ring/ec/curve25519/
ops.rs

1// Copyright 2015-2017 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
15//! Elliptic curve operations on the birationally equivalent curves Curve25519
16//! and Edwards25519.
17
18pub use super::scalar::{MaskedScalar, Scalar, SCALAR_LEN};
19use crate::{
20    bssl, error,
21    limb::{Limb, LIMB_BITS},
22};
23use core::marker::PhantomData;
24
25// Elem<T>` is `fe` in curve25519/internal.h.
26// Elem<L> is `fe_loose` in curve25519/internal.h.
27// Keep this in sync with curve25519/internal.h.
28#[repr(C)]
29pub struct Elem<E: Encoding> {
30    limbs: [Limb; ELEM_LIMBS], // This is called `v` in the C code.
31    encoding: PhantomData<E>,
32}
33
34pub trait Encoding {}
35pub struct T;
36impl Encoding for T {}
37
38const ELEM_LIMBS: usize = 5 * 64 / LIMB_BITS;
39
40impl<E: Encoding> Elem<E> {
41    fn zero() -> Self {
42        Self {
43            limbs: Default::default(),
44            encoding: PhantomData,
45        }
46    }
47}
48
49impl Elem<T> {
50    fn negate(&mut self) {
51        unsafe {
52            GFp_x25519_fe_neg(self);
53        }
54    }
55}
56
57// An encoding of a curve point. If on Curve25519, it should be encoded as
58// described in Section 5 of [RFC 7748]. If on Edwards25519, it should be
59// encoded as described in section 5.1.2 of [RFC 8032].
60//
61// [RFC 7748] https://tools.ietf.org/html/rfc7748#section-5
62// [RFC 8032] https://tools.ietf.org/html/rfc8032#section-5.1.2
63pub type EncodedPoint = [u8; ELEM_LEN];
64pub const ELEM_LEN: usize = 32;
65
66// Keep this in sync with `ge_p3` in curve25519/internal.h.
67#[repr(C)]
68pub struct ExtPoint {
69    x: Elem<T>,
70    y: Elem<T>,
71    z: Elem<T>,
72    t: Elem<T>,
73}
74
75impl ExtPoint {
76    pub fn new_at_infinity() -> Self {
77        Self {
78            x: Elem::zero(),
79            y: Elem::zero(),
80            z: Elem::zero(),
81            t: Elem::zero(),
82        }
83    }
84
85    pub fn from_encoded_point_vartime(encoded: &EncodedPoint) -> Result<Self, error::Unspecified> {
86        let mut point = Self::new_at_infinity();
87
88        Result::from(unsafe { GFp_x25519_ge_frombytes_vartime(&mut point, encoded) })
89            .map(|()| point)
90    }
91
92    pub fn into_encoded_point(self) -> EncodedPoint {
93        encode_point(self.x, self.y, self.z)
94    }
95
96    pub fn invert_vartime(&mut self) {
97        self.x.negate();
98        self.t.negate();
99    }
100}
101
102// Keep this in sync with `ge_p2` in curve25519/internal.h.
103#[repr(C)]
104pub struct Point {
105    x: Elem<T>,
106    y: Elem<T>,
107    z: Elem<T>,
108}
109
110impl Point {
111    pub fn new_at_infinity() -> Self {
112        Self {
113            x: Elem::zero(),
114            y: Elem::zero(),
115            z: Elem::zero(),
116        }
117    }
118
119    pub fn into_encoded_point(self) -> EncodedPoint {
120        encode_point(self.x, self.y, self.z)
121    }
122}
123
124fn encode_point(x: Elem<T>, y: Elem<T>, z: Elem<T>) -> EncodedPoint {
125    let mut bytes = [0; ELEM_LEN];
126
127    let sign_bit: u8 = unsafe {
128        let mut recip = Elem::zero();
129        GFp_x25519_fe_invert(&mut recip, &z);
130
131        let mut x_over_z = Elem::zero();
132        GFp_x25519_fe_mul_ttt(&mut x_over_z, &x, &recip);
133
134        let mut y_over_z = Elem::zero();
135        GFp_x25519_fe_mul_ttt(&mut y_over_z, &y, &recip);
136        GFp_x25519_fe_tobytes(&mut bytes, &y_over_z);
137
138        GFp_x25519_fe_isnegative(&x_over_z)
139    };
140
141    // The preceding computations must execute in constant time, but this
142    // doesn't need to.
143    bytes[ELEM_LEN - 1] ^= sign_bit << 7;
144
145    bytes
146}
147
148extern "C" {
149    fn GFp_x25519_fe_invert(out: &mut Elem<T>, z: &Elem<T>);
150    fn GFp_x25519_fe_isnegative(elem: &Elem<T>) -> u8;
151    fn GFp_x25519_fe_mul_ttt(h: &mut Elem<T>, f: &Elem<T>, g: &Elem<T>);
152    fn GFp_x25519_fe_neg(f: &mut Elem<T>);
153    fn GFp_x25519_fe_tobytes(bytes: &mut EncodedPoint, elem: &Elem<T>);
154    fn GFp_x25519_ge_frombytes_vartime(h: &mut ExtPoint, s: &EncodedPoint) -> bssl::Result;
155}
ring/ec/curve25519/ops.rs

ring/ec/curve25519/
ops.rs
