package cryptokit

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The Random module provides random and pseudo-random number generators suitable for generating cryptographic keys, nonces, or challenges.

class type rng = object ... end

Generic interface for a random number generator.

val string : rng -> int -> string

random_string rng len returns a string of len random bytes read from the generator rng.

val secure_rng : rng

A high-quality random number generator, using hard-to-predict system data to generate entropy. This generator either uses the OS-provided RNG, if any, or reads from /dev/random on systems that supports it, or interrogates the EGD daemon otherwise (see For EGD, the following paths are tried to locate the Unix socket used to communicate with EGD:

  • the value of the environment variable EGD_SOCKET;
  • $HOME/.gnupg/entropy;
  • /var/run/egd-pool; /dev/egd-pool; /etc/egd-pool.

The method secure_rng#random_bytes fails if no suitable RNG is available. secure_rng#random_bytes may block until enough entropy has been gathered. Do not use for generating large quantities of random data, otherwise you could exhaust the entropy sources of the system.

val system_rng : unit -> rng

system_rng () returns a random number generator derived from the OS-provided RNG. It raises Error No_entropy_source if the OS does not provide a secure RNG. Currently, this function is supported under Win32, and always fails under Unix.

val device_rng : string -> rng

device_rng devicename returns a random number generator that reads from the special file devicename, e.g. /dev/random or /dev/urandom.

val egd_rng : string -> rng

device_rng egd_socket returns a random number generator that uses the Entropy Gathering Daemon ( egd_socket is the path to the Unix socket that EGD uses for communication.

val hardware_rng : unit -> rng

A hardware random number generator based on the RDRAND instruction of the x86 architecture. Available only on recent Intel and AMD x86 processors in 64-bit mode. Raises Error No_entropy_source if not available.

val pseudo_rng : string -> rng

pseudo_rng seed returns a pseudo-random number generator seeded by the string seed. seed must contain at least 16 characters, and can be arbitrarily longer than this, except that only the first 32 characters are used. The seed is used as a key for the Chacha20 stream cipher. The generated pseudo-random data is the result of encrypting the all-zero input with Chacha20. While this generator is believed to have very good statistical properties, it still does not generate ``true'' randomness: the entropy of the byte strings it produces cannot exceed the entropy contained in the seed. As a typical use, Random.pseudo_rng (Random.string Random.secure_rng 20) returns a generator that can generate arbitrarily long strings of pseudo-random data without delays, and with a total entropy of approximately 160 bits.

val pseudo_rng_aes_ctr : string -> rng

This is another pseudo-random number generator, based on the AES block cipher in counter mode. It is slightly slower than pseudo_rng while having similar randomness characteristics. The only reason to use it instead of pseudo_rng is that AES has been cryptanalyzed even more than Chacha20. The seed argument must contain at least 16 characters. Only the first 16 characters are used, as an AES key. The generated pseudo-random data is the result of encrypting the 128-bit integers 0, 1, 2, ... with this key.