A first party has a first and a second cryptographic key. A second party has a third and a fourth cryptographic key, the fourth cryptographic key being derived from the first and third cryptographic keys thereby providing an association between the parties. To enable a third party to verify the existence of an association between the first and second parties, the second party generates a number that in association with the second cryptographic key, the third cryptographic key and the fourth cryptographic key define a first cryptographic parameter, a second cryptographic parameter and a third cryptographic parameter respectively. By using these parameters and the second and third cryptographic keys, the third party can verify if the first and second parties are associated.