1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
// Copyright 2015-2016 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.

//! X25519 Key agreement.

use super::{ops, scalar::SCALAR_LEN};
use crate::{agreement, constant_time, cpu, ec, error, rand};
use core::convert::TryInto;

static CURVE25519: ec::Curve = ec::Curve {
    public_key_len: PUBLIC_KEY_LEN,
    elem_scalar_seed_len: ELEM_AND_SCALAR_LEN,
    id: ec::CurveID::Curve25519,
    check_private_key_bytes: x25519_check_private_key_bytes,
    generate_private_key: x25519_generate_private_key,
    public_from_private: x25519_public_from_private,
};

/// X25519 (ECDH using Curve25519) as described in [RFC 7748].
///
/// Everything is as described in RFC 7748. Key agreement will fail if the
/// result of the X25519 operation is zero; see the notes on the
/// "all-zero value" in [RFC 7748 section 6.1].
///
/// [RFC 7748]: https://tools.ietf.org/html/rfc7748
/// [RFC 7748 section 6.1]: https://tools.ietf.org/html/rfc7748#section-6.1
pub static X25519: agreement::Algorithm = agreement::Algorithm {
    curve: &CURVE25519,
    ecdh: x25519_ecdh,
};

fn x25519_check_private_key_bytes(bytes: &[u8]) -> Result<(), error::Unspecified> {
    debug_assert_eq!(bytes.len(), PRIVATE_KEY_LEN);
    Ok(())
}

fn x25519_generate_private_key(
    rng: &dyn rand::SecureRandom,
    out: &mut [u8],
) -> Result<(), error::Unspecified> {
    rng.fill(out)
}

fn x25519_public_from_private(
    public_out: &mut [u8],
    private_key: &ec::Seed,
) -> Result<(), error::Unspecified> {
    let public_out = public_out.try_into()?;

    #[cfg(target_arch = "arm")]
    let cpu_features = private_key.cpu_features;

    let private_key: &[u8; SCALAR_LEN] = private_key.bytes_less_safe().try_into()?;
    let private_key = ops::MaskedScalar::from_bytes_masked(*private_key);

    #[cfg(all(not(target_os = "ios"), target_arch = "arm"))]
    {
        if cpu::arm::NEON.available(cpu_features) {
            static MONTGOMERY_BASE_POINT: [u8; 32] = [
                9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                0, 0, 0, 0,
            ];
            x25519_neon(public_out, &private_key, &MONTGOMERY_BASE_POINT);
            return Ok(());
        }
    }

    extern "C" {
        fn GFp_x25519_public_from_private_generic_masked(
            public_key_out: &mut PublicKey,
            private_key: &PrivateKey,
        );
    }
    unsafe {
        GFp_x25519_public_from_private_generic_masked(public_out, &private_key);
    }

    Ok(())
}

fn x25519_ecdh(
    out: &mut [u8],
    my_private_key: &ec::Seed,
    peer_public_key: untrusted::Input,
) -> Result<(), error::Unspecified> {
    let cpu_features = my_private_key.cpu_features;
    let my_private_key: &[u8; SCALAR_LEN] = my_private_key.bytes_less_safe().try_into()?;
    let my_private_key = ops::MaskedScalar::from_bytes_masked(*my_private_key);
    let peer_public_key: &[u8; PUBLIC_KEY_LEN] = peer_public_key.as_slice_less_safe().try_into()?;

    #[cfg_attr(
        not(all(not(target_os = "ios"), target_arch = "arm")),
        allow(unused_variables)
    )]
    fn scalar_mult(
        out: &mut ops::EncodedPoint,
        scalar: &ops::MaskedScalar,
        point: &ops::EncodedPoint,
        cpu_features: cpu::Features,
    ) {
        #[cfg(all(not(target_os = "ios"), target_arch = "arm"))]
        {
            if cpu::arm::NEON.available(cpu_features) {
                return x25519_neon(out, scalar, point);
            }
        }

        extern "C" {
            fn GFp_x25519_scalar_mult_generic_masked(
                out: &mut ops::EncodedPoint,
                scalar: &ops::MaskedScalar,
                point: &ops::EncodedPoint,
            );
        }
        unsafe {
            GFp_x25519_scalar_mult_generic_masked(out, scalar, point);
        }
    }

    scalar_mult(
        out.try_into()?,
        &my_private_key,
        peer_public_key,
        cpu_features,
    );

    let zeros: SharedSecret = [0; SHARED_SECRET_LEN];
    if constant_time::verify_slices_are_equal(out, &zeros).is_ok() {
        // All-zero output results when the input is a point of small order.
        return Err(error::Unspecified);
    }

    Ok(())
}

#[cfg(all(not(target_os = "ios"), target_arch = "arm"))]
fn x25519_neon(out: &mut ops::EncodedPoint, scalar: &ops::MaskedScalar, point: &ops::EncodedPoint) {
    extern "C" {
        fn GFp_x25519_NEON(
            out: &mut ops::EncodedPoint,
            scalar: &ops::MaskedScalar,
            point: &ops::EncodedPoint,
        );
    }
    unsafe { GFp_x25519_NEON(out, scalar, point) }
}

const ELEM_AND_SCALAR_LEN: usize = ops::ELEM_LEN;

type PrivateKey = ops::MaskedScalar;
const PRIVATE_KEY_LEN: usize = ELEM_AND_SCALAR_LEN;

// An X25519 public key as an encoded Curve25519 point.
type PublicKey = [u8; PUBLIC_KEY_LEN];
const PUBLIC_KEY_LEN: usize = ELEM_AND_SCALAR_LEN;

// An X25519 shared secret as an encoded Curve25519 point.
type SharedSecret = [u8; SHARED_SECRET_LEN];
const SHARED_SECRET_LEN: usize = ELEM_AND_SCALAR_LEN;