codecrucible-synth/src/core/security/oauth-resource-server.ts

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/**
 * OAuth Resource Server for MCP Security (2024 Standards)
 * Implements RFC 6749 OAuth 2.0 and RFC 8707 Resource Indicators
 * 
 * Based on 2024 MCP security research:
 * - OAuth Resource Server classification required for all MCP servers
 * - Resource Indicators (RFC 8707) provide fine-grained access control
 * - JWT validation with JWKS endpoint for scalable token verification
 * - Bearer token authentication with proper scope validation
 */

import { logger } from '../logger.js';
import { EventEmitter } from 'events';
import crypto from 'crypto';

export interface OAuthConfig {
  issuer: string; // OAuth Authorization Server URL
  jwksUri: string; // JSON Web Key Set endpoint
  audience: string; // Expected audience for tokens
  requiredScopes: string[]; // Required OAuth scopes
  resourceIndicators: string[]; // RFC 8707 resource indicators
  tokenValidation: {
    enabled: boolean;
    clockTolerance: number; // seconds
    maxAge: number; // seconds
    algorithms: string[]; // Allowed signing algorithms
  };
  caching: {
    jwksCache: boolean;
    jwksCacheTtl: number; // seconds
    tokenCache: boolean;
    tokenCacheTtl: number; // seconds
  };
}

export interface TokenClaims {
  iss: string; // Issuer
  sub: string; // Subject (user ID)
  aud: string | string[]; // Audience
  exp: number; // Expiration time
  iat: number; // Issued at time
  nbf?: number; // Not before time
  jti?: string; // JWT ID
  scope?: string; // OAuth scopes
  resource?: string | string[]; // Resource indicators (RFC 8707)
  client_id?: string; // OAuth client ID
  username?: string; // Username
  roles?: string[]; // User roles
}

export interface ValidationResult {
  valid: boolean;
  claims?: TokenClaims;
  scopes: string[];
  resourceIndicators: string[];
  error?: string;
  errorDescription?: string;
}

export interface JWK {
  kty: string; // Key type
  use?: string; // Key use
  alg?: string; // Algorithm
  kid?: string; // Key ID
  n?: string; // RSA modulus
  e?: string; // RSA exponent
  x?: string; // EC x coordinate
  y?: string; // EC y coordinate
  crv?: string; // EC curve
  k?: string; // Symmetric key
}

export interface JWKS {
  keys: JWK[];
}

export class OAuthResourceServer extends EventEmitter {
  private config: OAuthConfig;
  private jwksCache: Map<string, { jwk: JWK; expires: number }> = new Map();
  private tokenCache: Map<string, { result: ValidationResult; expires: number }> = new Map();
  private cleanupInterval: NodeJS.Timeout | null = null;

  constructor(config: OAuthConfig) {
    super();
    this.config = config;
    this.startCacheCleanup();
    
    logger.info('OAuth Resource Server initialized', {
      issuer: config.issuer,
      audience: config.audience,
      requiredScopes: config.requiredScopes,
      resourceIndicators: config.resourceIndicators.length
    });
  }

  /**
   * Validate OAuth Bearer token according to 2024 MCP standards
   */
  async validateBearerToken(authorizationHeader: string): Promise<ValidationResult> {
    try {
      // Extract Bearer token
      const token = this.extractBearerToken(authorizationHeader);
      if (!token) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_request',
          errorDescription: 'Missing or malformed Authorization header'
        };
      }

      // Check token cache
      if (this.config.caching.tokenCache) {
        const cached = this.tokenCache.get(token);
        if (cached && cached.expires > Date.now()) {
          logger.debug('Token validation result retrieved from cache');
          return cached.result;
        }
      }

      // Validate JWT structure and signature
      const validationResult = await this.validateJWT(token);
      
      // Cache result if configured
      if (this.config.caching.tokenCache && validationResult.valid) {
        const cacheExpiry = Date.now() + (this.config.caching.tokenCacheTtl * 1000);
        this.tokenCache.set(token, { result: validationResult, expires: cacheExpiry });
      }

      // Emit validation event
      this.emit('token-validated', {
        valid: validationResult.valid,
        subject: validationResult.claims?.sub,
        scopes: validationResult.scopes,
        resourceIndicators: validationResult.resourceIndicators
      });

      return validationResult;

    } catch (error) {
      logger.error('Bearer token validation failed', { error });
      return {
        valid: false,
        scopes: [],
        resourceIndicators: [],
        error: 'server_error',
        errorDescription: 'Token validation failed due to server error'
      };
    }
  }

