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// JWT Structure Visualization
interface JWTHeader {
    alg: string;  // Algorithm: "RS256", "ES256", "HS256", etc.
    typ: string;  // Type: typically "JWT"
    kid?: string; // Key ID: identifies which key signed this token
}
 
interface JWTPayload {
    // Standard claims (registered)
    iss?: string;  // Issuer: who created and signed this token
    sub?: string;  // Subject: who this token is about (user ID)
    aud?: string | string[]; // Audience: intended recipient(s)
    exp?: number;  // Expiration: unix timestamp when token expires
    nbf?: number;  // Not Before: token invalid before this time
    iat?: number;  // Issued At: when token was created
    jti?: string;  // JWT ID: unique identifier for this token
    
    // OIDC-specific
    nonce?: string;    // Prevents replay attacks
    auth_time?: number; // When user authenticated
    acr?: string;      // Auth context class
    amr?: string[];    // Auth methods used
    
    // Custom claims
    scope?: string;    // OAuth scopes
    roles?: string[];  // User roles
    permissions?: string[]; // Fine-grained permissions
    [key: string]: any; // Extension claims
}
 
// Decoding (NOT validating) a JWT
function decodeJWT(token: string): { header: JWTHeader; payload: JWTPayload } {
    const parts = token.split('.');
    if (parts.length !== 3) {
        throw new Error('Invalid JWT format');
    }
    
    const header = JSON.parse(
        Buffer.from(parts[0], 'base64url').toString()
    );
    const payload = JSON.parse(
        Buffer.from(parts[1], 'base64url').toString()
    );
    
    // WARNING: This only decodes, it does NOT validate!
    // Never use decoded claims without signature verification
    return { header, payload };
}

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// Secure Signature Verification with jose library
import * as jose from 'jose';
 
// ========================================
// Asymmetric Verification (RS256, ES256)
// ========================================
 
// For OIDC providers: Fetch public keys from JWKS
async function verifyWithJWKS(
    token: string,
    jwksUrl: string,
    expectedIssuer: string,
    expectedAudience: string
): Promise<jose.JWTPayload> {
    // Create JWKS client (handles key caching, rotation)
    const JWKS = jose.createRemoteJWKSet(new URL(jwksUrl));
    
    const { payload } = await jose.jwtVerify(
        token,
        JWKS,
        {
            issuer: expectedIssuer,
            audience: expectedAudience,
            // CRITICAL: Restrict allowed algorithms
            algorithms: ['RS256', 'ES256'],
            // Reject tokens from the future
            clockTolerance: 30, // 30 seconds tolerance for clock skew
        }
    );
    
    return payload;
}
 
// For self-managed keys: Use public key directly
async function verifyWithPublicKey(
    token: string,
    publicKeyPEM: string,
    expectedIssuer: string,
    expectedAudience: string
): Promise<jose.JWTPayload> {
    const publicKey = await jose.importSPKI(publicKeyPEM, 'RS256');
    
    const { payload } = await jose.jwtVerify(
        token,
        publicKey,
        {
            issuer: expectedIssuer,
            audience: expectedAudience,
            algorithms: ['RS256'], // Only accept expected algorithm
        }
    );
    
    return payload;
}
 
// ========================================
// Symmetric Verification (HS256)
// ========================================
 
// WARNING: Only for first-party tokens where validator = issuer
async function verifyWithSecret(
    token: string,
    secret: string,
    expectedIssuer: string,
    expectedAudience: string
): Promise<jose.JWTPayload> {
    // Secret must be sufficiently random (256+ bits for HS256)
    const secretKey = new TextEncoder().encode(secret);
    
    const { payload } = await jose.jwtVerify(
        token,
        secretKey,
        {
            issuer: expectedIssuer,
            audience: expectedAudience,
            algorithms: ['HS256'], // Explicit algorithm restriction
        }
    );
    
    return payload;
}
 
// ========================================
// Algorithm Confusion Prevention
// ========================================
 
// WRONG: Naive verification vulnerable to alg confusion
async function vulnerableVerify(token: string, keyOrSecret: unknown) {
    const { header } = jose.decodeJwt(token);
    
    // DANGEROUS: Trusting the token's claimed algorithm!
    if (header.alg === 'none') {
        // Attacker removes signature entirely
        return jose.decodeJwt(token); // No verification!
    }
    
    if (header.alg.startsWith('HS')) {
        // Attacker switches RS256 to HS256, uses public key as HMAC secret
        return jose.jwtVerify(token, keyOrSecret as Uint8Array);
    }
    
