Blum Blum Shub

Blum Blum Shub (B.B.S.) is a pseudorandom number generator proposed in 1986 by Lenore Blum, Manuel Blum and Michael Shub^[1] that is derived from Michael O. Rabin's oblivious transfer mapping.

Blum Blum Shub takes the form

x_{{n+1}}=x_{n}^{2}{\bmod M}

where M = pq is the product of two large primes p and q. At each step of the algorithm, some output is derived from x_n+1; the output is commonly either the bit parity of x_n+1 or one or more of the least significant bits of x_n+1.

The seed x₀ should be an integer that is co-prime to M (i.e. p and q are not factors of x₀) and not 1 or 0.

The two primes, p and q, should both be congruent to 3 (mod 4) (this guarantees that each quadratic residue has one square root which is also a quadratic residue) and gcd(φ(p − 1), φ(q − 1)) should be small (this makes the cycle length large).

An interesting characteristic of the Blum Blum Shub generator is the possibility to calculate any x_i value directly (via Euler's theorem):

x_{i}=\left(x_{0}^{{2^{i}{\bmod \lambda }(M)}}\right){\bmod M}

where $\lambda$ is the Carmichael function. (Here we have $\lambda(M) = \lambda(p\cdot q) = \operatorname{lcm}(p-1, q-1)$ ).

Security

There is a proof reducing its security to the computational difficulty of solving the quadratic residuosity problem.^[1] When the primes are chosen appropriately, and O(log log M) lower-order bits of each x_n are output, then in the limit as M grows large, distinguishing the output bits from random should be at least as difficult as solving the Quadratic residuosity problem modulo M.

Example

Let $p=11$ , $q=19$ and $s=3$ (where $s$ is the seed). We can expect to get a large cycle length for those small numbers, because ${\rm gcd}(\varphi(p-1), \varphi(q-1))=2$ . The generator starts to evaluate $x_{0}$ by using $x_{-1}=s$ and creates the sequence $x_{0}$ , $x_{1}$ , $x_{2}$ , $\ldots$ $x_5$ = 9, 81, 82, 36, 42, 92. The following table shows the output (in bits) for the different bit selection methods used to determine the output.

Even parity bit	Odd parity bit	Least significant bit
0 1 1 0 1 0	1 0 0 1 0 1	1 1 0 0 0 0

References

1 2 Blum, Lenore; Blum, Manuel; Shub, Mike (1 May 1986). "A Simple Unpredictable Pseudo-Random Number Generator". SIAM Journal on Computing. 15 (2): 364–383. doi:10.1137/0215025.

General

Blum, Lenore; Blum, Manuel; Shub, Mike (1982). "Comparison of Two Pseudo-Random Number Generators". Advances in Cryptology: Proceedings of CRYPTO '82. Plenum: 61–78.
Geisler, Martin; Krøigård, Mikkel; Danielsen, Andreas (December 2004). "About Random Bits". available as PDF and gzipped Postscript

External links

GMPBBS, a GPL'ed GMP-based implementation of Blum Blum Shub in C by Graffiti Plum
An implementation in Java
Randomness tests

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