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Security Fundamentals & Crypto · Intermediate

Cipher Block Chaining (CBC)

Block cipher mode in which each plaintext block is XORed with the previous ciphertext block before encryption.

Definition

CBC uses an initialization vector for the first block and chains every subsequent block, hiding patterns that ECB reveals. Encryption is sequential, while decryption can be parallelized. CBC without a MAC is vulnerable to padding oracle attacks, which is one reason TLS 1.3 removed CBC cipher suites.

Example

Older TLS 1.2 suites such as TLS_RSA_WITH_AES_128_CBC_SHA use CBC mode.

Exam tip

CBC needs an unpredictable IV and a separate integrity check; prefer AEAD modes.

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