libsession-util/include/session/random.hpp

#pragma once

#include <sodium/randombytes.h>

#include <algorithm>
#include <concepts>
#include <cstdint>
#include <limits>
#include <random>

#include "util.hpp"

namespace session {
/// rng type that uses libsodium's randombytes, which is cryptographically secure.  This object is
/// stateless and trivial, so you can either construct it on the fly (with no runtime cost --
/// compilers will be optimize the construction/destruction away), or you can use the slightly less
/// verbose session::csrng variable.
struct CSRNG {
    using result_type = uint64_t;

    static constexpr uint64_t min() { return std::numeric_limits<uint64_t>::min(); };

    static constexpr uint64_t max() { return std::numeric_limits<uint64_t>::max(); };

    uint64_t operator()() const {
        uint64_t i;
        randombytes((uint8_t*)&i, sizeof(i));
        return i;
    };
};

// Convenience instance.
inline constexpr CSRNG csrng{};

}  // namespace session

namespace session::random {

/// API: random/random
///
/// Wrapper around the randombytes_buf function.
///
/// Inputs:
/// - `size` -- the number of random bytes to be generated.
///
/// Outputs:
/// - random bytes of the specified length.
std::vector<unsigned char> random(size_t size);

/// API: random/random_base32
///
/// Return a random base32 string with the given length.
///
/// Inputs:
/// - `size` -- the number of characters to be generated.
///
/// Outputs:
/// - random base32 string of the specified length.
std::string random_base32(size_t size);

/// API: random/unique_id
///
/// Generates a unique id in the form of `{prefix}-{id_num}-{random_base32(4)}`.
///
/// Inputs:
/// - `prefix` -- a prefix to prepend to the generated id.
///
/// Outputs:
/// - generated id string.
std::string unique_id(std::string_view prefix);

/// API: random/get_uniform_distribution
///
/// Generates a cryptographically secure random integer within a given range (inclusive).
///
/// Inputs:
/// - `min` -- the minimum value for the range.
/// - `max` -- the maximum value for the range.
///
/// Outputs:
/// - A random integer in the specified range
template <std::integral T>
T get_uniform_distribution(T min, T max) {
    if (min > max)
        return min;

    return std::uniform_int_distribution<T>{min, max}(csrng);
}

}  // namespace session::random