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
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
// Copyright 2015-2017 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.

//! ECDH key agreement using the P-256 and P-384 curves.

use super::{ops::*, private_key::*, public_key::*};
use crate::{agreement, ec, error};

/// A key agreement algorithm.
macro_rules! ecdh {
    ( $NAME:ident, $curve:expr, $name_str:expr, $private_key_ops:expr,
      $public_key_ops:expr, $ecdh:ident ) => {
        #[doc = "ECDH using the NSA Suite B"]
        #[doc=$name_str]
        #[doc = "curve."]
        ///
        /// Public keys are encoding in uncompressed form using the
        /// Octet-String-to-Elliptic-Curve-Point algorithm in
        /// [SEC 1: Elliptic Curve Cryptography, Version 2.0]. Public keys are
        /// validated during key agreement according to
        /// [NIST Special Publication 800-56A, revision 2] and Appendix B.3 of
        /// the NSA's [Suite B Implementer's Guide to NIST SP 800-56A].
        ///
        /// [SEC 1: Elliptic Curve Cryptography, Version 2.0]:
        ///     http://www.secg.org/sec1-v2.pdf
        /// [NIST Special Publication 800-56A, revision 2]:
        ///     http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf
        /// [Suite B Implementer's Guide to NIST SP 800-56A]:
        ///     https://github.com/briansmith/ring/blob/main/doc/ecdh.pdf
        pub static $NAME: agreement::Algorithm = agreement::Algorithm {
            curve: $curve,
            ecdh: $ecdh,
        };

        fn $ecdh(
            out: &mut [u8],
            my_private_key: &ec::Seed,
            peer_public_key: untrusted::Input,
        ) -> Result<(), error::Unspecified> {
            ecdh(
                $private_key_ops,
                $public_key_ops,
                out,
                my_private_key,
                peer_public_key,
            )
        }
    };
}

ecdh!(
    ECDH_P256,
    &ec::suite_b::curve::P256,
    "P-256 (secp256r1)",
    &p256::PRIVATE_KEY_OPS,
    &p256::PUBLIC_KEY_OPS,
    p256_ecdh
);

ecdh!(
    ECDH_P384,
    &ec::suite_b::curve::P384,
    "P-384 (secp384r1)",
    &p384::PRIVATE_KEY_OPS,
    &p384::PUBLIC_KEY_OPS,
    p384_ecdh
);

fn ecdh(
    private_key_ops: &PrivateKeyOps,
    public_key_ops: &PublicKeyOps,
    out: &mut [u8],
    my_private_key: &ec::Seed,
    peer_public_key: untrusted::Input,
) -> Result<(), error::Unspecified> {
    // The NIST SP 800-56Ar2 steps are from section 5.7.1.2 Elliptic Curve
    // Cryptography Cofactor Diffie-Hellman (ECC CDH) Primitive.
    //
    // The "NSA Guide" steps are from section 3.1 of the NSA guide, "Ephemeral
    // Unified Model."

    // NSA Guide Step 1 is handled separately.

    // NIST SP 800-56Ar2 5.6.2.2.2.
    // NSA Guide Step 2.
    //
    // `parse_uncompressed_point` verifies that the point is not at infinity
    // and that it is on the curve, using the Partial Public-Key Validation
    // Routine.
    let peer_public_key = parse_uncompressed_point(public_key_ops, peer_public_key)?;

    // NIST SP 800-56Ar2 Step 1.
    // NSA Guide Step 3 (except point at infinity check).
    //
    // Note that the cofactor (h) is one since we only support prime-order
    // curves, so we can safely ignore the cofactor.
    //
    // It is impossible for the result to be the point at infinity because our
    // private key is in the range [1, n) and the curve has prime order and
    // `parse_uncompressed_point` verified that the peer public key is on the
    // curve and not at infinity. However, since the standards require the
    // check, we do it using `assert!`.
    //
    // NIST SP 800-56Ar2 defines "Destroy" thusly: "In this Recommendation, to
    // destroy is an action applied to a key or a piece of secret data. After
    // a key or a piece of secret data is destroyed, no information about its
    // value can be recovered." We interpret "destroy" somewhat liberally: we
    // assume that since we throw away the values to be destroyed, no
    // information about their values can be recovered. This doesn't meet the
    // NSA guide's explicit requirement to "zeroize" them though.
    // TODO: this only needs common scalar ops
    let my_private_key = private_key_as_scalar(private_key_ops, my_private_key);
    let product = private_key_ops.point_mul(&my_private_key, &peer_public_key);

