How does DES work?

Encryption of a block of the message takes place in 16 stages or rounds. From the input key, sixteen 48 bit keys are generated, one for each round. In each round, eight so-called S-boxes are used. These S-boxes are fixed in the specification of the standard. Using the S-boxes, groups of six bits are mapped to groups of four bits. The contents of these S-boxes has been determined by the U.S. National Security Agency (NSA). The S-boxes appear to be randomly filled, but this is not the case.Recently it has been discovered that these S-boxes, determined in the 1970s, are resistant against an attack called differential cryptanalysis which was first known in the 1990s. The block of the message is divided into two halves. The right half is expanded from 32 to 48 bits using another fixed table. The result is combined with the subkey for that round using the XOR operation. Using the S-boxes the 48 resulting bits are then transformed again to 32 bits, which are subsequently permutated again using yet another fixed table. This by now thoroughly shuffled right half is now combined with the left half using the XOR operation. In the next round, this combination is used as the new left half. The figure should hopefully make this process a bit more clear. In the figure, the left and right halves are denotes as L0 and R0, and in subsequent rounds as L1, R1, L2, R2 and so on. The function f is responsible for all the mappings described above.

The algorithm is repeated 16 times to produce the ciphertext. It has been found that the number of rounds is exponentially proportional to the amount of time required to find a key. So, as the number of rounds increases, the security of the algorithm increases exponentially. Encryption of a block of the message takes place in 16 stages or rounds. From the input key, sixteen 48 bit keys are generated, one for each round. In each round, eight so-called S-boxes are used. These S-boxes are fixed in the specification of the standard. Using the S-boxes, groups of six bits are mapped to groups of four bits. The contents of these S-boxes has been determined by the U.S. National Security Agency (NSA). The S-boxes appear to be randomly filled, but this is not the case. Recently it has been discovered that these S-boxes, determined in the 1970s, are resistant against an attack called differential cryptanalysis which was first known in the 1990s. The block of the message is divided into two halves. The right half is expanded from 32 to 48 bits using another fixed table. The result is combined with the subkey for that round using the XOR operation. Using the S-boxes the 48 resulting bits are then transformed again to 32 bits, which are subsequently permutated again using yet another fixed table. This by now thoroughly shuffled right half is now combined with the left half using the XOR operation. In the next round, this combination is used as the new left half. The figure should hopefully make this process a bit more clear. In the figure, the left and right halves are denotes as L0 and R0, and in subsequent rounds as L1, R1, L2, R2 and so on. The function f is responsible for all the mappings described above.

The algorithm is repeated 16 times to produce the ciphertext. It has been found that the number of rounds is exponentially proportional to the amount of time required to find a key. So, as the number of rounds increases, the security of the algorithm increases exponentially.