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Predicate crypto_n_random_bytes/2
Availability::- use_module(library(crypto)).(can be autoloaded)
Source[det]crypto_n_random_bytes(+N, -Bytes)
Bytes is unified with a list of N cryptographically secure pseudo-random bytes. Each byte is an integer between 0 and 255. If the internal pseudo-random number generator (PRNG) has not been seeded with enough entropy to ensure an unpredictable byte sequence, an exception is thrown.

One way to relate such a list of bytes to an integer is to use CLP(FD) constraints as follows: 

:- use_module(library(clpfd)).

bytes_integer(Bs, N) :-
        foldl(pow, Bs, 0-0, N-_).

pow(B, N0-I0, N-I) :-
        B in 0..255,
        N #= N0 + B*256^I0,
        I #= I0 + 1.

With this definition, you can generate a random 256-bit integer from a list of 32 random bytes: 

?- crypto_n_random_bytes(32, Bs),
   bytes_integer(Bs, I).
Bs = [98, 9, 35, 100, 126, 174, 48, 176, 246|...],
I = 109798276762338328820827...(53 digits omitted).

The above relation also works in the other direction, letting you translate an integer to a list of bytes. In addition, you can use hex_bytes/2 to convert bytes to tokens that can be easily exchanged in your applications. This also works if you have compiled SWI-Prolog without support for large integers.

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