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Home > 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 is logically required by the Pigeonhole principle and the fact that the cipher must be invertible — and it is simply undesireable 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 modeIn cryptography, a block cipher operates on blocks of fixed length, often 64 or 128 bits. To encrypt longer messages, several modes of operation may be used. While many modes provide only confidentiality, some modes also ensure message integrity. This dis, only B = 32 GB of data can be safely sent under one key. In practice we want odds a lot better than 50%, so we are restricted to 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 worse).

Consequently AES candidates were required to support a block length of 128 bits (16 bytes). This should be acceptable for up to B = 256 ExabyteAn exabyte (derived from the SI prefix exa ) is a unit of measurement in computers of one million million million bytes. Its abbreviation is EB . Because of irregularities in definition and usage of the kilobyte, the exact number in common practice coulds of data. That should be enough for quite a few years to come.

A few block ciphers, such as RC5RC5 is a block cipher notable for its simplicity. Designed by Ronald Rivest in 1994, RC stands for "Rivest Cipher", or alternatively, "Ron's Code" (compare RC2, RC4). The AES candidate RC6 was based on RC5. Unlike many schemes, RC5 has a variable block si, actually support a variable block size, while Joan Daemen 's 3-WayIn cryptography, 3-Way is a block cipher designed in 1994 by Joan Daemen, who also (with Vincent Rijmen) designed Rijndael, the winner of NIST's Advanced Encryption Standard (AES) contest. 3-Way has a block size of 96 bits, notably not a power of two such has a 96-bit block.

The Luby-Rackoff construction and the Outerbridge construction can both increase the effective block size of a cipher.


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Block Size (cryptography) Ciphers Length Data Input Output Bits

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Block cipherIn cryptography, a block cipher is a type of symmetric key cipher which operates on groups of bits of a fixed length, termed blocks''. This is in contrast to stream ciphers which encrypt each bit of the plaintext individually before moving on to the next.BlockFor blocks on Wikipedia, please see Blocking policy. The concept of a block as a group or a piece appears in various fields: The meaning in the physical world is any item with length, width, and height. It is often shaped like a rectagle, although it mayBlock check characterIn telecommunication, a block check character (BCC) is a character added to a transmission block to facilitate error detection. In longitudinal redundancy checking and cyclic redundancy checking, block check characters are computed for, and added to, each
BlockingFor blocking on Wikipedia, please see Blocking policy. In telecommunication, the term blocking has the following meanings: 1. The formatting of data into blocks for purposes of transmission, storage, checking, or other functions. Denying access to, or useBlock transfer attemptIn telecommunication, a block transfer attempt is a coordinated sequence of user and telecommunication system activities undertaken to effect transfer of an individual block from a source user to a destination user. A block transfer attempt begins when thBlock timeBlock time is one way of approaching the problem of the nature of time. Its name is derived from its description of spacetime as an unchanging four-dimensional "block", as opposed to a three-dimensional space that changes as it moves along a time axis.
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Block size (cryptography)Block size (data storage and transmission)Block size


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