Patent ID: 11876888
Assignee: MITSUBISHI ELECTRIC CORPORATION
Field: Digital communication (Electrical engineering)
Classification: CPC H | IPC H

Claim 15:
16. A non-transitory computer readable medium storing a decryption program which causes a computer to function as an decryption device that performs:
an acquisition process of acquiring a ciphertext C and an authenticator T′;
a division process of dividing the ciphertext C acquired by the acquisition process, every b bits from a beginning, the b bits being a block size of an encryption function E of a block cipher, thereby generating b-bit values C1, . . . , Cm-1 and a value Cm having 1 or more bits to b or less bits;
an S1 calculation process of assigning a b-bit value H1 to a value M0, and for each integer i of i=1, . . . , m in an ascending order, taking a value Mi-1 as input to the encryption function E, thereby calculating a value S1(i), and calculating a value Mi from the value S1(i) and a value Ci;
an S2 calculation process of assigning an r-bit value H2 to a value S2(0), and for each integer i of i=1, . . . , m in an ascending order, calculating a value S2(i) from the value S1(i) calculated by the S1 calculation process and from a value S2(i−1);
a plaintext generation process of generating a plaintext M from a value Mi for each integer i of i=1, . . . , m; and
an authenticator generation process of generating a (b+r)-bit authenticator T by using a value S1(m) and a value S2(m), and judging whether the authenticator T and the authenticator T′ coincide or not.