[ThinAir Home] [Table of Contents] [Generator Instantiations] [Bad Pseudo-Random Number Generators]

9 Special Generators

9.1 DwyerRandGen

 This class is derived from MultConModRandGen and based on an article by Jerry Dwyer in CUJ [2]. The algorithm used in this generator is supposedly immune to undefined effects of overflow in multiplications. Although, the MultConModRandGen class is also immune to these effects, its implementation is slower than Dwyer's. However, Dwyer's algorithm limits the size of the multipliers to the square root of the modulus. The constructor for this class has the following prototype:

DwyerRandGen( unsigned long mult, unsigned long mod, size_rand seed )

The value of a (supplied by mult), the modulus m (supplied by mod), and an initial value for X0 (supplied by seed). If no value is given for seed, a 1 is used.

In addition to the public interface, the DwyerRandGen class has a protected interface for use by its derived classes:

unsigned long      Quotient( void ) const;

Returns the quotient obtained by dividing the mod by the mult.

unsigned long      Remainder( void ) const;

Returns the remainder obtained by dividing the mod by the mult.

Dwyer's article listed four specific generators using his algorithm. These have been implemented as Dwyer1RandGen, Dwyer2RandGen, Dwyer3RandGen, and Dwyer4RandGen. All of these generators and their base class are implemented in the files rnddwyer.h and rnddwyer.cpp.

9.2 CompilerRandGen

All C/C++ compilers come with a PRNG implemented in their standard library. The CompilerRandGen class allows access to this generator through the interface specified by RandomGenerator. The CompilerRandGen class is derived from RandomGenerator and is implemented in the files randcomp.h and randcomp.cpp. The constructor for this class has the following prototype:

explicit CompilerRandGen( size_rand seed )

Requires a seed value. This seed defaults to 1 if none is supplied.

In addition to the public interface, the CompilerRandGen class has a protected interface for use by its derived classes.

size_rand      Seed( void ) const;

Returns the initial seed value used when the current generator was created.

void      SetSeed( size_rand seed );

Sets the initial seed value used when the current generator was created.

9.3 SHACtrRandGen

The SHACtrRandGen generator uses a counter to provide a variable source of numbers for use by the SHARandGen algorithm. The SHACtrRandGen class is derived from the SHARandGen class and is implemented in the files randsha.h and randsha.cpp. The constructors for this class have the following prototypes:

SHACtrRandGen( unsigned short size, const char *seedStr, size_rand seed )

Requires a buffer of data that the object uses as part of the message in the SHA algorithm. The length of the supplied buffer is size and seedStr points to the data. The constructor makes a copy of the data for the object's use. The initial value for the counter is supplied by seed. The default value for seed is 0.

explicit SHACtrRandGen( const char *seedStr, size_rand seed )

Simplifies the construction of a SHACtrRandGen object by allowing a null-terminated string as its argument. The argument seedStr has a default value of ``SHAPRNG''. The initial value for the counter is supplied by seed. The default value for seed is 0.

With many PRNGs, knowledge of the algorithm and a few consecutive numbers are enough to duplicate the rest of the pseudo-random number sequence. The output of the SHACtrRandGen generator should not be analyzable in this fashion.

9.4 SHARngRandGen

The SHARngRandGen generator uses a PRNG supplied at construction time to provide a variable source of numbers for use by the SHARandGen algorithm. The SHARngRandGen class is derived from the SHARandGen class and is implemented in the files randsha.h and randsha.cpp. The constructors for this class have the following prototypes:

SHARngRandGen( unsigned short size, const char *seedStr, RandomGenerator *pRNG )

Requires a buffer of data that the object uses as part of the message in the SHA algorithm. The length of the supplied buffer is size and seedStr points to the data. The constructor makes a copy of the data for the object's use. The constructor also requires a pointer to a heap-based PRNG object. The SHARngRandGen object owns this PRNG and deletes it when the object is destroyed.

SHARngRandGen( const char *seedStr, RandomGenerator *pRNG )

Simplifies the construction of a SHARngRandGen object by allowing a null-terminated string as one of its argument. The constructor also requires a pointer to a heap-based PRNG object. The SHARngRandGen object owns this PRNG and deletes it when the object is destroyed.

With many PRNGs, knowledge of the algorithm and a few consecutive numbers are enough to duplicate the rest of the pseudo-random number sequence. The output of the SHARngRandGen generator should not be analyzable in this fashion.

For further information, contact G. Wade Johnson (gwadej@anomaly.org).

© 1997 G. Wade Johnson. All rights reserved.
http://www.anomaly.org/ThinAir/specials.html