voprf-vx/src/group/mod.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.

//! Defines the Group trait to specify the underlying prime order group used in
//! OPAQUE's OPRF

mod expand;
#[cfg(feature = "p256")]
pub(crate) mod p256;
mod ristretto;

use crate::errors::InternalError;
use crate::hash::Hash;
use core::ops::{Add, Mul, Sub};
use generic_array::{ArrayLength, GenericArray};
use rand::{CryptoRng, RngCore};
use zeroize::Zeroize;

/// A prime-order subgroup of a base field (EC, prime-order field ...). This
/// subgroup is noted additively — as in the draft RFC — in this trait.
pub trait Group:
    Copy
    + Sized
    + for<'a> Mul<&'a <Self as Group>::Scalar, Output = Self>
    + for<'a> Add<&'a Self, Output = Self>
{
    /// The ciphersuite identifier as dictated by
    /// <https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-05.txt>
    const SUITE_ID: usize;

    /// transforms a password and domain separation tag (DST) into a curve point
    fn hash_to_curve<H: Hash>(msg: &[u8], dst: &[u8]) -> Result<Self, InternalError>;

    /// Hashes a slice of pseudo-random bytes to a scalar
    fn hash_to_scalar<H: Hash>(input: &[u8], dst: &[u8]) -> Result<Self::Scalar, InternalError>;

    /// The type of base field scalars
    type Scalar: Zeroize
        + Copy
        + for<'a> Add<&'a Self::Scalar, Output = Self::Scalar>
        + for<'a> Sub<&'a Self::Scalar, Output = Self::Scalar>
        + for<'a> Mul<&'a Self::Scalar, Output = Self::Scalar>;
    /// The byte length necessary to represent scalars
    type ScalarLen: ArrayLength<u8> + 'static;

    /// Return a scalar from its fixed-length bytes representation, without
    /// checking if the scalar is zero.
    fn from_scalar_slice_unchecked(
        scalar_bits: &GenericArray<u8, Self::ScalarLen>,
    ) -> Result<Self::Scalar, InternalError>;

    /// Return a scalar from its fixed-length bytes representation. If the scalar
    /// is zero, then return an error.
    fn from_scalar_slice(
        scalar_bits: &GenericArray<u8, Self::ScalarLen>,
    ) -> Result<Self::Scalar, InternalError> {
        let scalar = Self::from_scalar_slice_unchecked(scalar_bits)?;
        if Self::ct_equal_scalar(&scalar, &Self::scalar_zero()) {
            return Err(InternalError::ZeroScalarError);
        }
        Ok(scalar)
    }

    /// picks a scalar at random
    fn random_nonzero_scalar<R: RngCore + CryptoRng>(rng: &mut R) -> Self::Scalar;
    /// Serializes a scalar to bytes
    fn scalar_as_bytes(scalar: Self::Scalar) -> GenericArray<u8, Self::ScalarLen>;
    /// The multiplicative inverse of this scalar
    fn scalar_invert(scalar: &Self::Scalar) -> Self::Scalar;

    /// The byte length necessary to represent group elements
    type ElemLen: ArrayLength<u8> + 'static;

    /// Return an element from its fixed-length bytes representation. This is
    /// the unchecked version, which does not check for deserializing the identity
    /// element
    fn from_element_slice_unchecked(
        element_bits: &GenericArray<u8, Self::ElemLen>,
    ) -> Result<Self, InternalError>;

    /// Return an element from its fixed-length bytes representation. If the element
    /// is the identity element, return an error.
    fn from_element_slice(
        element_bits: &GenericArray<u8, Self::ElemLen>,
    ) -> Result<Self, InternalError> {
        let elem = Self::from_element_slice_unchecked(element_bits)?;

        if Self::ct_equal(&elem, &<Self as Group>::identity()) {
            // found the identity element
            return Err(InternalError::PointError);
        }

        Ok(elem)
    }

    /// Serializes the `self` group element
    fn to_arr(&self) -> GenericArray<u8, Self::ElemLen>;

    /// Get the base point for the group
    fn base_point() -> Self;

    /// Returns if the group element is equal to the identity (1)
    fn is_identity(&self) -> bool {
        self.ct_equal(&<Self as Group>::identity())
    }

    /// Returns the identity group element
    fn identity() -> Self;

    /// Returns the scalar representing zero
    fn scalar_zero() -> Self::Scalar;

    /// Compares in constant time if the group elements are equal
    fn ct_equal(&self, other: &Self) -> bool;

    /// Compares in constant time if the scalars are equal
    fn ct_equal_scalar(s1: &Self::Scalar, s2: &Self::Scalar) -> bool;
}

#[cfg(test)]
mod tests;