  /**
   * Extract Bearer token from Authorization header
   */
  private extractBearerToken(authHeader: string): string | null {
    if (!authHeader || typeof authHeader !== 'string') {
      return null;
    }

    const parts = authHeader.trim().split(' ');
    if (parts.length !== 2 || parts[0].toLowerCase() !== 'bearer') {
      return null;
    }

    return parts[1];
  }

  /**
   * Validate JWT token with comprehensive checks
   */
  private async validateJWT(token: string): Promise<ValidationResult> {
    try {
      // Parse JWT structure
      const parts = token.split('.');
      if (parts.length !== 3) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Malformed JWT structure'
        };
      }

      // Decode header and payload
      const header = this.decodeBase64Url(parts[0]);
      const payload = this.decodeBase64Url(parts[1]);
      
      const headerObj = JSON.parse(header);
      const claims: TokenClaims = JSON.parse(payload);

      // Validate algorithm
      if (!this.config.tokenValidation.algorithms.includes(headerObj.alg)) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: `Unsupported algorithm: ${headerObj.alg}`
        };
      }

      // Validate timing claims
      const now = Math.floor(Date.now() / 1000);
      const clockTolerance = this.config.tokenValidation.clockTolerance;

      if (claims.exp && claims.exp < (now - clockTolerance)) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Token has expired'
        };
      }

      if (claims.nbf && claims.nbf > (now + clockTolerance)) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Token not yet valid'
        };
      }

      if (claims.iat && this.config.tokenValidation.maxAge > 0) {
        const maxAge = this.config.tokenValidation.maxAge;
        if (claims.iat < (now - maxAge)) {
          return {
            valid: false,
            scopes: [],
            resourceIndicators: [],
            error: 'invalid_token',
            errorDescription: 'Token too old'
          };
        }
      }

      // Validate issuer
      if (claims.iss !== this.config.issuer) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Invalid issuer'
        };
      }

      // Validate audience
      const audiences = Array.isArray(claims.aud) ? claims.aud : [claims.aud];
      if (!audiences.includes(this.config.audience)) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Invalid audience'
        };
      }

      // Validate signature
      const signatureValid = await this.validateSignature(token, headerObj, claims);
      if (!signatureValid) {
        return {
          valid: false,
          scopes: [],
          resourceIndicators: [],
          error: 'invalid_token',
          errorDescription: 'Invalid token signature'
        };
      }

      // Extract and validate scopes
      const scopes = this.extractScopes(claims);
      const scopesValid = this.validateScopes(scopes);
      if (!scopesValid) {
        return {
          valid: false,
          scopes,
          resourceIndicators: [],
          error: 'insufficient_scope',
          errorDescription: 'Token lacks required scopes'
        };
      }

      // Extract and validate resource indicators (RFC 8707)
      const resourceIndicators = this.extractResourceIndicators(claims);
      const resourcesValid = this.validateResourceIndicators(resourceIndicators);
      if (!resourcesValid) {
        return {
          valid: false,
          scopes,
          resourceIndicators,
          error: 'invalid_resource',
          errorDescription: 'Token lacks required resource indicators'
        };
      }

      logger.debug('JWT validation successful', {
        subject: claims.sub,
        client: claims.client_id,
        scopes: scopes.length,
        resources: resourceIndicators.length
      });

      return {
        valid: true,
        claims,
        scopes,
        resourceIndicators
      };

    } catch (error) {
      logger.error('JWT validation error', { error });
      return {
        valid: false,
        scopes: [],
        resourceIndicators: [],
        error: 'invalid_token',
        errorDescription: 'Token validation failed'
      };
    }
  }

  /**
   * Validate JWT signature using JWKS
   */
  private async validateSignature(token: string, header: any, claims: TokenClaims): Promise<boolean> {
    try {
      // Get signing key from JWKS
      const jwk = await this.getSigningKey(header.kid);
      if (!jwk) {
        logger.warn('Signing key not found', { kid: header.kid });
        return false;
      }

      // Convert JWK to crypto key
      const publicKey = await this.jwkToCryptoKey(jwk);
      
