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::{arithmetic::limbs_from_hex, digest, error, limb};
16
17#[repr(transparent)]
18pub struct Scalar([u8; SCALAR_LEN]);
19
20pub const SCALAR_LEN: usize = 32;
21
22impl Scalar {
23    // Constructs a `Scalar` from `bytes`, failing if `bytes` encodes a scalar
24    // that not in the range [0, n).
25    pub fn from_bytes_checked(bytes: [u8; SCALAR_LEN]) -> Result<Self, error::Unspecified> {
26        const ORDER: [limb::Limb; SCALAR_LEN / limb::LIMB_BYTES] =
27            limbs_from_hex("1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed");
28
29        // `bytes` is in little-endian order.
30        let mut reversed = bytes;
31        reversed.reverse();
32
33        let mut limbs = [0; SCALAR_LEN / limb::LIMB_BYTES];
34        limb::parse_big_endian_in_range_and_pad_consttime(
35            untrusted::Input::from(&reversed),
36            limb::AllowZero::Yes,
37            &ORDER,
38            &mut limbs,
39        )?;
40
41        Ok(Self(bytes))
42    }
43
44    // Constructs a `Scalar` from `digest` reduced modulo n.
45    pub fn from_sha512_digest_reduced(digest: digest::Digest) -> Self {
46        prefixed_extern! {
47            fn x25519_sc_reduce(s: &mut UnreducedScalar);
48        }
49        let mut unreduced = [0u8; digest::SHA512_OUTPUT_LEN];
50        unreduced.copy_from_slice(digest.as_ref());
51        unsafe { x25519_sc_reduce(&mut unreduced) };
52        Self((&unreduced[..SCALAR_LEN]).try_into().unwrap())
53    }
54}
55
56#[repr(transparent)]
57pub struct MaskedScalar([u8; SCALAR_LEN]);
58
59impl MaskedScalar {
60    pub fn from_bytes_masked(bytes: [u8; SCALAR_LEN]) -> Self {
61        prefixed_extern! {
62            fn x25519_sc_mask(a: &mut [u8; SCALAR_LEN]);
63        }
64        let mut r = Self(bytes);
65        unsafe { x25519_sc_mask(&mut r.0) };
66        r
67    }
68}
69
70impl From<MaskedScalar> for Scalar {
71    fn from(MaskedScalar(scalar): MaskedScalar) -> Self {
72        Self(scalar)
73    }
74}
75
76type UnreducedScalar = [u8; UNREDUCED_SCALAR_LEN];
77const UNREDUCED_SCALAR_LEN: usize = SCALAR_LEN * 2;
ring/ec/curve25519/scalar.rs

ring/ec/curve25519/
scalar.rs
