There are several parts of this module:
The RNGs here are merely the deterministic part of a full random number generation suite. For proper operation, they need to be seeded with a high-quality entropy source.
Suitable entropy sources are provided by sub-libraries nocrypto.unix, nocrypto.lwt and nocrypto.xen. Although this module exposes a more fine-grained interface, allowing manual seeding of generators, this is intended either for implementing entropy-harvesting modules, or very specialized purposes. Users of this library should almost certainly use one of the above entropy libraries, and avoid manually managing the generator seeding.
Similarly, although it is possible to swap the default generator and gain control over the random stream, this is also intended for specialized applications such as testing or similar scenarios where the RNG needs to be fully deterministic, or as a component of deterministic algorithms which internally rely on pseudorandom streams.
In the general case, users should not maintain their local instances of g. All of the generators in a process have to compete for entropy, and it is likely that the overall result will have lower effective unpredictability.
The recommended way to use these functions is either to accept an optional
generator and pass it down, or to ignore the generator altogether, as
illustrated in the examples.
val create :
(module Nocrypto.Rng.S.Generator with type g = 'a) -> g
create moduleuses a module conforming to the Generator signature to instantiate the generic generator g.
g is the state to use, otherwise a fresh one is created.
seed can be provided to immediately reseed the generator with.
strict puts the generator into a more standards-conformant, but slighty
slower mode. Useful if the outputs need to match published test-vectors.
val generator :
generator is a way to subvert the random-generation process
e.g. to make it fully deterministic.
generator defaults to Fortuna.
val generate :
?g:g -> int -> Cstruct.t
val block :
g option -> int
with type t = int
with type t = int32
with type t = int64
with type t = Z.t
val prime :
?g:g -> ?msb:int -> int -> Z.t
prime ~g ~msb bitsgenerates a prime smaller than
msbmost significant bits set.
prime ~g ~msb:1 bits (the default) yields a prime in the interval
[2^(bits - 1), 2^bits - 1].
val safe_prime :
?g:g -> int -> Z.t * Z.t
safe_prime ~g bitsgives a prime pair
(g, p)such that
p = 2g + 1and
Generating a random 13-byte
let cs = Rng.generate 13
Generating a list of
Cstruct.ts, passing down an optional
let rec f1 ?g ~n i = if i < 1 then  else Rng.generate ?g n :: f1 ?g ~n (i - 1)
Z.t smaller than
10 and an
int64 in the range
let f2 ?g () = Rng.(Z.gen ?g ~$10, Int64.gen_r 3L 8L)
Creating a local Fortuna instance and using it as a key-derivation function:
let f3 secret = let g = Rng.(create ~seed:secret (module Generators.Fortuna)) in Rng.generate ~g 32
Generating a 17-bit prime with two leading bits set:
let p = Rng.prime ~msb:2 17
let f4 ?g arr = let n = Array.length arr in arr |> Array.iter @@ fun i -> let j = Rng.Int.gen_r ?g i n in let (a, b) = (arr.(i), arr.(j)) in arr.(i) <- b ; arr.(j) <- a