Patent ID: 7903820

Claim:
A system to determine a decryption key for a cryptoperiod C selected from a plurality of cryptoperiods indexed i, the cryptoperiod C being equal to a cryptoperiod A or a cryptoperiod B or a cryptoperiod between the cryptoperiod A and the cryptoperiod B, the cryptoperiod A being different from the cryptoperiod B, the system comprising: a physical computing device configured to: receive a key-component A- 1 associated with the cryptoperiod A, the key-component A- 1 forming part of a first hash-chain having a plurality of Y-key-components Y i , the index i of Y i indicating which one of the plurality of cryptoperiods i is assigned to Y i , the Y-key-components progressing from Y i to Y i+1 away from a root via a one-way function f 1 , the plurality of Y-key-components assigned to the plurality of cryptoperiods i with progressive Y-key-components assigned to later cryptoperiods; receive a key-component B- 2 associated with the cryptoperiod B, the key-component B- 2 forming part of a second hash-chain having a plurality of X-key-components X i , the index i of X i indicating which one of the plurality of cryptoperiods i is assigned to X i , the X-key-components progressing from X i to X i−1 away from a root via a one-way function f 2 , the plurality of X-key-components assigned to the same plurality of cryptoperiods i with progressive X-key-components assigned to earlier cryptoperiods; determine a key component C- 1 in the first hash-chain for the cryptoperiod C; determine a key-component C- 2 in the second hash-chain for the cryptoperiod C; and determine the decryption key for the cryptoperiod C based on the key component C- 1 and the key component C- 2 ; and a consumer device to decrypt content using the decryption key for the cryptoperiod C.