2 cissp ® Official Study Guide Eighth Edition

Data Encryption Standard 
The U.S. government published the Data Encryption Standard in 1977 as a proposed stan-
dard cryptosystem for all government communications. Because of fl aws in the algorithm, 
cryptographers and the federal government no longer consider DES secure. It is widely 
believed that intelligence agencies routinely decrypt DES-encrypted information. DES was 
superseded by the Advanced Encryption Standard in December 2001. It is still important to 
understand DES because it is the building block of Triple DES (3DES), a strong encryption 
algorithm discussed in the next section. 
DES is a 64-bit block cipher that has fi ve modes of operation: Electronic Code Book 
(ECB) mode, Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, output 
feedback (OFB) mode, and Counter (CTR) mode. These modes are explained in the fol-
lowing sections. All of the DES modes operate on 64 bits of plaintext at a time to generate 
64-bit blocks of ciphertext. The key used by DES is 56 bits long. 
DES uses a long series of exclusive OR (XOR) operations to generate the ciphertext. 
This process is repeated 16 times for each encryption/decryption operation. Each repetition 
is commonly referred to as a
round
of encryption, explaining the statement that DES per-
forms 16 rounds of encryption. 
As mentioned, DES uses a 56-bit key to drive the encryption and decryp-
tion process. However, you may read in some literature that DES uses a 
64-bit key. This is not an inconsistency—there’s a perfectly logical expla-
nation. The DES specification calls for a 64-bit key. However, of those 64 
bits, only 56 actually contain keying information. The remaining 8 bits are 
supposed to contain parity information to ensure that the other 56 bits are 
accurate. In practice, however, those parity bits are rarely used. You should 
commit the 56-bit figure to memory.