false-bottom/src/false_bottom.rs

pub mod fb128;

use crypto_bigint::{Limb, RandomMod, NonZero, Encoding};
use base64::prelude::{BASE64_STANDARD, Engine};
use crate::{FBError, Packing, FBKey, Encode};
use rand::{Rng, prelude::IteratorRandom};
use std::cmp::PartialOrd;

pub struct FBObj<T> {
    pub(crate) c: Vec<T>,
    pub(crate) r: Vec<T>,
}

pub trait FalseBottom<T>: FBBlockOperations<T>
where
    T: RandomMod + ModPrime<T> + Encoding + PartialOrd + Packing<T>
{
    const MODULUS: NonZero<T>;

    fn init(n: usize, k: usize) -> Result<FBObj<T>, FBError>
    where
        T: RandomMod
    {
		if n < k || k < 2 {
			return Err(FBError::InvalidParams);
		}
		let mut rng = rand::thread_rng();
		let r = (0..k)
			.map(|_| T::random_mod(&mut rng, &Self::MODULUS))
			.collect();
		let c = (0..n)
			.map(|_| T::random_mod(&mut rng, &Self::MODULUS))
			.collect();

		Ok(FBObj { c, r })
	}
    
    fn add(&mut self, msg: &[u8]) -> FBKey {
		let indices = T::pack(msg).into_iter()
			.map(|msg_uint| self.add_u128(&msg_uint))
			.collect();

		FBKey { indices }
	}

	fn decrypt(&self, key: &FBKey) -> Result<Vec<u8>, FBError> {
		let decr = key.indices.iter()
			.map(|index_row| self.decrypt_u128(&index_row))
			.collect::<Result<Vec<_>, _>>()?;
		let msg = T::unpack(&decr)?;

		Ok(msg)
	}
}

pub trait ModPrime<T> {
    const PRIME_POS: Limb;
    const PRIME: T;
    fn mul_mod_prime(&self, rhs: &Self) -> Self;
    fn add_mod_prime(&self, rhs: &Self) -> Self;
    fn sub_mod_prime(&self, rhs: &Self) -> Self;
    fn inv_mod_prime(&self) -> Self;
}


pub trait FBBlockOperations<T>
where
    T: RandomMod + ModPrime<T>
{
    const P: T;
    const P_POS: Limb;

    fn cipher(&self) -> &Vec<T>;
    fn cipher_mut(&mut self) -> &mut Vec<T>;
    fn keybase(&self) -> &Vec<T>;

    fn add_u128(&mut self, msg_uint: &T) -> Vec<(usize, usize)> {
        let c = self.cipher();
        let r = self.keybase();
		let mut rng = rand::thread_rng();
		let n = rng.gen_range(2..=r.len());
		let mut c_i = (0..c.len()).choose_multiple(&mut rng, n - 1);
		let r_i = (0..r.len()).choose_multiple(&mut rng, n);
		let mut sum = T::ZERO;
		for (&ci, &ri) in c_i.iter().zip( r_i.iter() ) {
			sum = sum.add_mod_prime( &c[ci].mul_mod_prime(&r[ri]) );
		}
		let ri_last = *r_i.last()
			.expect("r_i will contain at least 2 elements");
		let mod_inv = r[ri_last].inv_mod_prime();
		let c_new_el = msg_uint.sub_mod_prime(&sum)
			.mul_mod_prime(&mod_inv);
        let c = self.cipher_mut();
		c.push(c_new_el);
		c_i.push(c.len() - 1);
		let indices = c_i.into_iter()
			.zip(r_i.into_iter())
			.collect();

		indices
	}

	fn decrypt_u128(&self, indices: &[(usize, usize)]) -> Result<T, FBError> {
		let (r, c) = (self.keybase(), self.cipher());
		if indices.len() > r.len() {
			return Err(FBError::InvalidKey);
		}
		let mut msg = T::ZERO;
		for &(ci, ri) in indices {
			let c_el = c.get(ci).ok_or(FBError::InvalidKey)?;
			let r_el = r.get(ri).ok_or(FBError::InvalidKey)?;
			msg = msg.add_mod_prime(&c_el.mul_mod_prime(&r_el));
		}

		Ok(msg)
	}
}

impl<T> Encode<T> for FBObj<T>
where
	T: Encoding + crypto_bigint::Bounded,
<T as Encoding>::Repr: Iterator + for <'a> From<&'a [u8]>,
Vec<u8>: FromIterator<<<T as Encoding>::Repr as Iterator>::Item>
{
	const BYTES: usize = T::BYTES;

	fn export(&self) -> (String, String) {		
		let c_bytes: Vec<u8> = self.c.iter()
			.flat_map(|bigint| bigint.to_le_bytes())
			.collect();
		let r_bytes: Vec<u8> = self.r.iter()
			.flat_map(|bigint| bigint.to_le_bytes())
			.collect();

		(BASE64_STANDARD.encode(c_bytes), BASE64_STANDARD.encode(r_bytes))
	}

	fn import(cipher: &str, keybase: &str) -> Result<Self, FBError>
	where
		Self: Sized
	{
		let c_bytes = BASE64_STANDARD.decode(cipher)
			.map_err(|_| FBError::DecodeError)?;
		let c: Vec<T> = c_bytes.chunks_exact(Self::BYTES)
			.map(|chunk| T::from_le_bytes(chunk.try_into().unwrap()))
			.collect();
		let r_bytes = BASE64_STANDARD.decode(keybase)
			.map_err(|_| FBError::DecodeError)?;
		let r: Vec<T> = r_bytes.chunks_exact(Self::BYTES)
			.map(|chunk| T::from_le_bytes(chunk.try_into().unwrap()))
			.collect();
		if c.len() < r.len() || r.len() < 2 {
			return Err(FBError::InvalidParams);
		}

		Ok(Self {c, r})
	}
}