Imported from mukul975/anthropic-cybersecurity-skills (skills/implementing-jwt-signing-and-verification/SKILL.md). Install upstream with npx skills add mukul975/anthropic-cybersecurity-skills --skill implementing-jwt-signing-and-verification. Copyright stays with the author (Apache-2.0).

Implementing JWT Signing and Verification

Overview

JSON Web Tokens (JWT) defined in RFC 7519 are compact, URL-safe tokens used for authentication and authorization in web applications. This skill covers implementing secure JWT signing with HMAC-SHA256, RSA-PSS, and EdDSA algorithms, along with verification, token expiration, claims validation, and defense against common JWT attacks (algorithm confusion, none algorithm, key injection).

When to Use

• When deploying or configuring implementing jwt signing and verification capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Prerequisites

• Familiarity with cryptography concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

Objectives

• Implement JWT signing with HS256, RS256, ES256, and EdDSA
• Verify JWT signatures and validate standard claims
• Implement token expiration, not-before, and audience validation
• Defend against algorithm confusion and none algorithm attacks
• Implement JWT key rotation with JWK Sets
• Build a complete authentication middleware

Key Concepts

JWT Algorithms

Common JWT Attacks

• Algorithm confusion: Switching from RS256 to HS256, using public key as HMAC secret
• None algorithm: Setting alg=none to bypass signature verification
• Key injection: Embedding key in JWK header
• Weak secrets: Brute-forcing short HMAC secrets
• Token replay: Reusing valid tokens without expiration

Security Considerations

• Always validate the algorithm header against an allowlist
• Never accept alg=none in production
• Use asymmetric algorithms (RS256, ES256) for distributed systems
• Set short expiration times (15 min for access tokens)
• Implement token refresh mechanism
• Store secrets securely (not in source code)

Validation Criteria

• JWT signing produces valid tokens for all algorithms
• Signature verification rejects tampered tokens
• Expired tokens are rejected
• Algorithm confusion attack is prevented
• None algorithm is rejected
• JWK key rotation works correctly
• Claims validation enforces all required claims