error(t, err, "Public key only auth should not generate tokens")
+	// Should not generate token
+	_, err = verifier.GenerateToken(userID, claims)
 	assert.Equal(t, ErrTokenNoPrivateKey, err)
-
-	// Test algorithm mismatch
-	authHS256, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
-	require.NoError(t, err)
-
-	_, _, err = authHS256.ValidateToken(token)
-	assert.Error(t, err, "HS256 auth should not validate RS256 token")
-	// assert.True(t, errors.Is(err, ErrInvalidToken))
-	assert.True(t, errors.Is(err, ErrTokenAlgorithmMismatch))
-
-	fmt.Println(err)
 }
 
-func TestExpiredToken(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
+func TestJWTOptions(t *testing.T) {
+	secret := []byte("test-secret-key-must-be-32-bytes")
+
+	// Test custom lifetime
+	jwtMgr, err := NewJWT(secret,
+		WithTokenLifetime(1*time.Hour),
+		WithIssuer("test-issuer"),
+		WithAudience([]string{"api.example.com"}),
+	)
 	require.NoError(t, err)
 
-	userID := "user123"
-
-	// Generate normal token (should have 7 days expiry)
-	token, err := auth.GenerateToken(userID, nil)
+	token, err := jwtMgr.GenerateToken("user1", nil)
 	require.NoError(t, err)
 
-	_, extractedClaims, err := auth.ValidateToken(token)
-	require.NoError(t, err)
+	// Parse token to check claims
+	parsed, _ := jwt.Parse(token, func(token *jwt.Token) (any, error) {
+		return secret, nil
+	})
 
-	// Check expiry is in future (approximately 7 days)
-	expiry := extractedClaims["exp"].(float64)
-	now := time.Now().Unix()
+	claims := parsed.Claims.(jwt.MapClaims)
 
-	assert.Greater(t, expiry, float64(now), "Token expiry should be in future")
-	assert.InDelta(t, expiry, float64(now+7*24*60*60), 10,
-		"Token expiry should be approximately 7 days from now")
+	// 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 TestCorruptJWTParts(t *testing.T) {
-	auth, err := NewAuthenticator([]byte("test-secret-key-must-be-32-bytes"))
+func TestJWTErrors(t *testing.T) {
+	secret := []byte("test-secret-key-must-be-32-bytes")
+	jwtMgr, err := NewJWT(secret)
 	require.NoError(t, err)
 
-	// Test with missing parts
-	_, _, err = auth.ValidateToken("only.two")
-	assert.True(t, errors.Is(err, ErrTokenMalformed))
+	// Empty user ID
+	_, err = jwtMgr.GenerateToken("", nil)
+	assert.Equal(t, ErrTokenEmptyUserID, err)
 
-	// Test with invalid header encoding
-	_, _, err = auth.ValidateToken("not-base64!.valid.valid")
-	assert.Error(t, err)
+	// Invalid token format
+	_, _, err = jwtMgr.ValidateToken("invalid.token")
+	assert.ErrorIs(t, err, ErrTokenMalformed)
 
-	// Test with invalid claims encoding
-	validHeader := "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"
-	_, _, err = auth.ValidateToken(validHeader + ".not-base64!.valid")
-	assert.Error(t, err)
+	// 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)
 
-	// Test with invalid JSON in claims
-	invalidJSON := base64.RawURLEncoding.EncodeToString([]byte("{invalid json"))
-	_, _, err = auth.ValidateToken(validHeader + "." + invalidJSON + ".signature")
-	assert.Error(t, err)
+	// 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(),
+	})
+	tokenString, err := token.SignedString(secret)
+	require.NoError(t, err)
+
+	// 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)
+}
+
+func TestStandaloneFunctions(t *testing.T) {
+	secret := []byte("test-secret-key-must-be-32-bytes")
+	userID := "standalone-user"
+	claims := map[string]any{"test": "value"}
+
+	// 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)
+}
+
+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)
+
+	// 2. Encode the private key to PEM format
+	privateKeyPEM := pem.EncodeToMemory(&pem.Block{
+		Type:  "RSA PRIVATE KEY",
+		Bytes: x509.MarshalPKCS1PrivateKey(privateKey),
+	})
+
+	// 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,
+	})
+
+	// 4. Test the PEM constructor for the signer
+	jwtMgr, err := NewJWTRSAFromPEM(privateKeyPEM)
+	require.NoError(t, err)
+
+	token, err := jwtMgr.GenerateToken("user-from-pem", nil)
+	require.NoError(t, err)
+	assert.NotEmpty(t, token)
+
+	// 5. Test the PEM constructor for the verifier
+	verifier, err := NewJWTVerifierFromPEM(publicKeyPEM)
+	require.NoError(t, err)
+
+	userID, _, err := verifier.ValidateToken(token)
+	require.NoError(t, err)
+	assert.Equal(t, "user-from-pem", userID)
+
+	// 6. Test failure cases with invalid data
+	_, err = NewJWTRSAFromPEM([]byte("invalid pem data"))
+	assert.ErrorIs(t, err, ErrRSAInvalidPEM)
+
+	_, err = NewJWTVerifierFromPEM([]byte("invalid pem data"))
+	assert.ErrorIs(t, err, ErrRSAInvalidPEM)
 }
\ No newline at end of file
diff --git a/scram.go b/scram.go
index 51e7266..d9840e5 100644
--- a/scram.go
+++ b/scram.go
@@ -143,6 +143,10 @@ func DeriveCredential(username, password string, salt []byte, time, memory uint3
 		return nil, ErrSCRAMSaltTooShort
 	}
 
+	if time == 0 || memory == 0 || threads == 0 {
+		return nil, ErrSCRAMZeroParams
+	}
+
 	// Derive salted password using Argon2id
 	saltedPassword := argon2.IDKey([]byte(password), salt, time, memory, threads, DefaultArgonKeyLen)
 
diff --git a/scram_test.go b/scram_test.go
new file mode 100644
index 0000000..40c8fc2
--- /dev/null
+++ b/scram_test.go
@@ -0,0 +1,146 @@
+// FILE: auth/scram_test.go
+package auth
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// 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()
+	server.AddCredential(cred)
+
+	return server, username, password, cred
+}
+
+// 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")
+}
+
+// TestScram_FullRoundtrip_WrongPassword ensures authentication fails with an incorrect password.
+func TestScram_FullRoundtrip_WrongPassword(t *testing.T) {
+	server, username, _, _ := setupScramTest(t)
+	// 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")
+}
+
+// TestScram_FullRoundtrip_UserNotFound tests for user enumeration protection.
+// The server should not reveal whether a user exists or not in its first message.
+func TestScram_FullRoundtrip_UserNotFound(t *testing.T) {
+	server, _, _, _ := setupScramTest(t)
+	client := NewScramClient("unknown_user", "any_password")
+
+	clientFirst, err := client.StartAuthentication()
+	require.NoError(t, err)
+
+	// --- Step 2: Server should return an error, but also a FAKE response ---
+	// This prevents an attacker from knowing if the user exists based on the response structure.
+	serverFirst, err := server.ProcessClientFirstMessage(clientFirst.Username, clientFirst.ClientNonce)
+	assert.ErrorIs(t, err, ErrInvalidCredentials, "Server should return an error for an unknown user")
+	assert.NotEmpty(t, serverFirst.FullNonce, "Server must still provide a nonce to prevent enumeration")
+	assert.NotEmpty(t, serverFirst.Salt, "Server must still provide a salt to prevent enumeration")
+
+	t.Log("SCRAM correctly protected against user enumeration")
+}
+
+// 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")
+}
+
+// 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")
+}
\ No newline at end of file