Random-Number Subsystem Architecture (The Libgcrypt Reference Manual)

17.6 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 a multi-precision integer using the gcry_create_nonce function.

There are three 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.
The Deterministic Random Bits Generator (DRBG), based on the specification by NIST SP800-90A. Implemented in random/random-drbg.c and used if Libgcrypt is in FIPS mode, or Libgcrypt is configured by GCRYCTL_SET_PREFERRED_RNG_TYPE with GCRY_RNG_TYPE_FIPS.
Direct access to native RNG on the system. Implemented in random/random-system.c and used if Libgcrypt is configured by GCRYCTL_SET_PREFERRED_RNG_TYPE with GCRY_RNG_TYPE_SYSTEM.

All generators make use of so-called entropy gathering modules:

rndgetentropy: Uses the operating system provided getentropy function.
rndoldlinux: Uses the operating system provided /dev/random and /dev/urandom devices. The /dev/gcrypt/random.conf config option only-urandom can be used to inhibit the use of the blocking /dev/random device.
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.
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 supported hardware RNG is the Padlock engine of VIA (Centaur) CPUs and x86 CPUs with the RDRAND instruction.
rndjent: Extra module to collect additional entropy using a CPU jitter based approach. The /dev/gcrypt/random.conf config option disable-jent can be used to inhibit the use of this module.