Block Ciphers: CBC

Example of using CBC: Suppose that your plaintext is: 010 110, and the initial vector is 101. Also, suppose that you are given the following encryption function E(b) as a table:

Plaintext	000	001	010	011	100	101	110	111
Ciphertext	101	010	000	001	111	100	011	110

• To encrypt the plaintext, you'll work on one block at a time.
  • First, we XOR the 1st plaintext block with the initial value to get: 010 ⊕ 101 = 111, and then plug the result into the table to get the ciphertext: E(111) = 110.
  • To get the 2nd ciphertext block, we first XOR the 2nd plaintext block with the previous ciphertext, 110, to get: 110 ⊕ 110 = 000, and then plug the result into the table to get the ciphertext: E(000) = 101.
• To decrypt the ciphertext, you perform the actions above in the opposite order: you first solve E(b₁) = 110, and XOR b₁ with the initial value to get the 1st plaintext block. Then, you solve E(b₂) = 101, and XOR b₂ with the 1st ciphertext block to get the 2nd plaintext block.