Glossary - Cryptology Unlocked - page 497

RC5 A very simple and fast block algorithm that uses variable parameters (block
length, key length, number of rounds); see Section 5.4 as well as Section 5.4.3
for a discussion of RC5a.
RC6 The successor of RC5 (see Section 5.4.4), and one of the five final AES candi-
dates.
Reduced key space We speak of a reduced key space when a potentially good algo-
rithm uses only relatively few keys out of the theoretically possible number of
keys due to poor implementation. It represents a vulnerability to dictionary attacks.
Good examples are older versions of Netscape Navigator (Section 5.1.4); see also
Section 3.3.
Replay attack A special cryptanalytic attack. Though the attacker may not be able to
decrypt an intercepted message, he can copy it and replay it later, perhaps authen-
ticating himself by mimicking somebody else. The idea is to intentionally disturb
or forge data traffic, or to break into a third-party system. This was exploited in
Novell Netware (encrypted passwords had no sequential number or timestamp).
Residual class Any set of all integers that leave the same remainder with regard to a
given module (see also 'congruence').
Reversing drum A stationary rotor used in the German ciphering machine, the
Enigma, which permutes the output and returns it backwards across the rotors
(see Section 2.5.1).
Right rotation See 'rotation'.
Rijndael The Belgian algorithm that won the AES challenge and the accepted DES
successor. It is very fast, very small, and very simple. No vulnerability has become
known to date (see Section 5.5).
RIPE (RACE Integrity Primitives Evaluation) A collection of European security
standards proposed within the RACE (Research and Development in Advanced
Communication Technologies) Initiative.
RIPE-MD, RIPE-MD160 One-way hash functions used in RIPE. In addition to SHA
(see 'DSA'), RIPE-MD160 is considered to be very secure and preferred over
RIPE-MD.
ROT13 A Caesar cipher (used in news readers) where each letter is substituted by its
13th successor. Applying the method twice reproduces the original text. ROT13
does not allegedly offer cryptological security, but makes the undesirable reading
out of character strings from program texts harder.