Issue No. 02 - February (1987 vol. 13)
J.H. Moore , Department of Applied Mathematics, Sandia National Laboratories
Certain DES keys have been called weak or semiweak based upon the number of distinct round keys which they produce. For the weak keys, all 16 round keys are identical and encryption is the same as decryption. For the semiweak keys, there are only two distinct round keys but no specific weakness of the DES with these keys has been demonstrated.
weak and semiweak keys, Cryptography, cycle testing, data encryption standard (DES), fixed points and antifixed points, palindromic and antipalindromic sequences of round keys
J.H. Moore, G.J. Simmons, "Cycle Structure of the DES for Keys Having Palindromic (or Antipalindromic) Sequences of Round Keys", IEEE Transactions on Software Engineering, vol. 13, no. , pp. 262-273, February 1987, doi:10.1109/TSE.1987.233150