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use rand::{Rng, RngCore, SeedableRng};
pub struct MersenneTwister {
state: [u32; 624],
index: u32,
}
pub struct MersenneTwisterSeed([u8; 2500]);
impl Default for MersenneTwisterSeed {
fn default() -> MersenneTwisterSeed {
MersenneTwisterSeed([0; 2500])
}
}
impl AsMut<[u8]> for MersenneTwisterSeed {
fn as_mut(&mut self) -> &mut [u8] {
&mut self.0
}
}
fn mt_seed_to_state(seed: MersenneTwisterSeed) -> MersenneTwister {
let mut state = [0; 624];
for i in 0..624 {
let idx = i * 4;
state[i] = u32::from_ne_bytes([
seed.0[idx],
seed.0[idx + 1],
seed.0[idx + 2],
seed.0[idx + 3],
]);
}
let index = u32::from_ne_bytes([
seed.0[2496],
seed.0[2497],
seed.0[2498],
seed.0[2499],
]) % 624;
MersenneTwister { state, index }
}
fn mt_state_to_seed(state: [u32; 624], index: u32) -> MersenneTwisterSeed {
let mut seed = MersenneTwisterSeed([0; 2500]);
let mut idx = 0;
for i in &state[..] {
let bytes = u32::to_ne_bytes(*i);
seed.0[idx as usize] = bytes[0];
seed.0[(idx + 1) as usize] = bytes[1];
seed.0[(idx + 2) as usize] = bytes[2];
seed.0[(idx + 3) as usize] = bytes[3];
idx += 4;
}
let bytes = u32::to_ne_bytes(index);
seed.0[2496] = bytes[0];
seed.0[2497] = bytes[1];
seed.0[2498] = bytes[2];
seed.0[2499] = bytes[3];
seed
}
impl MersenneTwister {
fn new_unseeded() -> MersenneTwister {
MersenneTwister {
state: [0; 624],
index: 0,
}
}
pub fn from_u32(seed: u32) -> MersenneTwister {
let mut state = [0; 624];
state[0] = seed;
for i in 1..624 {
let prev = state[i - 1];
state[i] = 1812433253u32
.wrapping_mul(prev ^ (prev >> 30))
.wrapping_add(i as u32);
}
MersenneTwister::from_seed(mt_state_to_seed(state, 0))
}
pub fn from_state(state: [u32; 624], index: u32) -> MersenneTwister {
MersenneTwister::from_seed(mt_state_to_seed(state, index))
}
}
impl RngCore for MersenneTwister {
fn next_u32(&mut self) -> u32 {
if self.index == 0 {
for i in 0..624 {
let y = (self.state[i] & 0x80000000)
.wrapping_add(self.state[(i + 1) % 624] & 0x7fffffff);
self.state[i] = self.state[(i + 397) % 624] ^ (y >> 1);
if (y % 2) != 0 {
self.state[i] = self.state[i] ^ 0x9908b0df;
}
}
}
let mut y = self.state[self.index as usize];
y = y ^ (y >> 11);
y = y ^ ((y << 7) & 0x9d2c5680);
y = y ^ ((y << 15) & 0xefc60000);
y = y ^ (y >> 18);
self.index = (self.index + 1) % 624;
return y;
}
fn next_u64(&mut self) -> u64 {
rand_core::impls::next_u64_via_u32(self)
}
fn fill_bytes(&mut self, dest: &mut [u8]) {
rand_core::impls::fill_bytes_via_next(self, dest)
}
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand::Error> {
Ok(rand_core::impls::fill_bytes_via_next(self, dest))
}
}
impl SeedableRng for MersenneTwister {
type Seed = MersenneTwisterSeed;
fn from_seed(seed: MersenneTwisterSeed) -> MersenneTwister {
let mut mt = MersenneTwister::new_unseeded();
let MersenneTwister { state, index } = mt_seed_to_state(seed);
for i in 0..624 {
mt.state[i] = state[i];
}
mt.index = index;
mt
}
}
impl Clone for MersenneTwister {
fn clone(&self) -> MersenneTwister {
MersenneTwister {
state: self.state,
index: self.index,
}
}
}
impl std::fmt::Debug for MersenneTwister {
fn fmt(
&self,
f: &mut std::fmt::Formatter,
) -> Result<(), std::fmt::Error> {
write!(f, "MersenneTwister {{ ")?;
std::fmt::Debug::fmt(&&self.state[..], f)?;
write!(f, ", ")?;
std::fmt::Debug::fmt(&self.index, f)?;
write!(f, " }}")
}
}
pub fn mt19937_stream_cipher(ciphertext: &[u8], key: u32) -> Vec<u8> {
let mut mt = MersenneTwister::from_u32(key);
let keystream: Vec<u8> = mt
.sample_iter(&rand::distributions::Standard)
.take(ciphertext.len())
.collect();
return crate::primitives::fixed_xor(ciphertext, &keystream[..]);
}
#[test]
fn test_mt19937_stream_cipher() {
let key = rand::thread_rng().gen();
let plaintext = b"Summertime and the wind is blowing outside in lower \
Chelsea and I don't know what I'm doing in the city, the \
sun is always in my eyes";
let ciphertext = mt19937_stream_cipher(&plaintext[..], key);
assert!(&plaintext[..] != &ciphertext[..]);
let plaintext2 = mt19937_stream_cipher(&ciphertext[..], key);
assert_eq!(&plaintext[..], &plaintext2[..]);
}
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