ring/ec/curve25519/
scalar.rs

1// Copyright 2015-2019 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15use crate::{digest, error, limb};
16use core::convert::TryInto;
17
18#[repr(transparent)]
19pub struct Scalar([u8; SCALAR_LEN]);
20
21pub const SCALAR_LEN: usize = 32;
22
23impl Scalar {
24    // Constructs a `Scalar` from `bytes`, failing if `bytes` encodes a scalar
25    // that not in the range [0, n).
26    pub fn from_bytes_checked(bytes: [u8; SCALAR_LEN]) -> Result<Self, error::Unspecified> {
27        const ORDER: [limb::Limb; SCALAR_LEN / limb::LIMB_BYTES] =
28            limbs![0x5cf5d3ed, 0x5812631a, 0xa2f79cd6, 0x14def9de, 0, 0, 0, 0x10000000];
29
30        // `bytes` is in little-endian order.
31        let mut reversed = bytes;
32        reversed.reverse();
33
34        let mut limbs = [0; SCALAR_LEN / limb::LIMB_BYTES];
35        limb::parse_big_endian_in_range_and_pad_consttime(
36            untrusted::Input::from(&reversed),
37            limb::AllowZero::Yes,
38            &ORDER,
39            &mut limbs,
40        )?;
41
42        Ok(Self(bytes))
43    }
44
45    // Constructs a `Scalar` from `digest` reduced modulo n.
46    pub fn from_sha512_digest_reduced(digest: digest::Digest) -> Self {
47        extern "C" {
48            fn GFp_x25519_sc_reduce(s: &mut UnreducedScalar);
49        }
50        let mut unreduced = [0u8; digest::SHA512_OUTPUT_LEN];
51        unreduced.copy_from_slice(digest.as_ref());
52        unsafe { GFp_x25519_sc_reduce(&mut unreduced) };
53        Self((&unreduced[..SCALAR_LEN]).try_into().unwrap())
54    }
55}
56
57#[repr(transparent)]
58pub struct MaskedScalar([u8; SCALAR_LEN]);
59
60impl MaskedScalar {
61    pub fn from_bytes_masked(bytes: [u8; SCALAR_LEN]) -> Self {
62        extern "C" {
63            fn GFp_x25519_sc_mask(a: &mut [u8; SCALAR_LEN]);
64        }
65        let mut r = Self(bytes);
66        unsafe { GFp_x25519_sc_mask(&mut r.0) };
67        r
68    }
69}
70
71impl From<MaskedScalar> for Scalar {
72    fn from(MaskedScalar(scalar): MaskedScalar) -> Self {
73        Self(scalar)
74    }
75}
76
77type UnreducedScalar = [u8; UNREDUCED_SCALAR_LEN];
78const UNREDUCED_SCALAR_LEN: usize = SCALAR_LEN * 2;
ring/ec/curve25519/scalar.rs

ring/ec/curve25519/
scalar.rs
