13.6 Random-Number Subsystem Architecture
Libgcrypt provides 3 levels or random quality: The level
GCRY_VERY_STRONG_RANDOM usually used for key generation, the
level GCRY_STRONG_RANDOM for all other strong random
requirements and the function gcry_create_nonce which is used
for weaker usages like nonces. There is also a level
GCRY_WEAK_RANDOM which in general maps to
GCRY_STRONG_RANDOM except when used with the function
gcry_mpi_randomize, where it randomizes an
multi-precision-integer using the gcry_create_nonce function.
There are two distinct random generators available:
- The Continuously Seeded Pseudo Random Number Generator (CSPRNG), which
is based on the classic GnuPG derived big pool implementation.
Implemented in
random/random-csprng.c and used by default.
- A FIPS approved ANSI X9.31 PRNG using AES with a 128 bit key. Implemented in
random/random-fips.c and used if Libgcrypt is in FIPS mode.
Both generators make use of so-called entropy gathering modules:
- rndlinux
- Uses the operating system provided
/dev/random and /dev/urandom devices.
- rndunix
- Runs several operating system commands to collect entropy from sources
like virtual machine and process statistics. It is a kind of
poor-man's
/dev/random implementation. It is not available in
FIPS mode.
- rndegd
- Uses the operating system provided Entropy Gathering Daemon (EGD).
The EGD basically uses the same algorithms as rndunix does. However
as a system daemon it keeps on running and thus can serve several
processes requiring entropy input and does not waste collected entropy
if the application does not need all the collected entropy. It is not
available in FIPS mode.
- rndw32
- Targeted for the Microsoft Windows OS. It uses certain properties of
that system and is the only gathering module available for that OS.
- rndhw
- Extra module to collect additional entropy by utilizing a hardware
random number generator. As of now the only supported hardware RNG is
the Padlock engine of VIA (Centaur) CPUs. It is not available in FIPS
mode.