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    // 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.

    //! ECDSA Signatures using the P-256 and P-384 curves.

    use arithmetic::montgomery::*;
    use core;
    use {der, digest, ec, error, pkcs8, private, rand, signature};
    use super::verify_jacobian_point_is_on_the_curve;
    use super::ops::*;
    use super::public_key::*;
    use untrusted;

    /// An ECDSA signing algorithm.
    pub struct ECDSASigningAlgorithm {
        curve: &'static ec::Curve,
        pkcs8_template: &'static pkcs8::Template,
    }

    /// An ECDSA verification algorithm.
    pub struct ECDSAVerificationAlgorithm {
        ops: &'static PublicScalarOps,
        digest_alg: &'static digest::Algorithm,
        split_rs:
            for<'a> fn(ops: &'static ScalarOps, input: &mut untrusted::Reader<'a>)
                       -> Result<(untrusted::Input<'a>, untrusted::Input<'a>),
                                 error::Unspecified>,
        id: ECDSAVerificationAlgorithmID,
    }

    #[allow(non_camel_case_types)]
    enum ECDSAVerificationAlgorithmID {
        ECDSA_P256_SHA256_ASN1,
        ECDSA_P256_SHA256_FIXED,
        ECDSA_P256_SHA384_ASN1,
        ECDSA_P384_SHA256_ASN1,
        ECDSA_P384_SHA384_ASN1,
        ECDSA_P384_SHA384_FIXED,
    }

    impl core::fmt::Debug for ECDSAVerificationAlgorithm {
        fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
            use self::ECDSAVerificationAlgorithmID::*;
            write!(f, "ring::signature::{}", match self.id {
                ECDSA_P256_SHA256_ASN1 => "ECDSA_P256_SHA256_ASN1",
                ECDSA_P256_SHA256_FIXED => "ECDSA_P256_SHA256_FIXED",
                ECDSA_P256_SHA384_ASN1 => "ECDSA_P256_SHA384_ASN1",
                ECDSA_P384_SHA256_ASN1 => "ECDSA_P384_SHA256_ASN1",
                ECDSA_P384_SHA384_ASN1 => "ECDSA_P384_SHA384_ASN1",
                ECDSA_P384_SHA384_FIXED => "ECDSA_P384_SHA384_FIXED",
            })
        }
    }

    impl signature::VerificationAlgorithm for ECDSAVerificationAlgorithm {
        // Verify an ECDSA signature as documented in the NSA Suite B Implementer's
        // Guide to ECDSA Section 3.4.2: ECDSA Signature Verification.
        fn verify(&self, public_key: untrusted::Input, msg: untrusted::Input,
                  signature: untrusted::Input) -> Result<(), error::Unspecified> {
            let public_key_ops = self.ops.public_key_ops;
            let scalar_ops = self.ops.scalar_ops;

            // NSA Guide Prerequisites:
            //
            //    Prior to accepting a verified digital signature as valid the
            //    verifier shall have:
            //
            //       1. assurance of the signatory’s claimed identity,
            //       2. an authentic copy of the domain parameters, (q, FR, a, b,
            //          SEED, G, n, h),
            //       3. assurance of the validity of the public key, and
            //       4. assurance that the claimed signatory actually possessed the
            //          private key that was used to generate the digital signature
            //          at the time that the signature was generated.
            //
            // Prerequisites #1 and #4 are outside the scope of what this function
            // can do. Prerequisite #2 is handled implicitly as the domain
            // parameters are hard-coded into the source. Prerequisite #3 is
            // handled by `parse_uncompressed_point`.
            let peer_pub_key = parse_uncompressed_point(public_key_ops, public_key)?;

            let (r, s) = signature.read_all(
                error::Unspecified, |input| (self.split_rs)(scalar_ops, input))?;

            // NSA Guide Step 1: "If r and s are not both integers in the interval
            // [1, n − 1], output INVALID."
            let r = scalar_parse_big_endian_variable(public_key_ops.common,
                                                     AllowZero::No, r)?;
            let s = scalar_parse_big_endian_variable(public_key_ops.common,
                                                     AllowZero::No, s)?;

            // NSA Guide Step 2: "Use the selected hash function to compute H =
            // Hash(M)."
            // NSA Guide Step 3: "Convert the bit string H to an integer e as
            // described in Appendix B.2."
            let e = digest_scalar(scalar_ops, self.digest_alg, msg);