    // This code is INSECURE - never use in production
}
 
// CORRECT: Algorithm is enforced, not trusted from token
async function secureVerify(
    token: string,
    key: jose.KeyLike,
    expectedAlgorithm: string
): Promise<jose.JWTPayload> {
    const { payload } = await jose.jwtVerify(token, key, {
        algorithms: [expectedAlgorithm], // You decide the algorithm, not the token
    });
    return payload;
}

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// Comprehensive Claims Validation
interface ValidationConfig {
    issuer: string;
    audience: string | string[];
    clockToleranceSeconds: number;
    maxTokenAgeSeconds?: number;
    requiredScopes?: string[];
    requiredClaims?: string[];
}
 
interface ValidationResult {
    valid: boolean;
    payload?: Record<string, any>;
    error?: string;
}
 
class ClaimsValidator {
    constructor(private config: ValidationConfig) {}
    
    validate(payload: Record<string, any>, expectedNonce?: string): ValidationResult {
        try {
            // 1. Validate issuer (exact match required)
            this.validateIssuer(payload);
            
            // 2. Validate audience
            this.validateAudience(payload);
            
            // 3. Validate temporal claims
            this.validateTemporalClaims(payload);
            
            // 4. Validate nonce if provided (OIDC)
            if (expectedNonce) {
                this.validateNonce(payload, expectedNonce);
            }
            
            // 5. Validate required scopes
            if (this.config.requiredScopes) {
                this.validateScopes(payload, this.config.requiredScopes);
            }
            
            // 6. Validate required claims are present
            if (this.config.requiredClaims) {
                this.validateRequiredClaims(payload, this.config.requiredClaims);
            }
            
            return { valid: true, payload };
        } catch (error) {
            return { 
                valid: false, 
                error: error instanceof Error ? error.message : 'Validation failed' 
            };
        }
    }
    
    private validateIssuer(payload: Record<string, any>): void {
        // Exact match - trailing slashes, protocol all matter
        if (payload.iss !== this.config.issuer) {
            throw new Error(
                `Invalid issuer: expected '${this.config.issuer}', got '${payload.iss}'`
            );
        }
    }
    
    private validateAudience(payload: Record<string, any>): void {
        const tokenAudience = Array.isArray(payload.aud) 
            ? payload.aud 
            : [payload.aud];
        const expectedAudiences = Array.isArray(this.config.audience)
            ? this.config.audience
            : [this.config.audience];
        
        const hasValidAudience = tokenAudience.some(
            aud => expectedAudiences.includes(aud)
        );
        
        if (!hasValidAudience) {
            throw new Error(
                `Token not intended for this audience. Expected: ${expectedAudiences.join(', ')}`
            );
        }
        
        // For multi-audience tokens, check azp (authorized party)
        if (tokenAudience.length > 1 && payload.azp) {
            if (!expectedAudiences.includes(payload.azp)) {
                throw new Error('Token authorized party does not match');
            }
        }
    }
    
    private validateTemporalClaims(payload: Record<string, any>): void {
        const now = Math.floor(Date.now() / 1000);
        const tolerance = this.config.clockToleranceSeconds;
        
        // Token must not be expired
        if (payload.exp !== undefined) {
            if (now > payload.exp + tolerance) {
                throw new Error('Token has expired');
            }
        }
        
        // Token must be valid now (not before)
        if (payload.nbf !== undefined) {
            if (now < payload.nbf - tolerance) {
                throw new Error('Token not yet valid');
            }
        }
        
        // Token must have been issued in the past
        if (payload.iat !== undefined) {
            if (payload.iat > now + tolerance) {
                throw new Error('Token issued in the future');
            }
            
            // Optional: Reject very old tokens
            if (this.config.maxTokenAgeSeconds) {
                const tokenAge = now - payload.iat;
                if (tokenAge > this.config.maxTokenAgeSeconds) {
                    throw new Error('Token is too old');
                }
            }
        }
    }
    
    private validateNonce(payload: Record<string, any>, expectedNonce: string): void {
        if (payload.nonce !== expectedNonce) {
            throw new Error('Nonce mismatch - possible token replay or injection');
        }
    }
    
    private validateScopes(payload: Record<string, any>, required: string[]): void {
        const tokenScopes = (payload.scope || '').split(' ').filter(Boolean);
        
        for (const requiredScope of required) {
            if (!tokenScopes.includes(requiredScope)) {
                throw new Error(`Missing required scope: ${requiredScope}`);
            }
        }
    }
    
    private validateRequiredClaims(
        payload: Record<string, any>, 
        required: string[]
    ): void {
        for (const claim of required) {
            if (!(claim in payload) || payload[claim] == null) {
                throw new Error(`Missing required claim: ${claim}`);
            }
        }
    }
}
 
