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Security Fundamentals & Crypto · Foundational

Block Cipher

A symmetric cipher that encrypts fixed-size blocks of data, such as 128 bits, using a key.

Definition

Block ciphers process plaintext in fixed-length blocks and use a mode of operation to handle messages longer than one block. Examples include AES (128-bit blocks), DES and 3DES (64-bit blocks), and Twofish. The mode of operation strongly affects security properties.

Example

AES encrypts a 1 MB file as 65,536 128-bit blocks using GCM mode.

Exam tip

Small 64-bit blocks (DES, 3DES, Blowfish) are vulnerable to birthday-bound attacks such as Sweet32.

Don't confuse it with

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