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better wikilink, I think; "logic" here has nothing to do with the notion of "mathematical logic"
 
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{{more footnotes|date=September 2013}}
{{primary sources|date=September 2013}}
'''Blum Blum Shub''' ('''B.B.S.''') is a [[pseudorandom number generator]] proposed in 1986 by [[Lenore Blum]], [[Manuel Blum]] and [[Michael Shub]].<ref name=blum1986>
{{cite journal
|last=Blum
|first=Lenore
|coauthors=Blum, Manuel; Shub, Mike
|title=A Simple Unpredictable Pseudo-Random Number Generator
|journal=SIAM Journal on Computing
|date=1 May 1986
|volume=15
|issue=2
|pages=364–383
|doi=10.1137/0215025
|url=http://epubs.siam.org/doi/abs/10.1137/0215025}}
</ref>


Blum Blum Shub takes the form:


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:<math>x_{n+1} = x_n^2 \bmod M</math>
 
where ''M''=''pq'' is the product of two large [[prime number|primes]] ''p'' and ''q''. At each step of the algorithm, some output is derived from ''x''<sub>''n''+1</sub>; the output is commonly either the [[parity bit|bit parity]] of ''x''<sub>''n''+1</sub> or one or more of the least significant bits of ''x''<sub>''n''+1''</sub>.
 
The [[random seed|seed]] ''x''<sub>0</sub> should be an integer that is co-prime to ''M'' (i.e. ''p'' and ''q'' are not factors of ''x''<sub>0</sub>) and not 1 or 0.
 
The two primes, ''p'' and ''q'', should both be [[Congruence relation|congruent]] to 3 (mod 4) (this guarantees that each [[quadratic residue]] has one [[square root]] which is also a quadratic residue) and [[greatest common divisor|gcd]]([[Euler's totient function|''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''<sub>''i''</sub> value directly (via [[Euler's Theorem]]):
 
:<math>x_i = \left( x_0^{2^i \bmod \lambda(M)} \right) \bmod M</math>
 
where <math>\lambda</math> is the [[Carmichael function]]. (Here we have <math>\lambda(M) = \lambda(p\cdot q) = \operatorname{lcm}(p-1, q-1)</math>).
 
==Security==
The generator is very slow. However, there is a proof reducing its security to the [[Computational complexity theory|computational difficulty]] of the computing modular square roots, a problem whose difficulty is equivalent to factoring. When the primes are chosen appropriately, and [[big O notation|O]]([[logarithm|log]] log ''M'') lower-order bits of each ''x<sub>n</sub>'' are output, then in the limit as ''M'' grows large, distinguishing the output bits from random should be at least as difficult as factoring ''M''.
 
==Example==
Let <math>p=11</math>, <math>q=19</math> and <math>s=3</math> (where <math>s</math> is the seed.) We can expect to get a large cycle length for those small numbers, because <math>{\rm gcd}(\varphi(p-1), \varphi(q-1))=2</math>.
The generator starts to evaluate <math>x_0</math> by using <math>x_{-1}=s</math> and creates the sequence <math>x_0</math>, <math>x_1</math>, <math>x_2</math>, <math>\ldots</math> <math>x_5</math> = 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.
 
{| class="wikitable"
|-
! 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==
{{Reflist}}
 
;General
* {{cite journal|last=Blum|first=Lenore|coauthors=Blum, Manuel; Shub, Mike|title=Comparison of Two Pseudo-Random Number Generators|series=Advances in Cryptology: Proceedings of CRYPTO '82|pages=61--78|publisher=Plenum|year=1982|url=http://www.iacr.org/cryptodb/data/paper.php?pubkey=1751}}
* {{cite journal|last=Geisler|first=Martin|coauthors=Krøigård, Mikkel; Danielsen, Andreas|title=About Random Bits|date=December 2004|url=http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.90.3779}} available as [http://www.daimi.au.dk/~mg/mamian/random-bits.pdf PDF] and [http://daimi.au.dk/~mg/mamian/random-bits.ps.gz Gzipped Postscript]
*
 
==External links==
* [http://web.archive.org/web/20090524070627/http://firefly.is-a-geek.org/gmpbbs/ GMPBBS] (archived 2009-05-24 at the [[Wayback Machine]]), a [[GNU General Public License|GPL]]'ed [[GNU Multi-Precision Library|GMP]]-based implementation of Blum Blum Shub by Mark Rossmiller. Retrieved 2011-09-05.
* [http://code.google.com/p/javarng/ An implementation in Java]
* [http://www.ciphersbyritter.com/NEWS2/TESTSBBS.HTM Randomness tests]
 
[[Category:Pseudorandom number generators]]
[[Category:Cryptographically secure pseudorandom number generators]]

Revision as of 05:53, 3 February 2014

Template:More footnotes Template:Primary sources Blum Blum Shub (B.B.S.) is a pseudorandom number generator proposed in 1986 by Lenore Blum, Manuel Blum and Michael Shub.[1]

Blum Blum Shub takes the form:

where M=pq is the product of two large primes p and q. At each step of the algorithm, some output is derived from xn+1; the output is commonly either the bit parity of xn+1 or one or more of the least significant bits of xn+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 xi value directly (via Euler's Theorem):

where is the Carmichael function. (Here we have ).

Security

The generator is very slow. However, there is a proof reducing its security to the computational difficulty of the computing modular square roots, a problem whose difficulty is equivalent to factoring. When the primes are chosen appropriately, and O(log log M) lower-order bits of each xn are output, then in the limit as M grows large, distinguishing the output bits from random should be at least as difficult as factoring M.

Example

Let , and (where is the seed.) We can expect to get a large cycle length for those small numbers, because . The generator starts to evaluate by using and creates the sequence , , , = 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

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External links

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