How MAC {Message Authentication Code} Calculation works works
Let us start with this input data: 01234567890123450123456789012345
We will take the same value as the key:
01234567890123450123456789012345
We will take the setting of encryption as triple DES: 3DES
And the finish block will also be encrypted triple DES:3DES
Triple DES needs 3 keys, so up to 48 hex characters. We only
provided 32 hex characters, 16 bytes double-key, the most used
scenario
The first key is the first 8 bytes, first 16 hex characters(0-16);
The second key is the next 8 bytes, next 16 hex characters(16-32);
The third key is the first 8 bytes, first 16 hex characters(32-48
or 0-16) because we do not have a triple-length key;
k1:0123456789012345
k2:0123456789012345
k3:0123456789012345
We split the entire input data into blocks of 8 byes, or 16 hex
characters
block:0123456789012345
We xor this block with the previous block. If this is the first
block, we xor it with a block of all -zeroes
xor:0123456789012345
We perform a DES encryption of the block above with the first key.
If 3DES is selected, we perform 3DES with the 3 keys
des:D5D16164F4B82E0F
Next block of 8 bytes:
block:0123456789012345
XOR with the previous block (CBC mode)
xor:D4F224037DB90D4A
And then again 3DES or DES. Given that this is the last block, the
algorithm of the finishing selection will be applied.
des:04D62E1385AAF220
We are out of data to encrypt, so this is the resulting MAC
MAC:04D62E1385AAF220