All Topics  
Block size (cryptography)

 

 
   Email Print
   Bookmark   Link
 

 

Block size (cryptography)
 
 
  Topics Block size (cryptography) Topic Home

In modern cryptography
Cryptography

Cryptography is the practice and study of hiding information. In modern times cryptography is considered a branch of both mathematics and computer science and is affiliated closely with information theory, computer security and engineering....
, symmetric key cipher
Cipher

In cryptography, a cipher is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure....
s are generally divided into stream cipher
Stream cipher

In cryptography, a stream cipher is a symmetric key algorithm cipher where plaintext bits are combined with a pseudorandom cipher bit stream , typically by an exclusive-or operation....
s and block cipher
Block cipher

In cryptography, a block cipher is a symmetric key algorithm cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation....
s. Block cipher
Block cipher

In cryptography, a block cipher is a symmetric key algorithm cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation....
s operate on a fixed length string of bit
Bit

A bit is a binary numeral system numerical digit, taking a value of either 0 or 1. Binary digits are a basic unit of information Computer data storage and transmission in digital computing and digital information theory....
s. The length of this bit string is the block size. Both the input (plaintext
Plaintext

In cryptography, plaintext is the information which the sender wishes to transmit to the receiver. Before the computer era, plaintext simply meant text in the language of the communicating parties....
) and output (ciphertext) are the same length; the output cannot be shorter than the input — this is logically required by the Pigeonhole principle
Pigeonhole principle

In mathematics, the pigeonhole principle, also known as Dirichlet's box principle, is exemplified by such things as the fact that in a family of three children there must be at least two of the same gender....
 and the fact that the cipher must be invertible — and it is simply undesirable for the output to be longer than the input.

Until the announcement of NIST's AES contest
Advanced Encryption Standard process

The Advanced Encryption Standard , the block cipher ratified as a standard by National Institute of Standards and Technology of the United States , was chosen using a process markedly more open and transparent than its predecessor, the aging Data Encryption Standard ....
, the majority of block ciphers followed the example of the DES
Data Encryption Standard

The Data Encryption Standard is a block cipher that was selected by National Bureau of Standards as an official Federal Information Processing Standard for the United States in 1976 and which has subsequently enjoyed widespread use internationally....
 in using a block size of 64 bits (8 byte
Byte

A byte is a basic unit of measurement of Computer storage in computer science. In many computer architectures it is a Byte addressing memory address space....
s).






Discussion
Ask a question about 'Block size (cryptography)'
Start a new discussion about 'Block size (cryptography)'
Answer questions from other users
Full Discussion Forum



Encyclopedia


In modern cryptography
Cryptography

Cryptography is the practice and study of hiding information. In modern times cryptography is considered a branch of both mathematics and computer science and is affiliated closely with information theory, computer security and engineering....
, symmetric key cipher
Cipher

In cryptography, a cipher is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure....
s are generally divided into stream cipher
Stream cipher

In cryptography, a stream cipher is a symmetric key algorithm cipher where plaintext bits are combined with a pseudorandom cipher bit stream , typically by an exclusive-or operation....
s and block cipher
Block cipher

In cryptography, a block cipher is a symmetric key algorithm cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation....
s. Block cipher
Block cipher

In cryptography, a block cipher is a symmetric key algorithm cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation....
s operate on a fixed length string of bit
Bit

A bit is a binary numeral system numerical digit, taking a value of either 0 or 1. Binary digits are a basic unit of information Computer data storage and transmission in digital computing and digital information theory....
s. The length of this bit string is the block size. Both the input (plaintext
Plaintext

In cryptography, plaintext is the information which the sender wishes to transmit to the receiver. Before the computer era, plaintext simply meant text in the language of the communicating parties....
) and output (ciphertext) are the same length; the output cannot be shorter than the input — this is logically required by the Pigeonhole principle
Pigeonhole principle

In mathematics, the pigeonhole principle, also known as Dirichlet's box principle, is exemplified by such things as the fact that in a family of three children there must be at least two of the same gender....
 and the fact that the cipher must be invertible — and it is simply undesirable for the output to be longer than the input.

Until the announcement of NIST's AES contest
Advanced Encryption Standard process

The Advanced Encryption Standard , the block cipher ratified as a standard by National Institute of Standards and Technology of the United States , was chosen using a process markedly more open and transparent than its predecessor, the aging Data Encryption Standard ....
, the majority of block ciphers followed the example of the DES
Data Encryption Standard

The Data Encryption Standard is a block cipher that was selected by National Bureau of Standards as an official Federal Information Processing Standard for the United States in 1976 and which has subsequently enjoyed widespread use internationally....
 in using a block size of 64 bits (8 byte
Byte

A byte is a basic unit of measurement of Computer storage in computer science. In many computer architectures it is a Byte addressing memory address space....
s). However the Birthday paradox
Birthday paradox

In probability theory, the birthday problem, or birthday paradox pertains to the probability that in a set of randomly chosen people some pair of them will have the same birthday....
 tells us that after accumulating a number of blocks equal to the square root
Square root

In mathematics, a square root of a number x is a number r such that r2 = x, or, in other words, a number r whose square is x....
 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
Block cipher modes of operation

In cryptography, a block cipher operates on blocks of fixed length, often 64 or 128 bits. Because messages may be of any length, and because encrypting the same plaintext under the same key always produces the same output , several modes of operation have been invented which allow block ciphers to provide confidentiality for messages of arbit...
, only 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
Block cipher modes of operation

In cryptography, a block cipher operates on blocks of fixed length, often 64 or 128 bits. Because messages may be of any length, and because encrypting the same plaintext under the same key always produces the same output , several modes of operation have been invented which allow block ciphers to provide confidentiality for messages of arbit...
 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 Exabyte
Exabyte

An exabyte is a unit of information or computer storage equal to one quintillion bytes. It is commonly abbreviated EB. When used with byte multiples, the SI prefix may indicate a power of either 1000 or 1024, so the exact number may be either:...
s of data, and should suffice for quite a few years to come. The winner of the AES contest, Rijndael, supports block sizes of 128, 192, and 256 bits, although the extra block sizes were not adopted by the AES
Advanced Encryption Standard

In cryptography, the Advanced Encryption Standard is an encryption standard adopted by the Federal government of the United States. The standard comprises three block ciphers, AES-128, AES-192 and AES-256, adopted from a larger collection originally published as Rijndael. Each AES cipher has a 128 bit block size, with key sizes of 128...
 standard.

A few block ciphers, such as RC5
RC5

In cryptography, RC5 is a block cipher notable for its simplicity. Designed by Ron Rivest in 1994, RC stands for "Rivest Cipher", or alternatively, "Ron's Code" ....
, 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
Joan Daemen

Joan Daemen is a Belgian cryptographer and one of the designers of Rijndael, the Advanced Encryption Standard , together with Vincent Rijmen. He has also designed or co-designed the MMB, Square , SHARK , NOEKEON, 3-Way, and BaseKing block ciphers....
's 3-Way
3-Way

In cryptography, 3-Way is a block cipher designed in 1994 by Joan Daemen, who also designed Rijndael, the winner of NIST's Advanced Encryption Standard contest....
 and BaseKing
BaseKing

In cryptography, BaseKing is a block cipher designed in 1994 by Joan Daemen. It is very closely related to 3-Way; indeed, the two are variants of the same general cipher technique....
 have unusual block sizes of 96 and 192 bits, respectively.