Patent ID: 7676677

Claim:
A digital-signature method for simultaneously enabling signature verification of digitally-signed data exchanged between at least first and second parties that have associated respective public/private key-pairs; the method comprising: (a) the first party operating a computing apparatus to perform the steps of: generating first cryptographic data, herein a keystone, and from this, second cryptographic data, herein a keystone fix; generating a first signature in a first way using at least the keystone fix, the private key of the first-party's key-pair, the public key of at least one other public/private key-pair, and first subject data; making available, at least to the second party, a first data set comprising the first subject data and the first signature including a first set of at least one keystone-fix-related element; and b) the second party operating a computing apparatus to perform the steps of: generating a second signature in said first way using at least one said keystone-fix-related element made available by the first party, the private key of the second party's key-pair, the public key of at least one other public/private key-pair, and second subject data; making available, at least to the first party, a second data set comprising the second subject data and the second signature including a second set of at least one keystone-fix-related element; each of the first and second signatures being such that an independent signature-checking party, without knowledge of the keystone, can only verify the signature as being one of a set of signatures comprising: a signature created in said first way, and a respective signature created in a second way by a possessor of the private key of the or each relevant said other key-pair; knowledge of the keystone being necessary for the signature-checking party to determine that the first and second signatures were created by the first and second parties respectively.