Frequently Asked Questions For
Information Security Intelligence: Cryptographic Principles & Applications

Show that DES encryption and decryption are identical except for the order of the 48-bit keys.

DES uses the same algorithm for encryption and decryption except that the keys must be taken in reverse order to maintain the symmetry { k 16 , k 15 , k 14 ,... k 1 }. This is true because cycle j derives from j - 1 in the following manner.

L j = R j-1
R j = L j-1 ( R j-1 , k j )

These two equations show that the results of each cycle depends only on the previous cycle. By rewriting these equations in terms of R j-1 and L j we get:

R j-1 = L j and L j-1 = R j ( R j-1 , k j )

substituting gives

L j-1 = R j ( L j , k j )

These results show that later rounds (j) in process can be used to decrypt to earlier rounds (j-1) using only elements found in the j rounds. Thus this proves that the DES process is reversible (given that we present the keys in the correct oder each time as is appropriate to the giving process).


Many of the answers to FAQs are from lectures presented at JWU.