    // NIST SP 800-56Ar2 Steps 2, 3, 4, and 5.
    // NSA Guide Steps 3 (point at infinity check) and 4.
    //
    // Again, we have a pretty liberal interpretation of the NIST's spec's
    // "Destroy" that doesn't meet the NSA requirement to "zeroize."
    // `big_endian_affine_from_jacobian` verifies that the result is not at
    // infinity and also does an extra check to verify that the point is on
    // the curve.
    big_endian_affine_from_jacobian(private_key_ops, Some(out), None, &product)

    // NSA Guide Step 5 & 6 are deferred to the caller. Again, we have a
    // pretty liberal interpretation of the NIST's spec's "Destroy" that
    // doesn't meet the NSA requirement to "zeroize."
}

#[cfg(test)]
mod tests {
    use super::super::ops;
    use crate::{agreement, ec, limb, test};

    static SUPPORTED_SUITE_B_ALGS: [(&str, &agreement::Algorithm, &ec::Curve, &ops::CommonOps); 2] = [
        (
            "P-256",
            &agreement::ECDH_P256,
            &super::super::curve::P256,
            &super::super::ops::p256::COMMON_OPS,
        ),
        (
            "P-384",
            &agreement::ECDH_P384,
            &super::super::curve::P384,
            &super::super::ops::p384::COMMON_OPS,
        ),
    ];

    #[test]
    fn test_agreement_suite_b_ecdh_generate() {
        // Generates a string of bytes 0x00...00, which will always result in
        // a scalar value of zero.
        let random_00 = test::rand::FixedByteRandom { byte: 0x00 };

        // Generates a string of bytes 0xFF...FF, which will be larger than the
        // group order of any curve that is supported.
        let random_ff = test::rand::FixedByteRandom { byte: 0xff };

        for &(_, alg, curve, ops) in SUPPORTED_SUITE_B_ALGS.iter() {
            // Test that the private key value zero is rejected and that
            // `generate` gives up after a while of only getting zeros.
            assert!(agreement::EphemeralPrivateKey::generate(alg, &random_00).is_err());

            // Test that the private key value larger than the group order is
            // rejected and that `generate` gives up after a while of only
            // getting values larger than the group order.
            assert!(agreement::EphemeralPrivateKey::generate(alg, &random_ff).is_err());

            // Test that a private key value exactly equal to the group order
            // is rejected and that `generate` gives up after a while of only
            // getting that value from the PRNG.
            let mut n_bytes = [0u8; ec::SCALAR_MAX_BYTES];
            let num_bytes = curve.elem_scalar_seed_len;
            limb::big_endian_from_limbs(ops.n_limbs(), &mut n_bytes[..num_bytes]);
            {
                let n_bytes = &mut n_bytes[..num_bytes];
                let rng = test::rand::FixedSliceRandom { bytes: n_bytes };
                assert!(agreement::EphemeralPrivateKey::generate(alg, &rng).is_err());
            }

            // Test that a private key value exactly equal to the group order
            // minus 1 is accepted.
            let mut n_minus_1_bytes = n_bytes;
            {
                let n_minus_1_bytes = &mut n_minus_1_bytes[..num_bytes];
                n_minus_1_bytes[num_bytes - 1] -= 1;
                let rng = test::rand::FixedSliceRandom {
                    bytes: n_minus_1_bytes,
                };
                let key = agreement::EphemeralPrivateKey::generate(alg, &rng).unwrap();
                assert_eq!(n_minus_1_bytes, key.bytes());
            }

            // Test that n + 1 also fails.
            let mut n_plus_1_bytes = n_bytes;
            {
                let n_plus_1_bytes = &mut n_plus_1_bytes[..num_bytes];
                n_plus_1_bytes[num_bytes - 1] += 1;
                let rng = test::rand::FixedSliceRandom {
                    bytes: n_plus_1_bytes,
                };
                assert!(agreement::EphemeralPrivateKey::generate(alg, &rng).is_err());
            }

            // Test recovery from initial RNG failure. The first value will be
            // n, then n + 1, then zero, the next value will be n - 1, which
            // will be accepted.
            {
                let bytes = [
                    &n_bytes[..num_bytes],
                    &n_plus_1_bytes[..num_bytes],
                    &[0u8; ec::SCALAR_MAX_BYTES][..num_bytes],
                    &n_minus_1_bytes[..num_bytes],
                ];
                let rng = test::rand::FixedSliceSequenceRandom {
                    bytes: &bytes,
                    current: core::cell::UnsafeCell::new(0),
                };
                let key = agreement::EphemeralPrivateKey::generate(alg, &rng).unwrap();
                assert_eq!(&n_minus_1_bytes[..num_bytes], key.bytes());
            }
        }
    }
}