src/ec/curve25519/scalar.rs

// Copyright 2015-2019 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

use crate::{digest, error, limb};
use core::convert::TryInto;

#[repr(transparent)]
pub struct Scalar([u8; SCALAR_LEN]);

pub const SCALAR_LEN: usize = 32;

impl Scalar {
    // Constructs a `Scalar` from `bytes`, failing if `bytes` encodes a scalar
    // that not in the range [0, n).
    pub fn from_bytes_checked(bytes: [u8; SCALAR_LEN]) -> Result<Self, error::Unspecified> {
        const ORDER: [limb::Limb; SCALAR_LEN / limb::LIMB_BYTES] =
            limbs![0x5cf5d3ed, 0x5812631a, 0xa2f79cd6, 0x14def9de, 0, 0, 0, 0x10000000];

        // `bytes` is in little-endian order.
        let mut reversed = bytes;
        reversed.reverse();

        let mut limbs = [0; SCALAR_LEN / limb::LIMB_BYTES];
        limb::parse_big_endian_in_range_and_pad_consttime(
            untrusted::Input::from(&reversed),
            limb::AllowZero::Yes,
            &ORDER,
            &mut limbs,
        )?;

        Ok(Self(bytes))
    }

    // Constructs a `Scalar` from `digest` reduced modulo n.
    pub fn from_sha512_digest_reduced(digest: digest::Digest) -> Self {
        prefixed_extern! {
            fn x25519_sc_reduce(s: &mut UnreducedScalar);
        }
        let mut unreduced = [0u8; digest::SHA512_OUTPUT_LEN];
        unreduced.copy_from_slice(digest.as_ref());
        unsafe { x25519_sc_reduce(&mut unreduced) };
        Self((&unreduced[..SCALAR_LEN]).try_into().unwrap())
    }
}

#[repr(transparent)]
pub struct MaskedScalar([u8; SCALAR_LEN]);

impl MaskedScalar {
    pub fn from_bytes_masked(bytes: [u8; SCALAR_LEN]) -> Self {
        prefixed_extern! {
            fn x25519_sc_mask(a: &mut [u8; SCALAR_LEN]);
        }
        let mut r = Self(bytes);
        unsafe { x25519_sc_mask(&mut r.0) };
        r
    }
}

impl From<MaskedScalar> for Scalar {
    fn from(MaskedScalar(scalar): MaskedScalar) -> Self {
        Self(scalar)
    }
}

type UnreducedScalar = [u8; UNREDUCED_SCALAR_LEN];
const UNREDUCED_SCALAR_LEN: usize = SCALAR_LEN * 2;