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)
  • AES
  • Blowfish
  • DES
  • Triple DES
  • Serpent
  • Twofish
Less common
  • Camellia
  • CAST-128
  • IDEA
  • RC2
  • RC5
  • SEED
  • Skipjack
  • TEA
  • XTEA
  • 3-Way
  • Akelarre
  • Anubis
  • ARIA
  • BaseKing
  • BassOmatic
  • BEAR and LION
  • CAST-256
  • CIKS-1
  • CMEA
  • Cobra
  • Crab
  • Cryptomeria/C2
  • CS-Cipher
  • DEAL
  • DES-X
  • DFC
  • E2
  • FEAL
  • FEA-M
  • FROG
  • G-DES
  • GOST
  • Grand Cru
  • Hasty Pudding cipher
  • Hierocrypt
  • ICE
  • Intel Cascade Cipher
  • Iraqi
  • KeeLoq
  • Khufu and Khafre
  • KN-Cipher
  • Ladder-DES
  • Libelle
  • LOKI97
  • LOKI89/91
  • Lucifer
  • M6
  • M8
  • MacGuffin
  • Madryga
  • MARS
  • Mercy
  • MESH
  • MISTY1
  • MMB
  • MULTI2
  • MultiSwap
  • New Data Seal
  • NewDES
  • Nimbus
  • NUSH
  • Q
  • RC6
  • Red Pike
  • S-1
  • SC2000
  • SMS4
  • Spectr-H64
  • Square
  • Threefish
  • Treyfer
  • UES
  • Xenon
  • xmx
  • Zodiac
  • Feistel network
  • Key schedule
  • Product cipher
  • S-box
  • P-box
  • SPN
  • Brute force
  • MITM
  • Linear
  • Differential (Impossible
  • Truncated
  • Higher-order)
  • Integral
  • Boomerang
  • Mod n
  • Related-key
  • Slide
  • Rotational
  • Timing
  • XSL
  • Interpolation
  • Partitioning
  • Davies'
  • AES process
  • Avalanche effect
  • Block size
  • Initialization vector
  • Key size
  • Modes of operation
  • Padding
  • Piling-up lemma
  • Weak key
  • EFF DES cracker
  • Key whitening
  • 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

Famous quotes containing the words block and/or size:

    Of course I lie to people. But I lie altruistically—for our mutual good. The lie is the basic building block of good manners. That may seem mildly shocking to a moralist—but then what isn’t?
    Quentin Crisp (b. 1908)

    One writes of scars healed, a loose parallel to the pathology of the skin, but there is no such thing in the life of an individual. There are open wounds, shrunk sometimes to the size of a pin-prick but wounds still. The marks of suffering are more comparable to the loss of a finger, or the sight of an eye. We may not miss them, either, for one minute in a year, but if we should there is nothing to be done about it.
    F. Scott Fitzgerald (1896–1940)