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SYNOPSIS
# Replace rand().
use Math::Random::Secure qw(rand);
# Get a random number between 0 and 1
my $float = rand();
# Get a random integer (faster than int(rand))
use Math::Random::Secure qw(irand);
my $int = irand();
# Random integer between 0 and 9 inclusive.
$int = irand(10);
# Random floating-point number greater than or equal to 0.0 and
# less than 10.0.
$float = rand(10);
DESCRIPTION
This module is intended to provide a cryptographically-secure
replacement for Perl's built-in rand function. "Crytographically
secure", in this case, means:
* No matter how many numbers you see generated by the random number
generator, you cannot guess the future numbers, and you cannot guess
the seed.
* There are so many possible seeds that it would take decades,
centuries, or millenia for an attacker to try them all.
* The seed comes from a source that generates relatively strong
random data on your platform, so the seed itself will be as random as
possible.
See "IMPLEMENTATION DETAILS" for more information about the
underlying systems used to implement all of these guarantees, and
some important caveats if you're going to use this module for some
very-high-security purpose.
METHODS
rand
Should work exactly like Perl's built-in rand. Will automatically call
srand if srand has never been called in this process or thread.
There is one limitation--Math::Random::Secure is backed by a 32-bit
random number generator. So if you are on a 64-bit platform and you
specify a limit that is greater than 2^32, you are likely to get
less-random data.
srand
Note: Under normal circumstances, you should not call this function, as
rand and irand will automatically call it for you the first time they
are used in a thread or process.
Seeds the random number generator, much like Perl's built-in srand,
except that it uses a much larger and more secure seed. The seed should
be passed as a string of bytes, at least 8 bytes in length, and more
ideally between 32 and 64 bytes. (See "seed" in
Math::Random::Secure::RNG for more info.)
If you do not pass a seed, a seed will be generated automatically using
a secure mechanism. See "IMPLEMENTATION DETAILS" for more information.
This function returns the seed that generated (or the seed that was
passed in, if you passed one in).
irand
Works somewhat like "rand", except that it returns a 32-bit integer
between 0 and 2^32. Should be faster than doing int(rand).
Note that because it returns 32-bit integers, specifying a limit
greater than 2^32 will have no effect.
IMPLEMENTATION DETAILS
Currently, Math::Random::Secure is backed by Math::Random::ISAAC, a
cryptographically-strong random number generator with no known serious
weaknesses. If there are significant weaknesses found in ISAAC, we will
change our backend to a more-secure random number generator. The goal
is for Math::Random::Secure to be cryptographically strong, not to
represent some specific random number generator.
Math::Random::Secure seeds itself using Crypt::Random::Source. The
underlying implementation uses /dev/urandom on Unix-like platforms, and
the RtlGenRandom or CryptGenRandom functions on Windows 2000 and above.
(There is no support for versions of Windows before Windows 2000.) If
any of these seeding sources are not available and you have other
Crypt::Random::Source modules installed, Math::Random::Secure will use
those other sources to seed itself.
Making Math::Random::Secure Even More Secure
We use /dev/urandom on Unix-like systems, because one of the
requirements of duplicating rand is that we never block waiting for
seed data, and /dev/random could do that. However, it's possible that
/dev/urandom could run out of "truly random" data and start to use its
built-in pseudo-random number generator to generate data. On most
systems, this should still provide a very good seed for nearly all
uses, but it may not be suitable for very high-security cryptographic
circumstances.
For Windows, there are known issues with CryptGenRandom on Windows 2000
and versions of Windows XP before Service Pack 3. However, there is no
other built-in method of getting secure random data on Windows, and I
suspect that these issues will not be significant for most applications
of Math::Random::Secure.
If either of these situations are a problem for your use, you can
create your own Math::Random::Secure::RNG object with a different
"seeder" argument, and set $Math::Random::Secure::RNG to your own
instance of Math::Random::Secure::RNG. The "seeder" is an instance of
Crypt::Random::Source::Base, which should allow you to use most
random-data sources in existence for your seeder, should you wish.
Seed Exhaustion
Perl's built-in srand reads 32 bits from /dev/urandom. By default, we
read 512 bits. This means that we are more likely to exhaust available
truly-random data than the built-in srand is, and cause /dev/urandom to
fall back on its psuedo-random number generator. Normally this is not a
problem, since "srand" is only called once per Perl process or thread,
but it is something that you should be aware of if you are going to be
in a situation where you have many new Perl processes or threads and
you have very high security requirements (on the order of generating
private SSH or GPG keypairs, SSL private keys, etc.).
SEE ALSO
http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator
Describes the requirements and nature of a cryptographically-secure
random number generator.
http://en.wikipedia.org/wiki/CryptGenRandom,
More information about the Windows functions we use to seed
ourselves. The article also has some information about the weaknesses
in Windows 2000's CryptGenRandom implementation.
http://www.computerworld.com/s/article/9048438/Microsoft_confirms_that_XP_contains_random_number_generator_bug
A news article about the Windows 2000/XP CryptGenRandom weakness,
fixed in Vista and XP Service Pack 3.
http://en.wikipedia.org/wiki/Random_number_generator_attack
A description of ways to attack a random number generator, which can
help in understanding why such a generator needs to be secure.
Math::Random::Secure::RNG
The underlying random-number generator and seeding code for
Math::Random::Secure.
Crypt::Source::Random
Crypt::Random
Math::TrulyRandom
All of these modules contain generators for "truly random" data, but
they don't contain a simple rand replacement and they can be very
slow.
SUPPORT
Right now, the best way to get support for Math::Random::Secure is to
email the author using the email address in the "AUTHORS" section
below.
BUGS
Math::Random::Secure is relatively new, as of December 2010, but the
modules that underlie it are very well-tested and have a long history.
However, the author still welcomes all feedback and bug reports,
particularly those having to do with the security assurances provided
by this module.
You can report a bug by emailing bug-Math-Random-Secure@rt.cpan.org or
by using the RT web interface at
https://rt.cpan.org/Ticket/Display.html?Queue=Math-Random-Secure. If
your bug report is security-sensitive, you may also email it directly
to the author using the email address in the "AUTHORS" section below.
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