# data encryption standard algorithm (DESA)

The Data Encryption Standard Algorithm (DESA) is a product cipher that uses the substitutioncipher and the transposition cipher as elementary encryptions.

In the substitution cipher, characters or bit groups are replaced by other bit groups, and in the transposition cipher, the order of characters, bits, and bit groups is changed. The cryptoalgorithm is run in multiple rounds, with each run applying a different working key for control, selected from the actual key using a specific function. The trend here is to keep the key length and number of rounds variable. The increasing risk over time that the algorithm could be determined by increasing the computing capacity can be reduced again by increasing the key length and/or the number of rounds.

The DES algorithm is a block algorithm that converts 64-bit plaintext to 64-bit ciphertext and vice versa. The key length is 64 bits, but only 56 bits of these are significant, while the remaining 8 bits have the function of parity bits (7 bits each are assigned odd parity). The 64-bit keyword is used to convert plaintext blocks into 64-bit key blocks. To do this, the source text is first subjected to a series of permutations and substitutions. The result is then linked to the original plaintext by a logical X- OR. This encryption sequence is repeated 16 times, each time with a different arrangement of the key bits. A further increase of the already high security is possible by a triple encryption.

The fact that the encryption key can be exchanged at will results in `2^56` possibilities of encryption, which is about 18 trillion different combinations, probably a sufficient level of security.

DES chips are readily available from European and German companies, from the USA only with difficulty. Products containing the DES algorithm or any other algorithm for encryption generally require an export license. It does not matter whether the algorithm is implemented in hardware or software. According to the data sheet, the fastest DES chip currently commercially available achieves an encryption rate of 12 MB/s, i.e. almost 100 Mbit/s. The DES algorithm is included as standard in many smart cards.