// Usage
const validator = new ClaimsValidator({
    issuer: 'https://auth.example.com',
    audience: 'my-app-client-id',
    clockToleranceSeconds: 30,
    maxTokenAgeSeconds: 3600, // 1 hour
    requiredScopes: ['read', 'write'],
    requiredClaims: ['sub', 'email'],
});
 
const result = validator.validate(tokenPayload, sessionNonce);
if (!result.valid) {
    throw new AuthError(`Token validation failed: ${result.error}`);
}

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// Scope and Permission Validation System
type ScopePattern = string; // e.g., "users:read", "payments:*"
 
interface ScopeValidationConfig {
    scopeClaim: string; // Usually 'scope' or 'scp'
    hierarchical: boolean; // Does 'users:*' include 'users:read'?
    caseSensitive: boolean;
}
 
class ScopeValidator {
    constructor(private config: ScopeValidationConfig = {
        scopeClaim: 'scope',
        hierarchical: true,
        caseSensitive: true,
    }) {}
    
    // Extract scopes from token payload
    extractScopes(payload: Record<string, any>): string[] {
        const scopeValue = payload[this.config.scopeClaim];
        
        if (!scopeValue) {
            return [];
        }
        
        // Handle both space-delimited string and array formats
        if (Array.isArray(scopeValue)) {
            return scopeValue;
        }
        
        return scopeValue.split(' ').filter(Boolean);
    }
    
    // Check if token has specific scope
    hasScope(tokenScopes: string[], required: ScopePattern): boolean {
        const normalizedRequired = this.normalize(required);
        
        return tokenScopes.some(tokenScope => {
            const normalized = this.normalize(tokenScope);
            
            // Exact match
            if (normalized === normalizedRequired) {
                return true;
            }
            
            // Hierarchical match (admin includes everything)
            if (this.config.hierarchical) {
                // Check wildcard patterns
                if (this.matchesWildcard(normalizedRequired, normalized)) {
                    return true;
                }
                
                // Check hierarchy (users:read:own covered by users:read:*)
                if (this.matchesHierarchy(normalizedRequired, normalized)) {
                    return true;
                }
            }
            
            return false;
        });
    }
    
    // Check if token has ALL required scopes
    hasAllScopes(tokenScopes: string[], required: ScopePattern[]): boolean {
        return required.every(scope => this.hasScope(tokenScopes, scope));
    }
    
    // Check if token has ANY of the scopes
    hasAnyScope(tokenScopes: string[], required: ScopePattern[]): boolean {
        return required.some(scope => this.hasScope(tokenScopes, scope));
    }
    
    private normalize(scope: string): string {
        return this.config.caseSensitive ? scope : scope.toLowerCase();
    }
    
    private matchesWildcard(required: string, tokenScope: string): boolean {
        // users:* matches users:read, users:write, etc.
        if (tokenScope.endsWith('*')) {
            const prefix = tokenScope.slice(0, -1);
            return required.startsWith(prefix);
        }
        return false;
    }
    
    private matchesHierarchy(required: string, tokenScope: string): boolean {
        // admin:users includes admin:users:read
        const requiredParts = required.split(':');
        const tokenParts = tokenScope.split(':');
        
        // Token scope must be equal or more general
        if (tokenParts.length > requiredParts.length) {
            return false;
        }
        
        return tokenParts.every((part, index) => {
            if (part === '*') return true;
            return part === requiredParts[index];
        });
    }
}
 
// Express middleware for scope validation
function requireScopes(...requiredScopes: string[]) {
    return (req: Request, res: Response, next: NextFunction) => {
        const tokenScopes = req.tokenPayload?.scope?.split(' ') || [];
        const validator = new ScopeValidator();
        
        if (!validator.hasAllScopes(tokenScopes, requiredScopes)) {
            return res.status(403).json({
                error: 'insufficient_scope',
                required: requiredScopes,
                provided: tokenScopes,
            });
        }
        
        next();
    };
}
 
// Route-level scope enforcement
app.get('/api/users', 
    authenticateToken,
    requireScopes('users:read'),
    async (req, res) => {
        // Token has users:read scope
        const users = await getUsers();
        res.json(users);
    }
);
 
app.post('/api/payments',
    authenticateToken,
    requireScopes('payments:write', 'financial:access'),
    async (req, res) => {
        // Token has both payments:write AND financial:access
        const payment = await createPayment(req.body);
        res.json(payment);
    }
);

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// Production Revocation Checking Implementation
import Redis from 'ioredis';
 
const redis = new Redis(process.env.REDIS_URL);
 
