voprf-vx/src/group/elliptic_curve.rs

// Copyright (c) Facebook, Inc. and its affiliates.
//
// This source code is licensed under both the MIT license found in the
// LICENSE-MIT file in the root directory of this source tree and the Apache
// License, Version 2.0 found in the LICENSE-APACHE file in the root directory
// of this source tree.

use digest::core_api::BlockSizeUser;
use digest::OutputSizeUser;
use elliptic_curve::group::cofactor::CofactorGroup;
use elliptic_curve::hash2curve::{ExpandMsgXmd, FromOkm, GroupDigest};
use elliptic_curve::sec1::{FromEncodedPoint, ModulusSize, ToEncodedPoint};
use elliptic_curve::{
    AffinePoint, Field, FieldSize, Group as _, ProjectivePoint, PublicKey, Scalar, SecretKey,
};
use generic_array::sequence::Concat;
use generic_array::typenum::{IsLess, IsLessOrEqual, U256};
use generic_array::GenericArray;
use rand_core::{CryptoRng, RngCore};

use super::Group;
use crate::group::{STR_HASH_TO_GROUP, STR_HASH_TO_SCALAR};
use crate::voprf::{self, Mode};
use crate::{CipherSuite, Error, Result};

impl<C> Group for C
where
    C: GroupDigest,
    ProjectivePoint<Self>: CofactorGroup,
    FieldSize<Self>: ModulusSize,
    AffinePoint<Self>: FromEncodedPoint<Self> + ToEncodedPoint<Self>,
    Scalar<Self>: FromOkm,
{
    type Elem = ProjectivePoint<Self>;

    type ElemLen = <FieldSize<Self> as ModulusSize>::CompressedPointSize;

    type Scalar = Scalar<Self>;

    type ScalarLen = FieldSize<Self>;

    // Implements the `hash_to_curve()` function from
    // https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-hash-to-curve-11#section-3
    fn hash_to_curve<CS: CipherSuite>(msg: &[&[u8]], mode: Mode) -> Result<Self::Elem>
    where
        <CS::Hash as OutputSizeUser>::OutputSize:
            IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
    {
        let dst =
            GenericArray::from(STR_HASH_TO_GROUP).concat(voprf::get_context_string::<CS>(mode));

        Self::hash_from_bytes::<ExpandMsgXmd<CS::Hash>>(msg, &dst).map_err(|_| Error::PointError)
    }

    // Implements the `HashToScalar()` function
    fn hash_to_scalar<CS: CipherSuite>(input: &[&[u8]], mode: Mode) -> Result<Self::Scalar>
    where
        <CS::Hash as OutputSizeUser>::OutputSize:
            IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
    {
        let dst =
            GenericArray::from(STR_HASH_TO_SCALAR).concat(voprf::get_context_string::<CS>(mode));

        <Self as GroupDigest>::hash_to_scalar::<ExpandMsgXmd<CS::Hash>>(input, &dst)
            .map_err(|_| Error::PointError)
    }

    fn base_elem() -> Self::Elem {
        ProjectivePoint::<Self>::generator()
    }

    fn identity_elem() -> Self::Elem {
        ProjectivePoint::<Self>::identity()
    }

    fn serialize_elem(elem: Self::Elem) -> GenericArray<u8, Self::ElemLen> {
        let point: AffinePoint<Self> = elem.into();
        let bytes = point.to_encoded_point(true);
        let bytes = bytes.as_bytes();
        let mut result = GenericArray::default();
        result[..bytes.len()].copy_from_slice(bytes);
        result
    }

    fn deserialize_elem(element_bits: &GenericArray<u8, Self::ElemLen>) -> Result<Self::Elem> {
        PublicKey::<Self>::from_sec1_bytes(element_bits)
            .map(|public_key| public_key.to_projective())
            .map_err(|_| Error::PointError)
    }

    fn random_scalar<R: RngCore + CryptoRng>(rng: &mut R) -> Self::Scalar {
        *SecretKey::<Self>::random(rng).to_nonzero_scalar()
    }

    fn invert_scalar(scalar: Self::Scalar) -> Self::Scalar {
        Option::from(scalar.invert()).unwrap()
    }

    #[cfg(test)]
    fn zero_scalar() -> Self::Scalar {
        Scalar::<Self>::zero()
    }

    fn serialize_scalar(scalar: Self::Scalar) -> GenericArray<u8, Self::ScalarLen> {
        scalar.into()
    }

    fn deserialize_scalar(scalar_bits: &GenericArray<u8, Self::ScalarLen>) -> Result<Self::Scalar> {
        SecretKey::<Self>::from_be_bytes(scalar_bits)
            .map(|secret_key| *secret_key.to_nonzero_scalar())
            .map_err(|_| Error::ScalarError)
    }
}