**Data Authentication Algorithm (DAA)**

Figure: DAA Block Diagram |

One of
the most widely used MACs is referred to as the Data Authentication Algorithm. The
algorithm is designed using the Cipher Block Chaining mode of operation of DES,
as shown in figure. The data (e.g., message, record, file, or program) to be
authenticated are grouped into fixed size 64-bit blocks: D1, D2,….., DN. If
necessary, the final block is padded on the right with zeroes to make a 64-bit blocks.
Using the DES encryption algorithm and a secret key, a data authentication code
(DAC) is calculated as follows:

The DAC consists of either the entire block ON or the leftmost M bits of the block, with 16 ≤ M ≤ 64.

**Cipher based message authentication code
(CMAC)**

Figure: CMAC Block Diagram |

Data
Authentication Algorithm (DAA), which is now obsolete. Then CMAC, which is
designed to overcome the deficiencies of DAA. Cipher-based Message
Authentication Code (CMAC) mode of operation for use with AES and triple DES. First,
let us define the operation of CMAC when the message is an integer multiple n
of the cipher block length b. For AES, b=128, and for triple DES, b=64. The
message is divided into n blocks (M1, M2,…, Mn ).

For AES,
the key size is 128, 192, or 256 bits; for triple DES, the key size is 112 or
168 bits. CMAC is calculated as follows:

C1 = E(K,
M1)

C2 = E(K,
[M2 ⊕ C1])

C3 = E(K,
[M3 ⊕ C2])

.

.

.

Cn = E(K,
[Mn ⊕ Cn-1 ⊕ K1])

T =
MSBTlen(Cn)

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