// ========================================
// Strategy 1: Deny List with Redis
// ========================================
 
class TokenDenyList {
    private readonly prefix = 'token:revoked:';
    
    async revokeToken(jti: string, expiresAt: number): Promise<void> {
        const now = Math.floor(Date.now() / 1000);
        const ttl = expiresAt - now;
        
        if (ttl > 0) {
            // Only store until token would have expired anyway
            await redis.set(`${this.prefix}${jti}`, '1', 'EX', ttl);
        }
    }
    
    async isRevoked(jti: string): Promise<boolean> {
        const result = await redis.get(`${this.prefix}${jti}`);
        return result !== null;
    }
    
    // Revoke all tokens for a user (logout everywhere)
    async revokeUserTokens(userId: string): Promise<void> {
        // This requires tracking all tokens per user, or...
        // Use token versioning instead (see below)
    }
}
 
// ========================================
// Strategy 2: Token Versioning
// ========================================
 
interface TokenVersionStore {
    getUserTokenVersion(userId: string): Promise<number>;
    incrementTokenVersion(userId: string): Promise<number>;
}
 
class TokenVersionValidator {
    constructor(private versionStore: TokenVersionStore) {}
    
    async isValidVersion(userId: string, tokenVersion?: number): Promise<boolean> {
        if (tokenVersion === undefined) {
            // Tokens without version are always invalid in this system
            return false;
        }
        
        const currentVersion = await this.versionStore.getUserTokenVersion(userId);
        return tokenVersion >= currentVersion;
    }
    
    async revokeAllUserTokens(userId: string): Promise<void> {
        await this.versionStore.incrementTokenVersion(userId);
    }
}
 
// ========================================
// Strategy 3: Token Introspection (RFC 7662)
// ========================================
 
interface IntrospectionResponse {
    active: boolean;
    scope?: string;
    client_id?: string;
    username?: string;
    token_type?: string;
    exp?: number;
    iat?: number;
    nbf?: number;
    sub?: string;
    aud?: string;
    iss?: string;
    jti?: string;
}
 
class TokenIntrospector {
    constructor(
        private introspectionEndpoint: string,
        private clientId: string,
        private clientSecret: string,
    ) {}
    
    async introspect(token: string): Promise<IntrospectionResponse> {
        const response = await fetch(this.introspectionEndpoint, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
                'Authorization': `Basic ${Buffer.from(
                    `${this.clientId}:${this.clientSecret}`
                ).toString('base64')}`,
            },
            body: new URLSearchParams({
                token: token,
                token_type_hint: 'access_token',
            }),
        });
        
        return response.json();
    }
    
    async isTokenActive(token: string): Promise<boolean> {
        const result = await this.introspect(token);
        return result.active === true;
    }
}
 
// ========================================
// Combined Validation with Revocation Check
// ========================================
 
class TokenValidator {
    constructor(
        private jwtVerifier: JWTVerifier,
        private denyList: TokenDenyList,
        private versionValidator: TokenVersionValidator,
    ) {}
    
    async validateToken(token: string): Promise<TokenPayload> {
        // Step 1: Cryptographic validation (signature, expiry, audience)
        const payload = await this.jwtVerifier.verify(token);
        
        // Step 2: Check deny list
        if (payload.jti) {
            const isRevoked = await this.denyList.isRevoked(payload.jti);
            if (isRevoked) {
                throw new TokenError('Token has been revoked');
            }
        }
        
        // Step 3: Check token version
        if (payload.sub) {
            const isValidVersion = await this.versionValidator.isValidVersion(
                payload.sub,
                payload.tokenVersion
            );
            if (!isValidVersion) {
                throw new TokenError('Token version is outdated');
            }
        }
        
        return payload;
    }
}

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// Production Authentication Middleware
import * as jose from 'jose';
import { Request, Response, NextFunction } from 'express';
 
interface AuthConfig {
    issuer: string;
    audience: string;
    jwksUri: string;
    algorithms: string[];
    clockToleranceSeconds: number;
    tokenLookupFn?: (req: Request) => string | undefined;
}
 
interface AuthenticatedRequest extends Request {
    auth: {
        token: string;
        payload: jose.JWTPayload;
        scopes: string[];
        userId: string;
    };
}
 
class AuthMiddleware {
    private jwks: jose.JWTVerifyGetKey;
    
    constructor(private config: AuthConfig) {
        this.jwks = jose.createRemoteJWKSet(new URL(config.jwksUri));
    }
    
    // Extract token from request
    private extractToken(req: Request): string | undefined {
        // Custom extraction function
        if (this.config.tokenLookupFn) {
            return this.config.tokenLookupFn(req);
        }
        
