How to verify an ES256 JWT in Ruby

1gem install jwt -v 3.3.0   # or in a Gemfile: gem "jwt", "3.3.0"
2 

1require "jwt"
2require "openssl"
3 
4public_key = OpenSSL::PKey.read(File.read("pub.pem"))
5token = File.read("token.txt").strip
6 
7payload, header = JWT.decode(
8  token, public_key, true,          # true: verify the signature and the claims
9  algorithm: "ES256",               # the only algorithm accepted
10  iss: "https://ecdsa.com", verify_iss: true,
11  required_claims: %w[exp sub],     # exp is checked only when present otherwise
12  leeway: 30                        # seconds of clock skew for exp and nbf
13)
14puts "valid #{header['alg']} token for #{payload['sub']}, expires #{Time.at(payload['exp']).utc}"
15 

How it works

1. OpenSSL::PKey.read loads the SPKI public key; ruby-jwt accepts the resulting OpenSSL::PKey::EC directly as the verification key.
2. The third argument true enables verification, and algorithm: "ES256" pins the algorithm — a token whose header says HS256 raises JWT::IncorrectAlgorithm.
3. iss: together with verify_iss: true compares the issuer; required_claims makes exp and sub mandatory; leeway allows clock skew for exp and nbf.
4. decode returns [payload, header] as plain hashes, or raises a JWT::DecodeError subclass — the run triggers VerificationError, IncorrectAlgorithm, ExpiredSignature and InvalidIssuerError.

Gotchas

• iss: "https://ecdsa.com" on its own does nothing: without verify_iss: true a token from any issuer is accepted. The same pattern applies to aud: and verify_aud.
• A token without exp is accepted unless exp is listed in required_claims — expiry is only enforced when the claim exists.
• JWT.decode(token, nil, false) returns the payload without checking anything; it is for debugging only, never for authorization.
• Keys from an identity provider rotate: pass jwks: (a JWT::JWK::Set or a loader lambda) instead of a fixed key, so the token's kid selects the current key.

Related recipes

• How to verify an ECDSA signature in Ruby
• How to convert an ECDSA signature between DER and raw in Ruby
• How to verify an ES256 JWT in PHP
• How to verify an ES256 JWT in Python
• How to verify an ES256 JWT in Node.js