Cryptography Reference
In-Depth Information
chapter
3
KEY SYSTEMS
AND BLOCK AND
STREAM CIPHERS
Cryptographic systems are generically classified (1)
by the mathematical operations through which the
information (called the “plaintext”) is concealed using
the encryption key—namely, transposition, substitu-
tion, or product ciphers in which two such operations
are cascaded; (2) according to whether the transmitter
and receiver use the same key (symmetric [single-key]
cryptosystem) or different keys (asymmetric [two-key or
public-key] cryptosystem); and (3) by whether they pro-
duce block or stream ciphers. These three types of system
are described in turn here.
single-key crypTography
Single-key cryptography is limited in practice by what
is known as the key distribution problem. Since all par-
ticipants must possess the same secret key, if they are
physically separated—as is usually the case—there is
the problem of how they get the key in the first place.
Diplomatic and military organizations traditionally use
couriers to distribute keys for the highest-level commu-
nications systems, which are then used to superencrypt
and distribute keys for lower-level systems. This is imprac-
tical, though, for most business and private needs. In
addition, key holders are compelled to trust each other
