TCP/IP Tutorial and Technical Overview - IBM Redbooks

Encryption and decryption: Cryptographic algorithms
Encryption is the transformation of a cleartext message into an unreadable form
in order to hide its meaning. The opposite transformation, which retrieves the
original cleartext, is the decryption. The mathematical function used for
encryption and decryption is the cryptographic algorithm or cipher.
The security of a cipher might be based entirely on keeping its functionality a
secret, in which case it is a restricted cipher. There are many drawbacks to
restricted ciphers. It is very difficult to keep an algorithm a secret when it is used
by many people. If it is incorporated in a commercial product, it is only a matter of
time and money before it is reverse engineered. For these reasons, the currently
used algorithms are keyed, that is, the encryption and decryption makes use of a
parameter, known as the key. The key can be chosen from a set of possible
values, called the keyspace. The keyspace usually is huge, the bigger the better.
The security of these algorithms rely entirely on the key, not on their internal
secrets. In fact, the algorithms themselves are usually public and are extensively
analyzed for possible weaknesses. The principle of keyed ciphers is shown in
Figure 22-2.
Note: Do not trust new, unknown, or unpublished algorithms.
cleartext
clea
cle
Cleartext
Secret
Key
Figure 22-2 Keyed encryption and decryption
Note: It is common in cryptographic literature to denote the first participant in
a protocol as Alice and the second one as Bob. They are the “crypto couple.”
778 TCP/IP Tutorial and Technical Overview
cleartext
clea
cle
Encryption Decryption
Ciphertext
Secret
Key
cleartext
clea
cle
Original
cleartext
Alice Bob