ISAAC (cipher)

Encyclopedia : I : IS : ISA : ISAAC (cipher)

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.

Contents

1 Operation
2 Cryptanalysis
3 References
4 External links

Operation

The ISAAC algorithm has similarities to 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 takes 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 [4.67 \times 10^] instead of [10^]. This result has had no practical impact on the security of ISAAC. As always with cryptographic primitives, future improvements, or another attack, might.

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, [link].

External links

[Official ISAAC website]

Stream ciphers [edit]

Theory: Shift register | LFSR | NLFSR | Shrinking generator

Misc: eSTREAM

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.