DES is a secret-key, symmetric cryptosystem: when used for communication,
both sender and receiver must know the same secret key, which is used both
to encrypt and decrypt the message. DES can also be used for single-user
encryption, such as to store files on a hard disk in encrypted form. In a
multi-user environment, secure key distribution may be difficult;
public-key cryptography was invented to solve this problem (see Question
3.1.3). DES operates on 64-bit blocks with a 56-bit key. It was
designed to be implemented in hardware, and its operation is relatively
fast. It works well for bulk encryption, that is, for encrypting a large
set of data.
NIST (see Question 3.7.1) has recertified DES as an official U.S. government encryption standard every five years; DES was last recertified in 1993, by default. NIST has indicated, however, that it may not recertify DES again.