            // NSA Guide Step 4: "Compute w = s**−1 mod n, using the routine in
            // Appendix B.1."
            let w = scalar_ops.scalar_inv_to_mont(&s);

            // NSA Guide Step 5: "Compute u1 = (e * w) mod n, and compute
            // u2 = (r * w) mod n."
            let u1 = scalar_ops.scalar_product(&e, &w);
            let u2 = scalar_ops.scalar_product(&r, &w);

            // NSA Guide Step 6: "Compute the elliptic curve point
            // R = (xR, yR) = u1*G + u2*Q, using EC scalar multiplication and EC
            // addition. If R is equal to the point at infinity, output INVALID."
            let product =
                twin_mul(self.ops.private_key_ops, &u1, &u2, &peer_pub_key);

            // Verify that the point we computed is on the curve; see
            // `verify_affine_point_is_on_the_curve_scaled` for details on why. It
            // would be more secure to do the check on the affine coordinates if we
            // were going to convert to affine form (again, see
            // `verify_affine_point_is_on_the_curve_scaled` for details on why).
            // But, we're going to avoid converting to affine for performance
            // reasons, so we do the verification using the Jacobian coordinates.
            let z2 = verify_jacobian_point_is_on_the_curve(public_key_ops.common,
                                                           &product)?;

            // NSA Guide Step 7: "Compute v = xR mod n."
            // NSA Guide Step 8: "Compare v and r0. If v = r0, output VALID;
            // otherwise, output INVALID."
            //
            // Instead, we use Greg Maxwell's trick to avoid the inversion mod `q`
            // that would be necessary to compute the affine X coordinate.
            let x = public_key_ops.common.point_x(&product);
            fn sig_r_equals_x(ops: &PublicScalarOps, r: &Elem<Unencoded>,
                              x: &Elem<R>, z2: &Elem<R>) -> bool {
                let cops = ops.public_key_ops.common;
                let r_jacobian = cops.elem_product(z2, r);
                let x = cops.elem_unencoded(x);
                ops.elem_equals(&r_jacobian, &x)
            }
            let r = self.ops.scalar_as_elem(&r);
            if sig_r_equals_x(self.ops, &r, &x, &z2) {
                return Ok(());
            }
            if self.ops.elem_less_than(&r, &self.ops.q_minus_n) {
                let r_plus_n =
                    self.ops.elem_sum(&r, &public_key_ops.common.n);
                if sig_r_equals_x(self.ops, &r_plus_n, &x, &z2) {
                    return Ok(());
                }
            }

            Err(error::Unspecified)
        }
    }

    impl private::Private for ECDSAVerificationAlgorithm {}

    /// An ECDSA key pair, used for signing.
    #[doc(hidden)]
    pub struct ECDSAKeyPair {
        #[allow(dead_code)] // XXX: Temporary, since signing isn't implemented yet.
        key_pair: ec::KeyPair,

        #[allow(dead_code)] // XXX: Temporary, since signing isn't implemented yet.
        alg: &'static ECDSASigningAlgorithm,
    }

    impl<'a> ECDSAKeyPair {
        /// Generates a new key pair and returns the key pair serialized as a
        /// PKCS#8 document.
        ///
        /// The PKCS#8 document will be a v1 `OneAsymmetricKey` with the public key
        /// included in the `ECPrivateKey` structure, as described in
        /// [RFC 5958 Section 2] and [RFC 5915]. The `ECPrivateKey` structure will
        /// not have a `parameters` field so the generated key is compatible with
        /// PKCS#11.
        ///
        /// [RFC 5915]: https://tools.ietf.org/html/rfc5915
        /// [RFC 5958 Section 2]: https://tools.ietf.org/html/rfc5958#section-2
        pub fn generate_pkcs8(alg: &'static ECDSASigningAlgorithm,
                              rng: &rand::SecureRandom)
                              -> Result<pkcs8::PKCS8Document, error::Unspecified> {
            let private_key = ec::PrivateKey::generate(alg.curve, rng)?;
            let mut public_key_bytes = [0; ec::PUBLIC_KEY_MAX_LEN];
            let public_key_bytes = &mut public_key_bytes[..alg.curve.public_key_len];
            (alg.curve.public_from_private)(public_key_bytes, &private_key)?;
            Ok(pkcs8::wrap_key(&alg.pkcs8_template, private_key.bytes(alg.curve),
                               public_key_bytes))
        }

