Patent ID: 7599492

Claim:
A method of recovering a key used to produce a ciphertext document from a plaintext document, the method comprising: configuring a processor to perform the functions of: identifying location of an M-bit control value in the ciphertext document encrypted using an N-bit key; converting the control value to an M-bit portion of a gamma that uniquely corresponds to (a) the ciphertext document and (b) the N-bit key; accessing a file that corresponds to the M-bit portion of the gamma of the cipher text document which serves as an index to select the file, wherein the file includes approximately 2 N-M keys that correspond to the M-bit portion of the gamma out of the 2 N keys; testing the 2 N-M keys using a cryptographic key validity function, until a valid key is found and recovered; and decrypting the ciphertext document using the valid recovered key to produce the plaintext document.