Block size (cryptography)
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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.