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Data Encryption Standard (DES) and Advanced Encryption Standard (AES)

key bit bits permutation

Definition: Data Encryption Standard (DES) and Advanced Encryption Standard (AES) are designed to encrypt a sequence of data elements.

The DES algorithm is designed to encipher and decipher 64-bit blocks of data under control of a 64-bit key, of which 56 bits are randomly generated and used directly by the algorithm. Deciphering must be accomplished by using the same key as for enciphering. The deciphering process is the reverse of the enciphering process. A 64-bit block to be enciphered is subject to an initial permutation to form L 0 and R 0 (32 bits each, respectively the left and right half of the 64-bit block generated by the initial permutation), then to 16-iteration key-dependent computation, and the final result of the computation ( L 16 and R 16 ) is subject to a permutation that is the inverse of the initial permutation. The 16 key-dependent computations can be simply defined as:

L n = R n -1

R n = L n -1 ? f ( R n -1 , K n )

where n is in the range of 1 to 16; f is the cipher function; L n and R n are 32-bits each and respectively the left and right half of the 64-bit iteration result; and K n is the 48-bit sub key generated by the following key schedule function KS :

K n = KS ( n , KEY )

where KEY is the 56-bit main key.

With the advance of computation power, a 56-bit key is no longer considered as secure. Triple DES (3DES) is a straightforward way of enhancing the security of DES. 3DES involves repeating the basic DES algorithm three times, using either two or thee unique keys, for a key size of 112 or 168 bits.

Both DES and 3DES are not efficient because they operate on a 64-bit block. As a replacement, the Advanced Encryption Standard (AES) specifies the Rijndael algorithm, a symmetric block cipher that can process data blocks of 128 bits, using cipher keys with lengths of 128, 192, and 256 bits.

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