false_bottom/
fbobj.rs

1// SPDX-License-Identifier: GPL-3.0-or-later
2use crate::{FbError, PrimeField};
3use crypto_bigint::{U128, U256, U512, Uint};
4use rand::{Rng, rngs::ThreadRng, seq::index};
5use std::sync::RwLock;
6
7// The False bottom object. It holds the ciphertext(c) and the keybase(r).
8pub struct FbObj<T> {
9	pub(crate) c: RwLock<Vec<T>>,
10	pub(crate) r: Vec<T>,
11}
12
13/// An FbObj with block size of 128 bits.
14pub type Fb128 = FbObj<U128>;
15/// An FbObj with a block size of 256 bits.
16pub type Fb256 = FbObj<U256>;
17/// An FbObj with a block size of 512 bits.
18pub type Fb512 = FbObj<U512>;
19
20impl<const LIMBS: usize> FbObj<Uint<LIMBS>>
21where
22	Uint<LIMBS>: PrimeField
23{
24	pub(crate) fn add_block(
25		&self,
26		rng: &mut ThreadRng,
27		msg_uint: &Uint<LIMBS>
28	) -> Vec<(usize, usize)> {
29		let r = &self.r;
30		let n = rng.gen_range(2..=r.len());
31		let r_i = index::sample(rng, r.len(), n);
32		let ri_last = r_i.iter().last()
33			.expect("r_i will contain at least 2 elements");
34		let ri_last_inv = r[ri_last].field_inv();
35		let c_i;
36		let c_len;
37		{
38			let mut c = self.c.write().unwrap();
39			c_i = index::sample(rng, c.len(), n - 1);
40			let sum = c_i.iter()
41				.zip(r_i.iter())
42				.map(|(ci, ri)| c[ci].field_mul(&r[ri]))
43				.reduce(|acc, i| acc.field_add(&i))
44				.unwrap();
45			let c_new_el = msg_uint.field_sub(&sum).field_mul(&ri_last_inv);
46			c.push(c_new_el);
47			c_len = c.len();
48		}
49		let indices = c_i.into_iter()
50			.chain([c_len - 1].into_iter())
51			.zip(r_i.into_iter())
52			.collect();
53
54		indices
55	}
56
57	pub(crate) fn decrypt_block(
58		&self,
59		indices: &[(usize, usize)]
60	) -> Result<Uint<LIMBS>, FbError> {
61		let (c, r) = (self.c.read().unwrap(), &self.r);
62		if indices.len() > r.len() {
63			return Err(FbError::InvalidKey);
64		}
65		let mut msg = Uint::<LIMBS>::ZERO;
66		for &(ci, ri) in indices {
67			let c_el = c.get(ci).ok_or(FbError::InvalidKey)?;
68			let r_el = r.get(ri).ok_or(FbError::InvalidKey)?;
69			msg = msg.field_add(&c_el.field_mul(&r_el));
70		}
71
72		Ok(msg)
73	}
74}
75
76#[cfg(test)]
77mod test {
78	use super::*;
79	use crate::FalseBottom;
80
81	#[test]
82	fn test_block_operations() {
83		let msg = U512::from_u16(369);
84		let fb = Fb512::init(12, 12);
85		let key = fb.add_block(&mut rand::thread_rng(), &msg);
86		let decrypted = fb.decrypt_block(&key).unwrap();
87		assert_eq!(msg, decrypted);
88	}
89}
false_bottom/fbobj.rs

false_bottom/
fbobj.rs
