# Crypto U4, Block Cipher, Cipher Block Chaining Mode (CBC)

Note by , created almost 6 years ago

## IYM002 (Unit 4 - Symmetric Crypto) Note on Crypto U4, Block Cipher, Cipher Block Chaining Mode (CBC), created by jjanesko on 03/04/2013.

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 Created by jjanesko almost 6 years ago
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### Page 1

Encryption Start with an initialization vector (IV). XOR the IV with the first block of plaintext. Pass the result and the key to the encryption function. The result of the operation is the first ciphertext block. XOR the resulting ciphertext block with the next plaintext block. Pass the result and the key to the encryption function. The result of the operation is the next ciphertext block. Go back to step 5 and repeat until there are no more plaintext blocks remaining.

Decryption Take the first ciphertext block. Pass the ciphertext block and the key to the decryption function. XOR the result of the operation with the initialization vector (IV). The result is the first plaintext block. Pass the next ciphertext block and the key to the decryption function. XOR the result of the operation with the ciphertext block from the previous decryption step. The result is the next plaintext block. Go back to step 5 and repeat until there are no more ciphertext blocks remaining.

Error Propagation When there is a 1 bit error, only two blocks are impacted; one right after the other. Block with the error is badly impacted because the faulty ciphertext is processed with decryption function. Succeeding block has error in the position where the ciphertext had the original error because it is only impacted by the XOR process.

Properties message dependence limited error propagation no synchronization necessary can be used to provide data origin identification requires encryption AND decryption requires padding

Encryption and Decryption

Error Propagation