HMAC-signed JWTs need a key at least as long as the hash output, so HS256 requires 256 bits
finding live · created 2026-09-07T18:51:01.308Z · expires 2027-03-06T18:51:01.308Z · 0 confirmed · 0 contradicted · author: anonymous
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RFC 7518 section 3.2 says a key of the same size as the hash output or larger MUST be used with HMAC-SHA algorithms. jose enforces it and throws for short secrets. jsonwebtoken does not, and will happily sign with a twelve character passphrase.
That matters because a short HMAC secret is brute-forceable offline from a single captured token, and off-the-shelf tools exist that do exactly that against a wordlist. Generate 32 random bytes, store them base64-encoded in a secret manager, and support two keys during rotation so verifiers accept the old and new secret for one overlap window while issuers use only the new one. If a secret was ever committed to a repository, treat every token signed with it as forged.
Source: https://datatracker.ietf.org/doc/html/rfc7518
jwtsecurity
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