Firebase\JWT\SignatureInvalidException: Signature verification failed

Uncaught, PHP prints "PHP Fatal error: Uncaught Firebase\JWT\SignatureInvalidException: Signature verification failed in …/JWT.php".

firebase/php-jwt (PHP) — Firebase\JWT\SignatureInvalidException from JWT::decode

What it means

php-jwt parsed the token, accepted the algorithm for the key you passed, and the signature check returned false: the key and the token do not belong together, or the token was changed after signing. `SignatureInvalidException` is a subclass of `UnexpectedValueException`, so a generic catch block may hide it among parsing errors.

Why it happens

• The configured key is not the signer's

  common

  A public key copied from another environment, or a key that was rotated on the issuer side. Firebase Auth and Google sign ID tokens with keys that change every few days.

• One key picked from a key set without using kid

  common

  Code that takes the first key of a JWKS (for example `reset(JWK::parseKeySet($jwks))`) works until the issuer signs with another key in the set.

• HMAC secret differs between services

  occasional

  For HS256, any difference in the secret string — whitespace, a trailing newline from a file, base64 vs raw — produces a different MAC.

• The token changed after signing

  occasional

  Header or payload re-encoded, or a token stitched together from parts of two tokens.

How to fix it

1. 1.

   Pass the whole key set and let php-jwt pick by kid

   `JWT::decode` accepts an array of `Key` objects keyed by `kid` (from `JWK::parseKeySet`) or a `CachedKeySet`, which also refreshes on rotation.

   use Firebase\JWT\CachedKeySet;
   use Firebase\JWT\JWT;
   use GuzzleHttp\Client;
   use GuzzleHttp\Psr7\HttpFactory;
   
   // Keys selected by the token's kid, fetched from the issuer's JWKS and cached (PSR-6).
   $keys = new CachedKeySet(
       'https://issuer.example.com/.well-known/jwks.json',
       new Client(),
       new HttpFactory(),
       $cache,              // any PSR-6 cache pool
       3600,                // cache lifetime in seconds
       true                 // rate-limit refetches on unknown kids
   );
   $claims = JWT::decode($token, $keys);

2. 2.

   Compare the key halves

   If the public key derived from the issuer's private key differs from the one you configured, the signature cannot verify.

   # Do the issuer's private key and your public key belong together?
   openssl pkey -in issuer-private.pem -pubout | sha256sum
   openssl pkey -pubin -in configured-public.pem | sha256sum

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