In a server of a public key system a list of fingerprints of public keys is stored and provided. Furthermore, a fingerprint of the fingerprint list is calculated for providing the calculated fingerprint of the list of fingerprints. A client terminal of the public key system receives a list of fingerprints of public keys and a first fingerprint thereof from a first source. The client terminal further receives a second fingerprint of the list of fingerprints from a second source and compares the first and the second received fingerprint.