      // Verify signature based on algorithm
      const [headerB64, payloadB64, signatureB64] = token.split('.');
      const signatureData = `${headerB64}.${payloadB64}`;
      const signature = this.decodeBase64UrlToBuffer(signatureB64);

      let isValid = false;

      switch (header.alg) {
        case 'RS256':
          isValid = crypto.verify('sha256', Buffer.from(signatureData), publicKey, signature);
          break;
        case 'ES256':
          isValid = crypto.verify('sha256', Buffer.from(signatureData), publicKey, signature);
          break;
        case 'HS256':
          // HMAC validation (symmetric key)
          const hmac = crypto.createHmac('sha256', jwk.k!);
          hmac.update(signatureData);
          const expectedSignature = hmac.digest();
          isValid = crypto.timingSafeEqual(signature, expectedSignature);
          break;
        default:
          logger.warn('Unsupported signing algorithm', { algorithm: header.alg });
          return false;
      }

      return isValid;

    } catch (error) {
      logger.error('Signature validation failed', { error });
      return false;
    }
  }

  /**
   * Get signing key from JWKS endpoint
   */
  private async getSigningKey(keyId?: string): Promise<JWK | null> {
    try {
      // Check cache first
      if (this.config.caching.jwksCache && keyId) {
        const cached = this.jwksCache.get(keyId);
        if (cached && cached.expires > Date.now()) {
          return cached.jwk;
        }
      }

      // Fetch JWKS from endpoint
      const jwks = await this.fetchJWKS();
      
      // Find appropriate key
      let selectedKey: JWK | null = null;
      
      if (keyId) {
        selectedKey = jwks.keys.find(key => key.kid === keyId) || null;
      } else {
        // Use first available key if no kid specified
        selectedKey = jwks.keys[0] || null;
      }

      // Cache the key
      if (selectedKey && keyId && this.config.caching.jwksCache) {
        const cacheExpiry = Date.now() + (this.config.caching.jwksCacheTtl * 1000);
        this.jwksCache.set(keyId, { jwk: selectedKey, expires: cacheExpiry });
      }

      return selectedKey;

    } catch (error) {
      logger.error('Failed to get signing key', { keyId, error });
      return null;
    }
  }

  /**
   * Fetch JWKS from the configured endpoint
   */
  private async fetchJWKS(): Promise<JWKS> {
    try {
      // In a real implementation, this would be an HTTP request
      // For now, return a mock JWKS for development
      const mockJWKS: JWKS = {
        keys: [{
          kty: 'RSA',
          use: 'sig',
          alg: 'RS256',
          kid: 'mock-key-1',
          n: 'mock-rsa-modulus',
          e: 'AQAB'
        }]
      };

      logger.debug('JWKS fetched successfully', { keysCount: mockJWKS.keys.length });
      return mockJWKS;

    } catch (error) {
      logger.error('Failed to fetch JWKS', { jwksUri: this.config.jwksUri, error });
      throw error;
    }
  }

  /**
   * Convert JWK to Node.js crypto key
   */
  private async jwkToCryptoKey(jwk: JWK): Promise<any> {
    // This is a simplified version - in production you'd use a proper JWK library
    switch (jwk.kty) {
      case 'RSA':
        // Convert JWK RSA key to PEM format
        // In production, use a library like node-jose or crypto.createPublicKey
        return `-----BEGIN PUBLIC KEY-----\n${jwk.n}\n-----END PUBLIC KEY-----`;
      
      case 'EC':
        // Convert JWK EC key to PEM format
        return `-----BEGIN PUBLIC KEY-----\n${jwk.x}${jwk.y}\n-----END PUBLIC KEY-----`;
      
      case 'oct':
        // Symmetric key
        return Buffer.from(jwk.k!, 'base64url');
      
      default:
        throw new Error(`Unsupported key type: ${jwk.kty}`);
    }
  }

  /**
   * Extract OAuth scopes from token claims
   */
  private extractScopes(claims: TokenClaims): string[] {
    if (!claims.scope) return [];
    
    // Scopes can be space-separated string or array
    if (typeof claims.scope === 'string') {
      return claims.scope.split(' ').filter(s => s.length > 0);
    }
    
    // Handle array case with explicit typing
    const scope = claims.scope as any;
    if (Array.isArray(scope)) {
      return scope.filter((s: any) => typeof s === 'string' && s.length > 0);
    }
    
    return [];
  }

  /**
   * Validate required OAuth scopes
   */
  private validateScopes(tokenScopes: string[]): boolean {
    // Check if token has all required scopes
    for (const requiredScope of this.config.requiredScopes) {
      if (!tokenScopes.includes(requiredScope)) {
        logger.debug('Missing required scope', { 
          required: requiredScope,
          available: tokenScopes 
        });
        return false;
      }
    }
    return true;
  }

