SOURCE TABLE

  +------------------+-------------+------------+--------------------+
  |      SOURCE      |  STRENGTH   |  BLOCKING  |       NOTE         |
  |------------------+-------------+------------+--------------------|
  | RtlGenRandom     |   Strong(1) |     No     | Default WinXP+     |
  |------------------+-------------+------------+--------------------|
  | CryptGenRandom   |   Strong(1) |     No     | Default Win2000    |
  |------------------+-------------+------------+--------------------|
  | EGD              |   Strong    |    Yes(2)  | also PRNGD, etc.   |
  |------------------+-------------+------------+--------------------|
  | /dev/random      |   Strong    |    Yes     | Typical UNIX       |
  |------------------+-------------+------------+--------------------|
  | /dev/urandom     |    Weak     |     No     | Typical UNIX NB    |
  |------------------+-------------+------------+--------------------|
  | TESHA2-strong    |   Strong    |     No     |                    |
  |------------------+-------------+------------+--------------------|
  | TESHA2-weak      |    Weak     |     No     |                    |
  +------------------+-------------+------------+--------------------+

The alias 'Win32' can be used in whitelist and blacklist and will match both the Win32 sources "RtlGenRandom" and "CryptGenRandom". The alias 'TESHA2' may be similarly used and matches both the weak and strong sources.

  1) Both CryptGenRandom and RtlGenRandom are considered strong by this
     package, even though both are seeded CSPRNGs so should be the equal of
     /dev/urandom in this respect.  The CryptGenRandom function used in
     Windows 2000 has some known issues so should be considered weaker.

  2) EGD is blocking, PRNGD is not.  We cannot tell the two apart.  There are
     other software products that use the same protocol, and each will act
     differently.  E.g. EGD mixes in system entropy on every request, while
     PRNGD mixes on a time schedule.

STRENGTH

Creating a satisfactory strength measurement is problematic. The Win32 Crypto API is considered "strong" by most customers and every other Perl module, however it is a well seeded CSPRNG according to the MSDN docs, so is not a strong source based on the definition in the previous paragraph. Similarly, almost all sources consider /dev/urandom to be weak, as once it runs out of entropy it returns a deterministic function based on its state (albeit one that cannot be run either direction from a returned result if the internal state is not known).

Because of this confusion, I have removed the "Weak" configuration option that was present in version 0.01. It will now be ignored. You should be able to use a combination of whitelist, blacklist, and the source's "is_strong" return value to decide if this meets your needs. On Win32, you really only have a choice of Win32 and TESHA2. The former is going to be what most people want, and can be chosen even with non-blocking set. On most UNIX systems, "/dev/random" will be chosen for blocking and "/dev/urandom" for non-blocking, which is what should be done in most cases.

BLOCKING

Win32, PRNGD, and /dev/urandom are fast nonblocking sources. When they run out of entropy, they use a CSPRNG to keep supplying data at high speed. However this means that there is no additional entropy being supplied.

TESHA2 is nonblocking, but can be very slow. /dev/random can be faster if run on a machine with lots of activity. On an isolated server, TESHA2 may be much faster. Also note that the blocking sources such as EGD and /dev/random both try to maintain reasonably large entropy pools, so small requests can be supplied without blocking.

IN PRACTICE

  my $source = Crypt::Random::Seed->new();

To get a good non-blocking source (Win32 or /dev/urandom):

  my $source = Crypt::Random::Seed->new(NonBlocking => 1);

new

Optional parameters are passed in as a hash and may be mixed.

NonBlocking => boolean

Only non-blocking sources will be allowed. In practice this means EGD and /dev/random will not be chosen (except on FreeBSD where it is non-blocking).

Only => [list of strings]

Takes an array reference containing one or more string source names. No source whose name does not match one of these strings will be chosen. The string 'Win32' will match either of the Win32 sources, and 'TESHA2' will match both the strong and weak versions.

Never => [list of strings]

Takes an array reference containing one or more string source names. No source whose name matches one of these strings will be chosen. The string 'Win32' will match either of the Win32 sources, and 'TESHA2' will match both the strong and weak versions.

Source => sub { ... }

Uses the given anonymous subroutine as the generator. The subroutine will be given an integer (the argument to "random_bytes") and should return random data in a string of the given length. For the purposes of the other object methods, the returned object will have the name 'User', and be considered non-blocking and non-strong.

Source => ['name', sub { ... }, is_blocking, is_strong]

Similar to the simpler source routine, but also allows the other source parameters to be defined. The name may not be one of the standard names listed in the "name" section.

random_bytes($n)

random_values($n)

name

is_strong

is_blocking

Crypt::Random::Source

Crypt::URandom

  my $source = Crypt::Random::Seed->new(
     NonBlocking => 1,
     Only => [qw(/dev/random /dev/urandom Win32)]
  );
  croak "No randomness source available" unless defined $source;

Or you can leave out the "Only" and have TESHA2 as a backup.

Crypt::Random

Data::Entropy

Upstream modules