## RABIN signature scheme

is a variant of RSA signature scheme

## Random Number Generator

is a piece of software or hardware that generates random numbers. Random numbers are the keystones of cryptography. All keys are based on randon numbers. Many protocols use nonces.

There are two types of random number generators:

- True Random Number Generators (TRNG) are generating truly random numbers. They use physical phenomenons such as thermal noise, or zener diode noise. Secure hardware have often such components. Using dices is a physical random generator. It was used in second world war for generating the secret keys.
- Pseudo Random Number Generators (PRNG) are generating random noise by calculations. They often use a seed. Using the same seed generates the same sequence of nonces. Yarrow is an example of such algorithm.

## RC2

is a variable key size cipher block designed by RIVEST. The size of the block is 64 bits. The algorithm is confidential and proprietary to RSA Data Security. They promote it as replacement of DES. One of its advantage is that is easy to get the US export approval.

## RC4

is a steam cipher designed by RIVEST. The algorithm is confidential and proprietary to RSA Data Security. The 40-bit version as been broken by brute force.

## RC5

is a block cipher designed by RIVEST. Its block size is 32, 64, or 128 bit long. The size of the key can be from 0 to 128 bits. The number of rounds is in the range 1 to 255. The encryption routine uses the basic operations: addition, bit XOR, and rotation. RC5 is patented by RSA Data Security ltd.

Papers (EUROCRYPT 98) seem to prove that RC5 may not be very secure.

## Repudiation

is to falsely deny having sent a given message. It is a drawback usually associated with secret key systems and also a primary drawback of Kerberos since it maintains all secret keys in a central database.

## Rijndael

is the block cipher chosen by NIST, at the end of the AES challenge, to replace old DES. The cipher has a variable block length and key length. The Belgian designers specified how to use keys with a length of 128, 192, or 256 bits to encrypt blocks with al length of 128, 192 or 256 bits. Both block length and key length can be extended very easily to multiples of 32 bits. Rijndael can be implemented very efficiently on a wide range of processors and in hardware.

## Rootkit

is a type of software who hides itself from the user. By interception of messages between the application and the kernel, it hides its presence. Rootkit is at the base of many malware such as Trojan. For more information, see http://rootkit.com or Security Newsletter #1.

## RSA

(RIVEST, SHAMIR, ADLEMAN) is a public key cryptosystem both for encryption, and authentication. It was invented in 1977 and is still considered as secure. RSA ltd patented this algorithm. . Since 2000, it is in the public domain.