Glossary - Cryptology Unlocked - page 500

Twinkle A yet hypothetical device based on an idea by Shamir, which could be used
to factor large numbers faster than currently possible. This is important for the
security of the RSA method (see Section 4.5.3).
Twofish A further developed variant of Blowfish, one of the five final AES candidates.
It is a very secure, flexible, and fast algorithm.
Unicity distance A concept that measures the amount of ciphertext required such
that there is only one reasonable plaintext. This number depends both on the
characteristics of the plaintext and the key length of the encryption algorithm. The
unicity distance for standard English ASCII text is 5.9 characters (approximately
47 bits) when using 40-bit keys. However, practical cryptanalysis requires much
more ciphertext. The unicity distance has information-theoretical significance.
Vernam cipher A bitwise Vigenere cipher, i.e., a bitwise stream cipher with one
periodic key stream. Vernam cipher is sometimes used synonymously for one-time
pads (see Section 2.4.2).
Vigenere cipher The simplest polyalphabetic substitution, where a keyword is repeat-
edly written over a plaintext. The ciphertext is produced by adding superimposed
characters. This topic mainly discussed XORing rather than addition (which is a
Vernam cipher in the closer sense). However, both methods can be treated equally
(see Section 2.4.1).
Weak keys Special keys which, when used, reduce the security of the encryption
method. An unusual example: every sequence of zero bytes is a (very) weak
key in the Vigenere cipher since it leaves the plaintext unchanged during the
encryption. If there are many weak keys in an encryption method, then it is
worthwhile for an attacker to assume that a weak key was used to achieve his
goal faster. If the attempt fails, the attacker might additionally use other methods
(see Section 4.4.3).
XOR A basic computer operation between two binary numbers: bits in the same
positions are XORed (exclusive XOR):
0XOR0=0,1XOR1=0,0XOR1=1,1XOR0=1.
This is usually denoted as ⊕ :a ⊕ b.