Key points are not available for this paper at this time.
The National Bureau of Standards (NBS) has implemented a Data Encryption Standard (DES), describing an encryption procedure to be used by Federal agencies and others to protect data against unauthorized access. This procedure encrypts 64-bit data sequences using a 56-bit key. Diffie, Hellman and others have objected that a 56-bit key may be inadequate to resist a brute force attack using a special purpose computer costing about 20 million. Others have estimated ten times that much. Whichever figure is correct, there is little safety margin in a 56-bit key.
Morris et al. (Fri,) studied this question.