ring/ec/curve25519/scalar.rs
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// 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::{arithmetic::limbs_from_hex, digest, error, limb};
#[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_from_hex("1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed");
// `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;