[boost] [uuid] basic_random_generator doesn't produce a fully randomized uuid when sizeof(unsigned long) == 4

Subject: [boost] [uuid] basic_random_generator doesn't produce a fully randomized uuid when sizeof(unsigned long) == 4
From: Sylvester-Bradley, Gareth (Gareth.Sylvester-Bradley_at_[hidden])
Date: 2010-03-02 06:21:31

Hi Andy,

I wonder if the following is a flaw in basic_random_generator<>
(boost/uuid/uuid_generators.hpp, line 366-374)?

On many platforms sizeof(unsigned long) == 4. This means that the first
8 bytes of the uuid are populated with random data, but in my tests the
remaining 8 bytes are populated with repeated data, and then only by
relying on the undefined behaviour of >> when its "right operand is
greater than or equal to the length in bits of the promoted left
operand."

For example, try the following test program with a Win32 build by Visual
C++ 9.0 -

#include <iostreams>
#include <boost/uuid/uuid_generators.hpp>
#include <boost/uuid/uuid_io.hpp>

int main( int, char*[] )
{
  std::cout << boost::uuids::random_generator()() << std::endl;
  return 0;
}

This produced for me:

f1633d75-ee94-47b9-ae94-37b9ee9437b9

Or wrapped to show the problem:

f1633d75-
ee94-47b9-
ae94-37b9
ee94 37b9

Is 8 bytes of randomness enough, or is this a mistake?

Best regards,
Gareth

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