http://flashmxup6d3qesb72j3ciurm6v2tgjjhdffndiv35o2ifk62wv7njid.onion/verify?lang=fr
The signature is produced by a service-owned PGP key. Compare its fingerprint above with the one inside the letter to confirm authenticity. Pasting the signed letter here submits it to the server, which verifies the signature against the published public key.