  /**
   * Extract resource indicators from token claims (RFC 8707)
   */
  private extractResourceIndicators(claims: TokenClaims): string[] {
    if (!claims.resource) return [];
    
    if (typeof claims.resource === 'string') {
      return [claims.resource];
    }
    
    if (Array.isArray(claims.resource)) {
      return claims.resource.filter(r => typeof r === 'string' && r.length > 0);
    }
    
    return [];
  }

  /**
   * Validate resource indicators (RFC 8707)
   */
  private validateResourceIndicators(tokenResources: string[]): boolean {
    if (this.config.resourceIndicators.length === 0) {
      return true; // No resources required
    }

    // Check if token has at least one required resource indicator
    for (const requiredResource of this.config.resourceIndicators) {
      if (tokenResources.includes(requiredResource)) {
        return true;
      }
    }

    logger.debug('Missing required resource indicators', {
      required: this.config.resourceIndicators,
      available: tokenResources
    });
    
    return false;
  }

  /**
   * Decode base64url without padding
   */
  private decodeBase64Url(input: string): string {
    // Add padding if needed
    let padded = input;
    while (padded.length % 4) {
      padded += '=';
    }
    
    // Replace URL-safe characters
    const base64 = padded.replace(/-/g, '+').replace(/_/g, '/');
    
    return Buffer.from(base64, 'base64').toString('utf8');
  }

  /**
   * Decode base64url to buffer
   */
  private decodeBase64UrlToBuffer(input: string): Buffer {
    let padded = input;
    while (padded.length % 4) {
      padded += '=';
    }
    
    const base64 = padded.replace(/-/g, '+').replace(/_/g, '/');
    return Buffer.from(base64, 'base64');
  }

  /**
   * Start cache cleanup interval
   */
  private startCacheCleanup(): void {
    if (this.cleanupInterval) return;

    this.cleanupInterval = setInterval(() => {
      this.cleanupExpiredEntries();
    }, 60000); // Clean every minute
  }

  /**
   * Clean up expired cache entries
   */
  private cleanupExpiredEntries(): void {
    const now = Date.now();
    let cleaned = 0;

    // Clean JWKS cache
    for (const [key, entry] of this.jwksCache.entries()) {
      if (entry.expires <= now) {
        this.jwksCache.delete(key);
        cleaned++;
      }
    }

    // Clean token cache
    for (const [key, entry] of this.tokenCache.entries()) {
      if (entry.expires <= now) {
        this.tokenCache.delete(key);
        cleaned++;
      }
    }

    if (cleaned > 0) {
      logger.debug('Cache cleanup completed', { entriesRemoved: cleaned });
    }
  }

  /**
   * Get OAuth server metrics
   */
  getMetrics(): {
    jwksCacheSize: number;
    tokenCacheSize: number;
    cacheHitRate: number;
    validationCount: number;
    errorCount: number;
  } {
    // In production, these would be tracked metrics
    return {
      jwksCacheSize: this.jwksCache.size,
      tokenCacheSize: this.tokenCache.size,
      cacheHitRate: 0.85, // Mock value
      validationCount: 0, // Would be tracked
      errorCount: 0 // Would be tracked
    };
  }

  /**
   * Shutdown OAuth Resource Server
   */
  shutdown(): void {
    if (this.cleanupInterval) {
      clearInterval(this.cleanupInterval);
      this.cleanupInterval = null;
    }

    this.jwksCache.clear();
    this.tokenCache.clear();
    this.removeAllListeners();

    logger.info('OAuth Resource Server shutdown completed');
  }
}

// Default OAuth configuration for MCP servers (2024 standards)
export const defaultOAuthConfig: OAuthConfig = {
  issuer: 'https://auth.example.com',
  jwksUri: 'https://auth.example.com/.well-known/jwks.json',
  audience: 'mcp-server',
  requiredScopes: ['mcp:read', 'mcp:execute'],
  resourceIndicators: ['urn:mcp:server:codecrucible-synth'],
  tokenValidation: {
    enabled: true,
    clockTolerance: 30, // 30 seconds
    maxAge: 3600, // 1 hour
    algorithms: ['RS256', 'ES256']
  },
  caching: {
    jwksCache: true,
    jwksCacheTtl: 300, // 5 minutes
    tokenCache: true,
    tokenCacheTtl: 60 // 1 minute
  }
};