Implementing JWT Signing and Verification Skill

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

| Algorithm | Type | Key | Security Level | |-----------|------|-----|---------------| | HS256 | Symmetric (HMAC) | Shared secret | 128-bit | | RS256 | Asymmetric (RSA) | RSA key pair | 112-bit | | ES256 | Asymmetric (ECDSA) | P-256 key pair | 128-bit | | EdDSA | Asymmetric (Ed25519) | Ed25519 pair | 128-bit |

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

Agent Skills: Implementing JWT Signing and Verification

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