1//! For LICENSE check out https://github.com/0xPARC/plonky2-crypto-gadgets/blob/main/LICENSE
  2//!
  3//! Implements Poseidon encryption as described at
  4//! https://drive.google.com/file/d/1EVrP3DzoGbmzkRmYnyEDcIQcXVU7GlOd/view .
  5
  6#![allow(incomplete_features)]
  7#![feature(generic_const_exprs)]
  8#![allow(dead_code)] // TMP
  9#![allow(non_snake_case)]
 10
 11use std::array;
 12
 13use num::bigint::BigUint;
 14use num_bigint::RandBigInt;
 15use plonky2::{
 16    field::{
 17        extension::{FieldExtension, quintic::QuinticExtension},
 18        goldilocks_field::GoldilocksField as F,
 19        types::{Field, Field64},
 20    },
 21    hash::{hashing::hash_n_to_m_no_pad, poseidon::PoseidonPermutation},
 22};
 23use pod2::backends::plonky2::primitives::ec::curve::{GROUP_ORDER, Point};
 24use rand::rngs::OsRng;
 25
 26pub mod circuit;
 27
 28// ECField
 29type Fq = QuinticExtension<F>;
 30
 31pub fn new_key() -> (BigUint, Point) {
 32    let k: BigUint = OsRng.gen_biguint_below(&GROUP_ORDER);
 33    let K = &k * Point::generator();
 34    (k, K)
 35}
 36pub fn expanded_key(K: Point) -> Point {
 37    let r: BigUint = OsRng.gen_biguint_below(&GROUP_ORDER);
 38    &r * K
 39}
 40
 41pub(crate) const TWO128: Fq =
 42    QuinticExtension::<F>([F(F::ORDER - 1), F(F::ORDER - 1), F::ZERO, F::ZERO, F::ZERO]);
 43
 44pub fn encrypt(ks: Point, msg: &[Fq], nonce: [F; 2]) -> Vec<Fq> {
 45    let mut m = msg.to_vec(); // pad m
 46    m.resize(m.len().next_multiple_of(3), Fq::ZERO);
 47    assert!(m.len() < F::ORDER as usize);
 48
 49    let l = Fq::from(F::from_canonical_u64(msg.len() as u64));
 50    let nonce_5: [F; 5] = [nonce[0], nonce[1], F::ZERO, F::ZERO, F::ZERO];
 51    let n = Fq::from_basefield_array(nonce_5);
 52    let nl128: Fq = n + l * TWO128;
 53    let mut s: [Fq; 4] = [Fq::ZERO, ks.x, ks.u, nl128];
 54
 55    let mut ct: Vec<Fq> = vec![Fq::ZERO; m.len() + 1];
 56    for i in 0..m.len() / 3 {
 57        s = hash_state(s);
 58
 59        // absorb
 60        s[1] += m[i * 3];
 61        s[2] += m[i * 3 + 1];
 62        s[3] += m[i * 3 + 2];
 63
 64        // release
 65        ct[i * 3] = s[1];
 66        ct[i * 3 + 1] = s[2];
 67        ct[i * 3 + 2] = s[3];
 68    }
 69    s = hash_state(s);
 70    ct[m.len()] = s[1];
 71    ct
 72}
 73
 74pub fn decrypt(ks: Point, ct: &[Fq], nonce: [F; 2], l: usize) -> Vec<Fq> {
 75    let l_fq = Fq::from(F::from_canonical_u64(l as u64));
 76    let nonce_5: [F; 5] = [nonce[0], nonce[1], F::ZERO, F::ZERO, F::ZERO];
 77    let n = Fq::from_basefield_array(nonce_5);
 78    let nl128: Fq = n + l_fq * TWO128;
 79    let mut s: [Fq; 4] = [Fq::ZERO, ks.x, ks.u, nl128];
 80
 81    let mut m: Vec<Fq> = vec![Fq::ZERO; ct.len() - 1];
 82    for i in 0..ct.len() / 3 {
 83        s = hash_state(s);
 84
 85        // release
 86        m[3 * i] = ct[3 * i] - s[1];
 87        m[3 * i + 1] = ct[3 * i + 1] - s[2];
 88        m[3 * i + 2] = ct[3 * i + 2] - s[3];
 89
 90        // modify state
 91        s[1] = ct[3 * i];
 92        s[2] = ct[3 * i + 1];
 93        s[3] = ct[3 * i + 2];
 94    }
 95
 96    if l > 3 {
 97        if l % 3 == 2 {
 98            assert_eq!(m[m.len() - 1], Fq::ZERO);
 99        } else if l % 3 == 1 {
100            assert_eq!(m[m.len() - 1], Fq::ZERO);
101            assert_eq!(m[m.len() - 2], Fq::ZERO);
102        }
103    }
104    s = hash_state(s);
105    assert_eq!(ct[ct.len() - 1], s[1]);
106    m[0..l].to_vec()
107}
108
109fn hash_state(s: [Fq; 4]) -> [Fq; 4] {
110    let elems: [F; 4 * 5] = array::from_fn(|i| s[i / 5].0[i % 5]);
111    // Note: here we're using plonky2's normal poseidon, not duplex-sponge
112    let h = hash_n_to_m_no_pad::<F, PoseidonPermutation<_>>(&elems, 4 * 5);
113    array::from_fn(|i| Fq::from_basefield_array(array::from_fn::<_, 5, _>(|j| h[j + 5 * i])))
114}
115
116#[cfg(test)]
117mod tests {
118    use plonky2::field::types::Sample;
119
120    use super::*;
121
122    #[test]
123    fn test_encrypt_decrypt() {
124        test_encrypt_decrypt_op(9);
125        test_encrypt_decrypt_op(10);
126        test_encrypt_decrypt_op(11);
127        test_encrypt_decrypt_op(12);
128        test_encrypt_decrypt_op(128);
129        test_encrypt_decrypt_op(129);
130        test_encrypt_decrypt_op(1024);
131        test_encrypt_decrypt_op(1023);
132        test_encrypt_decrypt_op(1025);
133    }
134    fn test_encrypt_decrypt_op(msg_len: usize) {
135        let mut rng = rand::thread_rng();
136
137        let (_k, K) = new_key();
138        let ks = expanded_key(K);
139        let nonce: [F; 2] = [F::rand(), F::rand()];
140        let msg: Vec<Fq> = (0..msg_len).map(|_| Fq::sample(&mut rng)).collect();
141        let l = msg.len();
142
143        let ct = encrypt(ks, &msg, nonce);
144
145        let m = decrypt(ks, &ct, nonce, l);
146        assert_eq!(m, msg);
147        assert_ne!(&msg, &ct[..msg_len]);
148    }
149}