        /// Constructs an ECDSA key pair by parsing an unencrypted PKCS#8 v1
        /// id-ecPublicKey `ECPrivateKey` key.
        ///
        /// The input must be in PKCS#8 v1 format. It must contain the public key in
        /// the `ECPrivateKey` structure; `from_pkcs8()` will verify that the public
        /// key and the private key are consistent with each other. The algorithm
        /// identifier must identify the curve by name; it must not use an
        /// "explicit" encoding of the curve. The `parameters` field of the
        /// `ECPrivateKey`, if present, must be the same named curve that is in the
        /// algorithm identifier in the PKCS#8 header.
        pub fn from_pkcs8(alg: &'static ECDSASigningAlgorithm,
                          input: untrusted::Input)
                          -> Result<ECDSAKeyPair, error::Unspecified> {
            let key_pair = ec::suite_b::key_pair_from_pkcs8(alg.curve,
                alg.pkcs8_template, input)?;
            Ok(ECDSAKeyPair { key_pair, alg })
        }

        /// Constructs an ECDSA key pair directly from the big-endian-encoded
        /// private key and public key bytes.
        ///
        /// This is intended for use by code that deserializes key pairs. It is
        /// recommended to use `ECDSAKeyPair::from_pkcs8()` (with a PKCS#8-encoded
        /// key) instead.
        pub fn from_private_key_and_public_key(alg: &'static ECDSASigningAlgorithm,
                                               private_key: untrusted::Input,
                                               public_key: untrusted::Input)
                          -> Result<ECDSAKeyPair, error::Unspecified> {
            let key_pair = ec::suite_b::key_pair_from_bytes(
                alg.curve, private_key, public_key)?;
            Ok(ECDSAKeyPair { key_pair, alg })
        }
    }

    fn split_rs_fixed<'a>(
            ops: &'static ScalarOps, input: &mut untrusted::Reader<'a>)
            -> Result<(untrusted::Input<'a>, untrusted::Input<'a>),
                      error::Unspecified> {
        let scalar_len = ops.scalar_bytes_len();
        let r = input.skip_and_get_input(scalar_len)?;
        let s = input.skip_and_get_input(scalar_len)?;
        Ok((r, s))
    }

    fn split_rs_asn1<'a>(
            _ops: &'static ScalarOps, input: &mut untrusted::Reader<'a>)
            -> Result<(untrusted::Input<'a>, untrusted::Input<'a>),
                      error::Unspecified> {
        der::nested(input, der::Tag::Sequence, error::Unspecified, |input| {
            let r = der::positive_integer(input)?;
            let s = der::positive_integer(input)?;
            Ok((r, s))
        })
    }

    /// Calculate the digest of `msg` using the digest algorithm `digest_alg`. Then
    /// convert the digest to a scalar in the range [0, n) as described in
    /// NIST's FIPS 186-4 Section 4.2. Note that this is one of the few cases where
    /// a `Scalar` is allowed to have the value zero.
    ///
    /// NIST's FIPS 186-4 4.2 says "When the length of the output of the hash
    /// function is greater than N (i.e., the bit length of q), then the leftmost N
    /// bits of the hash function output block shall be used in any calculation
    /// using the hash function output during the generation or verification of a
    /// digital signature."
    ///
    /// "Leftmost N bits" means "N most significant bits" because we interpret the
    /// digest as a bit-endian encoded integer.
    ///
    /// The NSA guide instead vaguely suggests that we should convert the digest
    /// value to an integer and then reduce it mod `n`. However, real-world
    /// implementations (e.g. `digest_to_bn` in OpenSSL and `hashToInt` in Go) do
    /// what FIPS 186-4 says to do, not what the NSA guide suggests.
    ///
    /// Why shifting the value right by at most one bit is sufficient: P-256's `n`
    /// has its 256th bit set; i.e. 2**255 < n < 2**256. Once we've truncated the
    /// digest to 256 bits and converted it to an integer, it will have a value
    /// less than 2**256. If the value is larger than `n` then shifting it one bit
    /// right will give a value less than 2**255, which is less than `n`. The
    /// analogous argument applies for P-384. However, it does *not* apply in
    /// general; for example, it doesn't apply to P-521.
    fn digest_scalar(ops: &ScalarOps, digest_alg: &'static digest::Algorithm,
                     msg: untrusted::Input) -> Scalar {
        let digest = digest::digest(digest_alg, msg.as_slice_less_safe());
        digest_scalar_(ops, digest.as_ref())
    }

