Page 469 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

are the most common in the English language, you can then test

several hypotheses:
If these letters are also the most common in the ciphertext, the

cipher was likely a transposition cipher, which rearranged the
characters of the plain text without altering them.

If other letters are the most common in the ciphertext, the cipher is
probably some form of substitution cipher that replaced the
plaintext characters.

This is a simple overview of frequency analysis, and many

sophisticated variations on this technique can be used against
polyalphabetic ciphers and other sophisticated cryptosystems.

Known Plaintext In the known plaintext attack, the attacker has a
copy of the encrypted message along with the plaintext message used
to generate the ciphertext (the copy). This knowledge greatly assists
the attacker in breaking weaker codes. For example, imagine the ease
with which you could break the Caesar cipher described in Chapter 6 if

you had both a plaintext copy and a ciphertext copy of the same
message.

Chosen Ciphertext In a chosen ciphertext attack, the attacker has
the ability to decrypt chosen portions of the ciphertext message and
use the decrypted portion of the message to discover the key.

Chosen Plaintext In a chosen plaintext attack, the attacker has the
ability to encrypt plaintext messages of their choosing and can then
analyze the ciphertext output of the encryption algorithm.

Meet in the Middle Attackers might use a meet-in-the-middle
attack to defeat encryption algorithms that use two rounds of
encryption. This attack is the reason that Double DES (2DES) was
quickly discarded as a viable enhancement to the DES encryption (it
was replaced by Triple DES, or 3DES).

In the meet-in-the-middle attack, the attacker uses a known plaintext

message. The plain text is then encrypted using every possible key
(k1), and the equivalent ciphertext is decrypted using all possible keys
(k2). When a match is found, the corresponding pair (k1, k2)
represents both portions of the double encryption. This type of attack