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