Question 1

How long should my JWT secret be?

Accepted Answer

RFC 7518 (JSON Web Algorithms) requires HMAC keys to be at least as long as the hash output. For HS256: at least 32 bytes (256 bits). For HS384: at least 48 bytes. For HS512: at least 64 bytes. Shorter keys are forbidden by the spec and many JWT libraries refuse to sign or verify with them. Using a key shorter than the hash output reduces effective security — an attacker only needs to brute-force the key's actual entropy, not the full hash output size.

Question 2

Should I use HS256, HS384, or HS512?

Accepted Answer

HS256 is the modern default and what most JWT libraries pick automatically. It's fast, secure, and supported everywhere. Use HS384 or HS512 only when you have a specific requirement for longer keys — high-security environments, compliance frameworks that mandate 384+ bits, or interoperability with systems that demand it. The security difference between HS256 and HS512 is practically zero — both are far beyond brute-force reach at full key length.

Question 3

Hex, base64, or base64url?

Accepted Answer

Base64 (or base64url) is standard for JWT secrets — most environment variables and config systems accept it without escaping. Base64url is the URL-safe variant that replaces + with -, / with _, and drops padding (=); use it when the secret might appear in a URL. Hex is twice as long as base64 for the same number of bytes (32 bytes = 64 hex chars vs 44 base64 chars), but some systems prefer it because it's free of special characters. They all encode the same cryptographic key — pick the one your tooling expects.

Question 4

Should I use HMAC or RSA/ECDSA for JWT signing?

Accepted Answer

Depends on your trust model. HMAC (HS256/HS384/HS512) uses a single shared secret — anyone with the secret can sign AND verify, so it's only appropriate when the signer and verifier are the same party (your auth service signing tokens for your own API). Public-key signing (RS256, ES256, EdDSA) lets you publish a public key that anyone can use to verify, while only the private key holder can sign — appropriate for federated auth, multi-service systems, or when you don't fully trust the verifier. For a single API behind a single auth service, HMAC is simpler and faster.

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