Block cipher

A block cipher is a symmetric-key cryptographic algorithm that transforms fixed-length blocks of plaintext into blocks of ciphertext of the same size, using a secret key. Unlike stream ciphers, which encrypt data bit-by-bit or byte-by-byte, block ciphers process data in discrete chunks — typically 64 or 128 bits — making them natural building blocks for file encryption, disk encryption, and network protocols. The Data Encryption Standard and the Advanced Encryption Standard are the two most widely deployed block ciphers in history, and their design tradeoffs illustrate a fundamental tension in cryptographic engineering: larger blocks resist certain attacks but require more memory and processing power, while smaller blocks leak information about message structure through repeated patterns.

The security of a block cipher does not depend solely on the algorithm itself but on the mode of operation — the protocol that determines how multiple blocks are chained together. A theoretically perfect block cipher can fail catastrophically when used in the wrong mode, a lesson demonstrated repeatedly by real-world breaches that exploited implementation errors rather than mathematical weaknesses.