ISAAC is a pseudorandom number generator designed by Bob Jenkins (1996) to be cryptographically secure. The name is an acronym for Indirection, Shift, Accumulate, Add, and Count.
Operation
The ISAAC algorithm is inspired by RC4. It uses an array of 256 4-byte integers (called mm) as the internal state, writing the results to another 256-integer array, from which they are read one at a time until empty, at which point they are recomputed. The computation consists of altering mm[i] with mm[i^128], two elements of mm found by indirection, an accumulator, and a counter, for all values of i from 0 to 255. Since it only take about 19 32-bit operations for each 32-bit output word, it is extremely fast on 32-bit computers.
Cryptanalysis
Cryptanalysis has been undertaken by Marina Pudovkina (2001). Her attack can recover the initial state with a complexity that is approximated to be less than the time needed for searching through the square root of all possible initial states. In practice this means that the attack needs 
instead of 102466. This has yet no impact on the practical security of ISAAC.
References
- Robert J. Jenkins Jr., ISAAC. Fast Software Encryption 1996, pp41–49.
- Marina Pudovkina, A known plaintext attack on the ISAAC keystream generator, 2001, Cryptology ePrint Archive: Report 2001/049, [1].
External links