Block Size (cryptography)

In modern cryptography, symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size. Both the input (plaintext) and output (ciphertext) are the same length; the output cannot be shorter than the input — this follows logically from the Pigeonhole principle and the fact that the cipher must be reversible — and it is undesirable for the output to be longer than the input.

Until the announcement of NIST's AES contest, the majority of block ciphers followed the example of the DES in using a block size of 64 bits (8 bytes). However the Birthday paradox tells us that after accumulating a number of blocks equal to the square root of the total number possible, there will be an approximately 50% chance of two or more being the same, which would start to leak information about the message contents. Thus even when used with a proper encryption mode (e.g. CBC or OFB), only 232 x 8 B = 32 GB of data can be safely sent under one key. In practice a greater margin of security is desired, restricting a single key to the encryption of much less data - say a few hundred megabytes. Once that seemed like a fair amount of data, but today it is easily exceeded. If the cipher mode does not properly randomise the input, the limit is even lower.

Consequently AES candidates were required to support a block length of 128 bits (16 bytes). This should be acceptable for up to 264 x 16 B = 256 Exabytes of data, and should suffice for quite a few years to come. The winner of the AES contest, Rijndael, supports block and key sizes of 128, 192, and 256 bits, but in AES the block size is always 128 bits. The extra block sizes were not adopted by the AES standard.

Many block ciphers, such as RC5, support a variable block size. The Luby-Rackoff construction and the Outerbridge construction can both increase the effective block size of a cipher.

Joan Daemen's 3-Way and BaseKing have unusual block sizes of 96 and 192 bits, respectively.

There are techniques for creating ciphers on unusual or fractional block sizes (i.e. domains whose size is not a power of two); see Format-Preserving Encryption.

Block ciphers (security summary)

Common algorithms	AES Blowfish DES Triple DES Serpent Twofish

Less common algorithms	Camellia CAST-128 IDEA RC2 RC5 SEED Skipjack TEA XTEA

Other algorithms	3-Way Akelarre Anubis ARIA BaseKing BassOmatic BATON BEAR and LION CAST-256 CIKS-1 CIPHERUNICORN-A CIPHERUNICORN-E CLEFIA CMEA Cobra COCONUT98 Crab Cryptomeria/C2 CRYPTON CS-Cipher DEAL DES-X DFC E2 FEAL FEA-M FROG G-DES GOST Grand Cru Hasty Pudding cipher Hierocrypt ICE IDEA NXT Intel Cascade Cipher Iraqi KASUMI KeeLoq KHAZAD Khufu and Khafre KN-Cipher Ladder-DES Libelle LOKI97 LOKI89/91 Lucifer M6 M8 MacGuffin Madryga MAGENTA MARS Mercy MESH MISTY1 MMB MULTI2 MultiSwap New Data Seal NewDES Nimbus NOEKEON NUSH PRESENT Q RC6 REDOC Red Pike S-1 SAFER SAVILLE SC2000 SHACAL SHARK SMS4 Spectr-H64 Square SXAL/MBAL Threefish Treyfer UES Xenon xmx XXTEA Zodiac

Design	Feistel network Key schedule Product cipher S-box P-box SPN

Attack (cryptanalysis)	Brute force MITM Linear Differential (Impossible Truncated Higher-order) Integral Boomerang Mod n Related-key Slide Rotational Timing XSL Interpolation Partitioning Davies'

Standardization	AES process CRYPTREC NESSIE

Misc	Avalanche effect Block size Initialization vector Key size Modes of operation Padding Piling-up lemma Weak key EFF DES cracker Key whitening

Cryptography

History of cryptography Cryptanalysis Cryptography portal Outline of cryptography

Symmetric-key algorithm Block cipher Stream cipher Public-key cryptography Cryptographic hash function Message authentication code Random numbers Steganography

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