        // Standard: Authorization header
        const authHeader = req.headers.authorization;
        if (authHeader?.startsWith('Bearer ')) {
            return authHeader.slice(7);
        }
        
        // Fallback: Query parameter (less secure, use sparingly)
        if (req.query.access_token) {
            return req.query.access_token as string;
        }
        
        return undefined;
    }
    
    // Main authentication middleware
    authenticate() {
        return async (req: Request, res: Response, next: NextFunction) => {
            try {
                const token = this.extractToken(req);
                
                if (!token) {
                    return res.status(401).json({
                        error: 'missing_token',
                        error_description: 'No access token provided',
                    });
                }
                
                // Verify token
                const { payload } = await jose.jwtVerify(token, this.jwks, {
                    issuer: this.config.issuer,
                    audience: this.config.audience,
                    algorithms: this.config.algorithms as any[],
                    clockTolerance: this.config.clockToleranceSeconds,
                });
                
                // Optional: Additional revocation checks
                // await this.checkRevocation(payload);
                
                // Attach to request
                (req as AuthenticatedRequest).auth = {
                    token,
                    payload,
                    scopes: (payload.scope as string || '').split(' ').filter(Boolean),
                    userId: payload.sub as string,
                };
                
                next();
            } catch (error) {
                if (error instanceof jose.errors.JWTExpired) {
                    return res.status(401).json({
                        error: 'token_expired',
                        error_description: 'Access token has expired',
                    });
                }
                
                if (error instanceof jose.errors.JWTInvalid) {
                    return res.status(401).json({
                        error: 'invalid_token',
                        error_description: 'Access token is invalid',
                    });
                }
                
                if (error instanceof jose.errors.JWTClaimValidationFailed) {
                    return res.status(401).json({
                        error: 'invalid_claims',
                        error_description: error.message,
                    });
                }
                
                console.error('Auth error:', error);
                return res.status(401).json({
                    error: 'authentication_error',
                    error_description: 'Unable to authenticate request',
                });
            }
        };
    }
    
    // Scope checking middleware
    requireScopes(...requiredScopes: string[]) {
        return (req: Request, res: Response, next: NextFunction) => {
            const authReq = req as AuthenticatedRequest;
            
            if (!authReq.auth) {
                return res.status(401).json({
                    error: 'unauthenticated',
                    error_description: 'Authentication required',
                });
            }
            
            const hasScopes = requiredScopes.every(
                scope => authReq.auth.scopes.includes(scope)
            );
            
            if (!hasScopes) {
                return res.status(403).json({
                    error: 'insufficient_scope',
                    error_description: `Required scopes: ${requiredScopes.join(', ')}`,
                    required_scopes: requiredScopes,
                });
            }
            
            next();
        };
    }
    
    // Optional authentication (doesn't fail if no token)
    optionalAuth() {
        return async (req: Request, res: Response, next: NextFunction) => {
            const token = this.extractToken(req);
            
            if (!token) {
                return next(); // Continue without auth
            }
            
            try {
                const { payload } = await jose.jwtVerify(token, this.jwks, {
                    issuer: this.config.issuer,
                    audience: this.config.audience,
                    algorithms: this.config.algorithms as any[],
                    clockTolerance: this.config.clockToleranceSeconds,
                });
                
                (req as AuthenticatedRequest).auth = {
                    token,
                    payload,
                    scopes: (payload.scope as string || '').split(' ').filter(Boolean),
                    userId: payload.sub as string,
                };
            } catch {
                // Invalid token on optional route - continue unauthenticated
            }
            
            next();
        };
    }
}
 
// Usage
const auth = new AuthMiddleware({
    issuer: 'https://auth.example.com',
    audience: 'my-api',
    jwksUri: 'https://auth.example.com/.well-known/jwks.json',
    algorithms: ['RS256'],
    clockToleranceSeconds: 30,
});
 
// Protected route
app.get('/api/protected',
    auth.authenticate(),
    (req, res) => {
        const authReq = req as AuthenticatedRequest;
        res.json({ userId: authReq.auth.userId });
    }
);
 
// Route with specific scope requirement
app.delete('/api/users/:id',
    auth.authenticate(),
    auth.requireScopes('users:delete', 'admin'),
    (req, res) => {
        // Has both users:delete AND admin scopes
    }
);
 
// Optional auth route (works with or without token)
app.get('/api/content',
    auth.optionalAuth(),
    (req, res) => {
        const authReq = req as AuthenticatedRequest;
        const isAuthenticated = !!authReq.auth;
        // Return different content based on auth status
    }
);