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//! CSPRNG based on the Reverie construction, a permutation-based PRNG
//! with forward security, instantiated with the Ascon(128,12,8) permutation.
//!
//! Compared to ChaCha, this PRNG has a much smaller state, and can be
//! a better choice for constrained environments.
//!
//! References:
//! - A Robust and Sponge-Like PRNG with Improved Efficiency https://eprint.iacr.org/2016/886.pdf
//! - Ascon https://ascon.iaik.tugraz.at/files/asconv12-nist.pdf
const std = @import("std");
const mem = std.mem;
const Self = @This();
const Ascon = std.crypto.core.Ascon(.little);
state: Ascon,
const rate = 16;
pub const secret_seed_length = 32;
/// The seed must be uniform, secret and `secret_seed_length` bytes long.
pub fn init(secret_seed: [secret_seed_length]u8) Self {
var self = Self{ .state = Ascon.initXof() };
self.addEntropy(&secret_seed);
return self;
}
/// Inserts entropy to refresh the internal state.
pub fn addEntropy(self: *Self, bytes: []const u8) void {
comptime std.debug.assert(secret_seed_length % rate == 0);
var i: usize = 0;
while (i + rate < bytes.len) : (i += rate) {
self.state.addBytes(bytes[i..][0..rate]);
self.state.permuteR(8);
}
if (i != bytes.len) self.state.addBytes(bytes[i..]);
self.state.permute();
}
/// Returns a `std.Random` structure backed by the current RNG.
pub fn random(self: *Self) std.Random {
return std.Random.init(self, fill);
}
/// Fills the buffer with random bytes.
pub fn fill(self: *Self, buf: []u8) void {
var i: usize = 0;
while (true) {
const left = buf.len - i;
const n = @min(left, rate);
self.state.extractBytes(buf[i..][0..n]);
if (left == 0) break;
self.state.permuteR(8);
i += n;
}
self.state.permuteRatchet(6, rate);
}
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