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

    /// 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/master/doc/ecdh.pdf
            pub static $NAME: agreement::Algorithm = agreement::Algorithm {
                i: ec::AgreementAlgorithmImpl {
                    curve: $curve,
                    ecdh: $ecdh,
                },
            };

            fn $ecdh(out: &mut [u8], my_private_key: &ec::PrivateKey,
                     peer_public_key: untrusted::Input)
                     -> Result<(), error::Unspecified> {
                ecdh($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::PrivateKey,
            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.
        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 core;
        use {agreement, ec, limb, test};
        use super::super::ops;

        static SUPPORTED_SUITE_B_ALGS:
            [(&'static str, &'static agreement::Algorithm,
              &'static ec::Curve,
              &'static 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_and_scalar_len;
                limb::big_endian_from_limbs_padded(&ops.n.limbs[..ops.num_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(curve));
                }

                // 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(curve));
                }
            }
        }
    }

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