spiral-spir/src/ot.rs

// Oblivious transfer

use subtle::Choice;
use subtle::ConditionallySelectable;

use aes::Block;

use sha2::Digest;
use sha2::Sha256;
use sha2::Sha512;

use curve25519_dalek::constants as dalek_constants;
use curve25519_dalek::ristretto::CompressedRistretto;
use curve25519_dalek::ristretto::RistrettoBasepointTable;
use curve25519_dalek::ristretto::RistrettoPoint;
use curve25519_dalek::scalar::Scalar;

use lazy_static::lazy_static;

// Generators of the Ristretto group (the standard B and another one C,
// for which the DL relationship is unknown), and their precomputed
// multiplication tables.  Used for the Oblivious Transfer protocol
lazy_static! {
    pub static ref OT_B: RistrettoPoint = dalek_constants::RISTRETTO_BASEPOINT_POINT;
    pub static ref OT_C: RistrettoPoint =
        RistrettoPoint::hash_from_bytes::<Sha512>(b"OT Generator C");
    pub static ref OT_B_TABLE: RistrettoBasepointTable = dalek_constants::RISTRETTO_BASEPOINT_TABLE;
    pub static ref OT_C_TABLE: RistrettoBasepointTable = RistrettoBasepointTable::create(&OT_C);
}

// 1-out-of-2 Oblivious Transfer (OT)

fn ot12_request(sel: Choice) -> ((Choice, Scalar), [u8; 32]) {
    let Btable: &RistrettoBasepointTable = &OT_B_TABLE;
    let C: &RistrettoPoint = &OT_C;
    let mut rng = rand07::thread_rng();
    let x = Scalar::random(&mut rng);
    let xB = &x * Btable;
    let CmxB = C - xB;
    let P = RistrettoPoint::conditional_select(&xB, &CmxB, sel);
    ((sel, x), P.compress().to_bytes())
}

fn ot12_serve(query: &[u8; 32], m0: &[u8; 16], m1: &[u8; 16]) -> [u8; 64] {
    let Btable: &RistrettoBasepointTable = &OT_B_TABLE;
    let Ctable: &RistrettoBasepointTable = &OT_C_TABLE;
    let mut rng = rand07::thread_rng();
    let y = Scalar::random(&mut rng);
    let yB = &y * Btable;
    let yC = &y * Ctable;
    let P = CompressedRistretto::from_slice(query).decompress().unwrap();
    let yP0 = y * P;
    let yP1 = yC - yP0;
    let mut HyP0 = Sha256::digest(yP0.compress().as_bytes());
    for i in 0..16 {
        HyP0[i] ^= m0[i];
    }
    let mut HyP1 = Sha256::digest(yP1.compress().as_bytes());
    for i in 0..16 {
        HyP1[i] ^= m1[i];
    }
    let mut ret = [0u8; 64];
    ret[0..32].copy_from_slice(yB.compress().as_bytes());
    ret[32..48].copy_from_slice(&HyP0[0..16]);
    ret[48..64].copy_from_slice(&HyP1[0..16]);
    ret
}

fn ot12_receive(state: (Choice, Scalar), response: &[u8; 64]) -> [u8; 16] {
    let yB = CompressedRistretto::from_slice(&response[0..32])
        .decompress()
        .unwrap();
    let yP = state.1 * yB;
    let mut HyP = Sha256::digest(yP.compress().as_bytes());
    for i in 0..16 {
        HyP[i] ^= u8::conditional_select(&response[32 + i], &response[48 + i], state.0);
    }
    HyP[0..16].try_into().unwrap()
}

// Obliviously fetch the key for element q of the database (which has
// 2^r elements total).  Each bit of q is used in a 1-out-of-2 OT to get
// one of the keys in each of the r pairs of keys on the server side.
// The resulting r keys are XORed together.

pub fn otkey_init() {
    // Resolve the lazy statics
    let _B: &RistrettoPoint = &OT_B;
    let _Btable: &RistrettoBasepointTable = &OT_B_TABLE;
    let _C: &RistrettoPoint = &OT_C;
    let _Ctable: &RistrettoBasepointTable = &OT_C_TABLE;
}

pub fn otkey_request(q: usize, r: usize) -> (Vec<(Choice, Scalar)>, Vec<[u8; 32]>) {
    let mut state: Vec<(Choice, Scalar)> = Vec::with_capacity(r);
    let mut query: Vec<[u8; 32]> = Vec::with_capacity(r);
    for i in 0..r {
        let bit = ((q >> i) & 1) as u8;
        let (si, qi) = ot12_request(bit.into());
        state.push(si);
        query.push(qi);
    }
    (state, query)
}

pub fn otkey_serve(query: Vec<[u8; 32]>, keys: &Vec<[u8; 16]>) -> Vec<[u8; 64]> {
    let r = query.len();
    assert!(keys.len() == 2 * r);
    let mut response: Vec<[u8; 64]> = Vec::with_capacity(r);
    for i in 0..r {
        response.push(ot12_serve(&query[i], &keys[2 * i], &keys[2 * i + 1]));
    }
    response
}

// XOR a 16-byte slice into a Block (which will be used as an AES key)
pub fn xor16(outar: &mut Block, inar: &[u8; 16]) {
    for i in 0..16 {
        outar[i] ^= inar[i];
    }
}

pub fn otkey_receive(state: Vec<(Choice, Scalar)>, response: &Vec<[u8; 64]>) -> Block {
    let r = state.len();
    assert!(response.len() == r);
    let mut key = Block::from([0u8; 16]);
    for i in 0..r {
        xor16(&mut key, &ot12_receive(state[i], &response[i]));
    }
    key
}