    // This is a separate function solely so that we can test specific digest
    // values like all-zero values and values larger than `n`.
    fn digest_scalar_(ops: &ScalarOps, digest: &[u8]) -> Scalar {
        let cops = ops.common;
        let num_limbs = cops.num_limbs;
        let digest = if digest.len() > num_limbs * LIMB_BYTES {
            &digest[..(num_limbs * LIMB_BYTES)]
        } else {
            digest
        };

        scalar_parse_big_endian_partially_reduced_variable_consttime(
            cops, AllowZero::Yes, untrusted::Input::from(digest)).unwrap()
    }

    fn twin_mul(ops: &PrivateKeyOps, g_scalar: &Scalar, p_scalar: &Scalar,
                p_xy: &(Elem<R>, Elem<R>)) -> Point {
        // XXX: Inefficient. TODO: implement interleaved wNAF multiplication.
        let scaled_g = ops.point_mul_base(g_scalar);
        let scaled_p = ops.point_mul(p_scalar, p_xy);
        ops.common.point_sum(&scaled_g, &scaled_p)
    }

    /// Signing of fixed-length (PKCS#11 style) ECDSA signatures using the
    /// P-256 curve and SHA-256.
    ///
    /// See "`ECDSA_*_FIXED` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    #[doc(hidden)]
    pub static ECDSA_P256_SHA256_FIXED_SIGNING: ECDSASigningAlgorithm =
            ECDSASigningAlgorithm {
        curve: &ec::suite_b::curve::P256,
        pkcs8_template: &EC_PUBLIC_KEY_P256_PKCS8_V1_TEMPLATE,
    };

    /// Verification of fixed-length (PKCS#11 style) ECDSA signatures using the
    /// P-256 curve and SHA-256.
    ///
    /// See "`ECDSA_*_FIXED` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P256_SHA256_FIXED: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p256::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA256,
        split_rs: split_rs_fixed,
        id: ECDSAVerificationAlgorithmID::ECDSA_P256_SHA256_FIXED,
    };

    /// Signing of fixed-length (PKCS#11 style) ECDSA signatures using the
    /// P-384 curve and SHA-384.
    ///
    /// See "`ECDSA_*_FIXED` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    #[doc(hidden)]
    pub static ECDSA_P384_SHA384_FIXED_SIGNING: ECDSASigningAlgorithm =
            ECDSASigningAlgorithm {
        curve: &ec::suite_b::curve::P384,
        pkcs8_template: &EC_PUBLIC_KEY_P384_PKCS8_V1_TEMPLATE,
    };

    /// Verification of fixed-length (PKCS#11 style) ECDSA signatures using the
    /// P-384 curve and SHA-384.
    ///
    /// See "`ECDSA_*_FIXED` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P384_SHA384_FIXED: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p384::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA384,
        split_rs: split_rs_fixed,
        id: ECDSAVerificationAlgorithmID::ECDSA_P384_SHA384_FIXED,
    };

    /// Signing of ASN.1 DER-encoded ECDSA signatures using the P-256 curve and
    /// SHA-256.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    #[doc(hidden)]
    pub static ECDSA_P256_SHA256_ASN1_SIGNING: ECDSASigningAlgorithm =
            ECDSASigningAlgorithm {
        curve: &ec::suite_b::curve::P256,
        pkcs8_template: &EC_PUBLIC_KEY_P256_PKCS8_V1_TEMPLATE,
    };

    /// Verification of ASN.1 DER-encoded ECDSA signatures using the P-256 curve
    /// and SHA-256.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P256_SHA256_ASN1: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p256::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA256,
        split_rs: split_rs_asn1,
        id: ECDSAVerificationAlgorithmID::ECDSA_P256_SHA256_ASN1,
    };

    /// *Not recommended*. Verification of ASN.1 DER-encoded ECDSA signatures using
    /// the P-256 curve and SHA-384.
    ///
    /// In most situations, P-256 should be used only with SHA-256 and P-384
    /// should be used only with SHA-384. However, in some cases, particularly TLS
    /// on the web, it is necessary to support P-256 with SHA-384 for compatibility
    /// with widely-deployed implementations that do not follow these guidelines.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P256_SHA384_ASN1: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p256::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA384,
        split_rs: split_rs_asn1,
        id: ECDSAVerificationAlgorithmID::ECDSA_P256_SHA384_ASN1,
    };

