Block size (cryptography)

In modern <a href="/facts/Cryptography/e94PEVau">cryptography</a>, <a href="/facts/Symmetric_key_algorithm/5zfe0b76">symmetric key</a> <a href="/facts/Cipher/eJnuftYe">ciphers</a> are generally divided into <a href="/facts/Stream_cipher/BxU0aWmJ">stream ciphers</a> and <a href="/facts/Block_cipher/MJywAyfQ">block ciphers</a>. <a href="/facts/Block_cipher/MJywAyfQ">Block ciphers</a> operate on a fixed length string of <a href="/facts/Bit/S972NSaD">bits</a>. The length of this bit string is the block size. Both the input (<a href="/facts/Plaintext/tWnBDCgE">plaintext</a>) and output (<a href="/facts/Ciphertext/luNTs4sY">ciphertext</a>) are the same length; the output cannot be shorter than the input – this follows logically from the <a href="/facts/Pigeonhole_principle/hDks9p4n">pigeonhole principle</a> 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 <a href="/facts/NIST/gr9Fnh85">NIST</a>'s <a href="/facts/Advanced_Encryption_Standard_process/UkHpAuEj">AES contest</a>, the majority of block ciphers followed the example of the <a href="/facts/Data_Encryption_Standard/STfBjTGb">DES</a> in using a block size of 64 bits (8 <a href="/facts/BytesForAll/NCxtqu9W">bytes</a>). However, the <a href="/facts/Birthday_problem/ct7ao4dF">birthday paradox</a> indicates that after accumulating several blocks equal to the <a href="/facts/Square_root/AfrzfBdQ">square root</a> 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 <a href="/facts/Block_cipher_modes_of_operation/vIUabB2z">encryption mode</a> (e.g. CBC or OFB), only 232 × 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. At one point that seemed like a fair amount of data, but today it is easily exceeded. If the <a href="/facts/Block_cipher_modes_of_operation/vIUabB2z">cipher mode</a> 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 × 16 B = 256 <a href="/facts/Exabyte/BNHUp9Qq">exabytes</a> of data, and would suffice for many years after introduction. The winner of the AES contest, <a href="/facts/Rijndael/HdXGBY2Q">Rijndael</a>, 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 <a href="/facts/Advanced_Encryption_Standard/HdXGBY2Q">AES</a> standard.
Many block ciphers, such as <a href="/facts/RC5/RjIGUnNk">RC5</a>, support a variable block size. The <a href="/facts/Feistel_cipher/CPtzaEET">Luby-Rackoff</a> construction and the Outerbridge construction can both increase the effective block size of a cipher.
<a href="/facts/Joan_Daemen/cqdIFHHG">Joan Daemen</a>'s <a href="/facts/3-Way/ys51uHk9">3-Way</a> and <a href="/facts/BaseKing/WurRuLJd">BaseKing</a> have unusual block sizes of 96 and 192 bits, respectively.

Block size (cryptography) open-in-new

Block size (cryptography)