Block size (cryptography) facts for kids

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 undesirable for the output to be longer than the input.

The DES is using a block size of 64 bits (8 bytes) while the AES is using a block size of 128 bits (16 bytes). Rijndael, supports block sizes of 128, 192, and 256 bits, although the extra block sizes were not adopted by the AES standard.

A few 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.

Few ciphers have unusual block sizes, for example the 3-Way block cipher have an unusual block size of 96 bits (12 bytes).