    /// *Not recommended*. Verification of ASN.1 DER-encoded ECDSA signatures using
    /// the P-384 curve and SHA-256.
    ///
    /// In most situations, P-256 should be used only with SHA-256 and P-384
    /// should be used only with SHA-384. However, in some cases, particularly TLS
    /// on the web, it is necessary to support P-256 with SHA-384 for compatibility
    /// with widely-deployed implementations that do not follow these guidelines.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P384_SHA256_ASN1: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p384::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA256,
        split_rs: split_rs_asn1,
        id: ECDSAVerificationAlgorithmID::ECDSA_P384_SHA256_ASN1,
    };

    /// Signing of ASN.1 DER-encoded ECDSA signatures using the P-384 curve and
    /// SHA-384.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    #[doc(hidden)]
    pub static ECDSA_P384_SHA384_ASN1_SIGNING: ECDSASigningAlgorithm =
            ECDSASigningAlgorithm {
        curve: &ec::suite_b::curve::P384,
        pkcs8_template: &EC_PUBLIC_KEY_P384_PKCS8_V1_TEMPLATE,
    };

    /// Verification of ASN.1 DER-encoded ECDSA signatures using the P-384 curve
    /// and SHA-384.
    ///
    /// See "`ECDSA_*_ASN1` Details" in `ring::signature`'s module-level
    /// documentation for more details.
    pub static ECDSA_P384_SHA384_ASN1: ECDSAVerificationAlgorithm =
            ECDSAVerificationAlgorithm {
        ops: &p384::PUBLIC_SCALAR_OPS,
        digest_alg: &digest::SHA384,
        split_rs: split_rs_asn1,
        id: ECDSAVerificationAlgorithmID::ECDSA_P384_SHA384_ASN1,
    };

    static EC_PUBLIC_KEY_P256_PKCS8_V1_TEMPLATE: pkcs8::Template = pkcs8::Template {
        bytes: include_bytes ! ("ecPublicKey_p256_pkcs8_v1_template.der"),
        alg_id_range: core::ops::Range { start: 8, end: 27 },
        curve_id_index: 9,
        private_key_index: 0x24,
    };

    static EC_PUBLIC_KEY_P384_PKCS8_V1_TEMPLATE: pkcs8::Template = pkcs8::Template {
        bytes: include_bytes!("ecPublicKey_p384_pkcs8_v1_template.der"),
        alg_id_range: core::ops::Range { start: 8, end: 24 },
        curve_id_index: 9,
        private_key_index: 0x23,
    };

    #[cfg(test)]
    mod tests {
        use {digest, test};
        use super::digest_scalar_;
        use super::super::ops::*;
        use untrusted;

        #[test]
        fn ecdsa_digest_scalar_test() {
            test::from_file("src/ec/suite_b/ecdsa_digest_scalar_tests.txt",
                            |section, test_case| {
                assert_eq!(section, "");

                let curve_name = test_case.consume_string("Curve");
                let digest_name = test_case.consume_string("Digest");
                let input = test_case.consume_bytes("Input");
                let output = test_case.consume_bytes("Output");

                let (ops, digest_alg) = match
                    (curve_name.as_str(), digest_name.as_str()) {
                    ("P-256", "SHA256") =>
                        (&p256::PUBLIC_SCALAR_OPS, &digest::SHA256),
                    ("P-256", "SHA384") =>
                        (&p256::PUBLIC_SCALAR_OPS, &digest::SHA384),
                    ("P-384", "SHA256") =>
                        (&p384::PUBLIC_SCALAR_OPS, &digest::SHA256),
                    ("P-384", "SHA384") =>
                        (&p384::PUBLIC_SCALAR_OPS, &digest::SHA384),
                    _ => {
                        panic!("Unsupported curve+digest: {}+{}", curve_name,
                               digest_name);
                    }
                };

                let num_limbs = ops.public_key_ops.common.num_limbs;
                assert_eq!(input.len(), digest_alg.output_len);
                assert_eq!(output.len(),
                           ops.public_key_ops.common.num_limbs * LIMB_BYTES);

                let expected = scalar_parse_big_endian_variable(
                    ops.public_key_ops.common, AllowZero::Yes,
                    untrusted::Input::from(&output)).unwrap();

                let actual = digest_scalar_(ops.scalar_ops, &input);

                assert_eq!(actual.limbs[..num_limbs], expected.limbs[..num_limbs]);

                Ok(())
            });
        }
    }

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