Patent ID: 7386124

Claim:
A reencrypting method for a series of blocks of input ciphertext data, comprising: enabling a first bank memory and a key scheduling circuit and disabling a second bank memory and an encrypting/decrypting circuit; transferring decryption key data to the key scheduling circuit; generating first round key data with the key scheduling circuit based on the decryption key data; storing the first round key data in the first bank memory; enabling the second bank memory and disabling the first bank memory; transferring encryption key data to the key scheduling circuit; generating second round key data with the key scheduling circuit based on encryption key data; storing the second round key data in the second bank memory; enabling the encrypting/decrypting circuit and the first bank memory and disabling the key scheduling circuit and the second bank memory; transferring the first round key data and a block of this series of blocks of input ciphertext data to the encrypting/decrypting circuit; decrypting the block of the series of blocks of input ciphertext data into plaintext data with the encrypting/decrypting circuit using the first round key data to create a block of plaintext data; enabling the second bank memory and disabling the first bank memory; transferring the second round key data and the block of plaintext data to the encrypting/decrypting circuit; and encrypting the block of plaintext data into re-encrypted data with the encrypting/decrypting circuit using the second round key data, wherein the decrypting and the encrypting are alternately carried out with the first and second key data stored in the first and second bank memory for every block of the series of blocks of input ciphertext data on a block-by-block basis after the enabling the encrypting/decrypting circuit.