Patent ID: 7315941

Claim:
A computer implemented method for generating computer data for verifying validity and/or invalidity of digital certificates in a plurality of periods of time, each digital certificate certifying that a cryptographic key is associated with an entity, the digital certificates including a first digital certificate, the method comprising: (a) obtaining first data defining a plurality of sets, each set being a set of one or more of the digital certificates, the plurality of sets comprising a first plurality of sets, wherein the first digital certificate belongs to each set in the first plurality, wherein at least one set of the first plurality comprises two or more digital certificates; (b) obtaining second data which define, for each of said certificates and each of said periods of time, associated proof data for proving validity or invalidity of the certificate in the period of time; (c) obtaining third data which define, for each said certificate and each said period of time T 1 , an encryption key for encrypting the proof data associated with the certificate and the period T 1 ; (c 1 ) wherein for each said set containing the certificate, the third data also define decryption key data associated with the set and the period T 1 , the decryption key data being for computing a decryption key for each certificate in the set and for the period T 1 from the decryption key data associated with the set and one or more of the periods of time which are not later than the period T 1 and which include T 1 , wherein the decryption key associated with the certificate and the period T 1 is computable from the decryption key data associated with any given one of the sets containing the certificate and with one or more of the periods of time which are not later than the period T 1 and which include the period T 1 ; (d) wherein for at least one said period of time, at least two sets of the first plurality of sets are associated with respective different decryption key data, and in at least one set of the first plurality, the first digital certificate and at least one other digital certificate are associated with different decryption keys.