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// Copyright (c) 2018, The Tor Project, Inc.
// Copyright (c) 2018, isis agora lovecruft
// See LICENSE for licensing information
//! Wrappers for Tor's pseudo-random number generator to provide implementations
//! of `rand_core` traits.
use rand_core::impls;
#[cfg(test)] use rand_core::CryptoRng;
use rand_core::Error;
use rand_core::RngCore;
use rand_core::SeedableRng;
/// A cryptographically-/insecure/ psuedo-random number generator based
/// on a mixed congruential generator.
///
/// Specifically the PRNG state, `X`, is mutated by the following
/// discontinuous linear equation:
///
/// ```text
/// X_{i} = (a X_{i-1} + b) mod n
/// ```
///
/// where, in our case, we reuse the same parameters as OpenBSD and glibc,
/// `a=1103515245`, `b=12345`, and `n=2147483647`, which should produce a
/// maximal period over the range `0..u32::MAX`.
///
/// # Note
///
/// We reimplement the C here, rather than wrapping it, as it's one line of
/// pure-Rust code (meaning it can also trivially be used in Rust tests without
/// running into potential linker issues), as opposed to a few lines of `unsafe`
/// calls to C.
///
/// # Warning
///
/// This should hopefully go without saying, but this PRNG is completely
/// insecure and should never be used for anything an adversary should be unable
/// to predict.
//
// C_RUST_COUPLED: `tor_weak_rng_t` /src/common/util.c
pub struct TorInsecurePrng {
state: u32,
}
impl SeedableRng for TorInsecurePrng {
type Seed = [u8; 4];
/// Create a new PRNG from a random 32-bit seed.
//
// C_RUST_COUPLED: `tor_init_weak_random()` /src/common/util.c
fn from_seed(seed: Self::Seed) -> Self {
let mut combined: u32 = seed[0].to_le() as u32;
// Rather than using std::mem::transmute, we'll just bitwise-OR them
// into each other.
combined = (seed[1].to_le() as u32) << 8 | combined;
combined = (seed[2].to_le() as u32) << 16 | combined;
combined = (seed[2].to_le() as u32) << 24 | combined;
TorInsecurePrng{ state: (combined & 0x7fffffff).to_le() }
}
}
impl TorInsecurePrng {
/// This is the equivalent function to `tor_weak_random()`.
//
// C_RUST_COUPLED: `tor_weak_random()` /src/common/util.c
pub fn next_i32(&mut self) -> i32 {
// The C code appears to purposefully overflow the 32-bit state integer.
self.state = (self.state.wrapping_mul(1103515245).wrapping_add(12345) & 0x7fffffff).to_le();
self.state as i32
}
}
impl RngCore for TorInsecurePrng {
// C_RUST_COUPLED: `tor_weak_random()` /src/common/util.c
fn next_u32(&mut self) -> u32 {
let x: u32 = self.next_i32() as u32;
let y: u32 = self.next_i32() as u32;
// We have to add two samples together due to modding 0x7fffffff
x + y
}
fn next_u64(&mut self) -> u64 {
impls::next_u64_via_u32(self)
}
fn fill_bytes(&mut self, dest: &mut [u8]) {
impls::fill_bytes_via_u32(self, dest);
}
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
Ok(self.fill_bytes(dest))
}
}
/// If we're running tests, it's fine to pretend this PRNG is cryptographically
/// secure. (This allows us to test which require an implementation of
/// `CryptoRng` without actually initialising all the OpenSSL C code.)
#[cfg(test)]
impl CryptoRng for TorInsecurePrng {}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn next_u32_shouldnt_return_same_number_twice_in_a_row() {
// This test will fail 1 out of 2^{64} times (5.42 e-20), but the
// probability of a particle radiating off a star and hitting your RAM
// is roughly 1.4 e-15 per byte of RAM per second, so if this fails,
// blame ~~Cosmic Rays~~ and not anyone named isis.
let mut prng: TorInsecurePrng = TorInsecurePrng::from_seed([0xDE, 0xAD, 0x15, 0x15]);
let one: u32 = prng.next_u32();
let two: u32 = prng.next_u32();
assert!(one != two);
}
#[test]
fn next_u32_should_have_uniform_distribution_average() {
let mut prng: TorInsecurePrng = TorInsecurePrng::from_seed([0xDE, 0xAD, 0x15, 0x15]);
let mut accumulator: Vec<u32> = Vec::new();
let n: u64 = 10_000;
for _ in 0 .. n as usize {
accumulator.push(prng.next_u32());
}
let total: u64 = accumulator.iter().fold(0, |acc,&x| acc + (x as u64));
let average = total / n;
println!("average is {:?}", average);
assert!(average <= 0x7fffffff + 0xf00000);
assert!(average >= 0x7fffffff - 0xf00000);
}
#[test]
fn next_u32_shouldnt_have_bit_bias() {
// Since the modulus in the mixed congruential generator isn't a power
// of two, the bits should not have any statistical bias.
let mut prng: TorInsecurePrng = TorInsecurePrng::from_seed([0xDE, 0xAD, 0x15, 0x15]);
let mut accumulator: Vec<u32> = Vec::new();
let n: u64 = 10_000;
for _ in 0 .. n as usize {
accumulator.push(prng.next_u32().count_ones());
}
let total: u64 = accumulator.iter().fold(0, |acc,&x| acc + (x as u64));
let average = total / n;
println!("average is {:?}", average);
assert!(average == 16);
}
#[test]
fn next_u64_shouldnt_return_same_number_twice_in_a_row() {
// This test will fail 1 out of 2^{128} times (2.94 e-39), but the
// probability of a particle radiating off a star and hitting your RAM
// is roughly 1.4 e-15 per byte of RAM per second, so if this fails,
// blame ~~Cosmic Rays~~ and not anyone named isis.
let mut prng: TorInsecurePrng = TorInsecurePrng::from_seed([0xDE, 0xAD, 0x15, 0x15]);
let one: u64 = prng.next_u64();
let two: u64 = prng.next_u64();
assert!(one != two);
}
#[test]
fn fill_bytes_shouldnt_leave_all_zeroes() {
// Again, 1 in 256^8 (5.42 e-20) chances this fails.
// ~~Cosmic Rays~~, I tell you.
let mut prng: TorInsecurePrng = TorInsecurePrng::from_seed([0xDE, 0xAD, 0x15, 0x15]);
let mut bytes: [u8; 8] = [0u8; 8];
prng.fill_bytes(&mut bytes);
assert!(bytes != [0u8; 8]);
}
}
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