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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.

    //! EdDSA Signatures.

    use core;
    use {der, digest, error, pkcs8, private, rand, signature, signature_impl};
    use super::ops::*;
    use untrusted;

    /// Parameters for EdDSA signing and verification.
    pub struct EdDSAParameters;

    impl core::fmt::Debug for EdDSAParameters {
        fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
            write!(f, "ring::signature::ED25519")
        }
    }

    /// An Ed25519 key pair, for signing.
    pub struct Ed25519KeyPair {
        // RFC 8032 Section 5.1.6 calls this *s*.
        private_scalar: Scalar,

        // RFC 8032 Section 5.1.6 calls this *prefix*.
        private_prefix: Prefix,

        // RFC 8032 Section 5.1.5 calls this *A*.
        public_key: PublicKey,
    }

    impl<'a> Ed25519KeyPair {
        /// Generates a new key pair and returns the key pair serialized as a
        /// PKCS#8 document.
        ///
        /// The PKCS#8 document will be a v2 `OneAsymmetricKey` with the public key,
        /// as described in [RFC 5958 Section 2]. See also
        /// https://tools.ietf.org/html/draft-ietf-curdle-pkix-04.
        ///
        /// [RFC 5958 Section 2]: https://tools.ietf.org/html/rfc5958#section-2
        pub fn generate_pkcs8(rng: &rand::SecureRandom)
                -> Result<[u8; ED25519_PKCS8_V2_LEN], error::Unspecified> {
            let mut seed = [0u8; SEED_LEN];
            rng.fill(&mut seed)?;
            let key_pair = Ed25519KeyPair::from_seed_(&seed);
            // TODO: Replace this with `wrap_key()` and return a `PKCS8Document`.
            let mut bytes = [0; ED25519_PKCS8_V2_LEN];
            pkcs8::wrap_key_(&PKCS8_TEMPLATE, &seed[..], key_pair.public_key_bytes(),
                             &mut bytes[..]);
            Ok(bytes)
        }

        /// Constructs an Ed25519 key pair by parsing an unencrypted PKCS#8 v2
        /// Ed25519 private key.
        ///
        /// The input must be in PKCS#8 v2 format, and in particular it must contain
        /// the public key in addition to the private key. `from_pkcs8()` will
        /// verify that the public key and the private key are consistent with each
        /// other.
        ///
        /// If you need to parse PKCS#8 v1 files (without the public key) then use
        /// `Ed25519KeyPair::from_pkcs8_maybe_unchecked()` instead.
        pub fn from_pkcs8(input: untrusted::Input)
                          -> Result<Ed25519KeyPair, error::Unspecified> {
            let (seed, public_key) = unwrap_pkcs8(pkcs8::Version::V2Only, input)?;
            Self::from_seed_and_public_key(seed, public_key.unwrap())
        }

        /// Constructs an Ed25519 key pair by parsing an unencrypted PKCS#8 v1 or v2
        /// Ed25519 private key.
        ///
        /// It is recommended to use `Ed25519KeyPair::from_pkcs8()`, which accepts
        /// only PKCS#8 v2 files that contain the public key.
        /// `from_pkcs8_maybe_unchecked()` parses PKCS#2 files exactly like
        /// `from_pkcs8()`. It also accepts v1 files. PKCS#8 v1 files do not contain
        /// the public key, so when a v1 file is parsed the public key will be
        /// computed from the private key, and there will be no consistency check
        /// between the public key and the private key.
        ///
        /// PKCS#8 v2 files are parsed exactly like `Ed25519KeyPair::from_pkcs8()`.
        pub fn from_pkcs8_maybe_unchecked(input: untrusted::Input)
                -> Result<Ed25519KeyPair, error::Unspecified> {
            let (seed, public_key) = unwrap_pkcs8(pkcs8::Version::V1OrV2, input)?;
            if let Some(public_key) = public_key {
                Self::from_seed_and_public_key(seed, public_key)
            } else {
                Self::from_seed_unchecked(seed)
            }
        }

        /// Constructs an Ed25519 key pair from the private key seed `seed` and its
        /// public key `public_key`.
        ///
        /// It is recommended to use `Ed25519KeyPair::from_pkcs8()` instead.
        ///
        /// The private and public keys will be verified to be consistent with each
        /// other. This helps avoid misuse of the key (e.g. accidentally swapping
        /// the private key and public key, or using the wrong private key for the
        /// public key). This also detects any corruption of the public or private
        /// key.
        pub fn from_seed_and_public_key(seed: untrusted::Input,
                                        public_key: untrusted::Input)
                -> Result<Ed25519KeyPair, error::Unspecified> {
            let pair = Self::from_seed_unchecked(seed)?;

            // This implicitly verifies that `public_key` is the right length.
            // XXX: This rejects ~18 keys when they are partially reduced, though
            // those keys are virtually impossible to find.
            if public_key != pair.public_key_bytes() {
                return Err(error::Unspecified);
            }

            Ok(pair)
        }

        /// Constructs a Ed25519 key pair from the private key seed `seed`.
        ///
        /// It is recommended to use `Ed25519KeyPair::from_pkcs8()` instead. When
        /// that is not practical, it is recommended to use
        /// `Ed25519KeyPair::from_seed_and_public_key()` instead.
        ///
        /// Since the public key is not given, the public key will be computed from
        /// the private key. It is not possible to detect misuse or corruption of
        /// the private key since the public key isn't given as input.
        pub fn from_seed_unchecked(seed: untrusted::Input)
                                   -> Result<Ed25519KeyPair, error::Unspecified> {
            let seed = slice_as_array_ref!(seed.as_slice_less_safe(), SEED_LEN)?;
            Ok(Self::from_seed_(seed))
        }

        fn from_seed_(seed: &Seed) -> Ed25519KeyPair {
            let h = digest::digest(&digest::SHA512, seed);
            let (scalar_encoded, prefix_encoded) = h.as_ref().split_at(SCALAR_LEN);

            let mut scalar = [0u8; SCALAR_LEN];
            scalar.copy_from_slice(&scalar_encoded);
            unsafe { GFp_curve25519_scalar_mask(&mut scalar) };

            let mut prefix = [0u8; PREFIX_LEN];
            prefix.copy_from_slice(prefix_encoded);

            let mut a = ExtPoint::new_at_infinity();
            unsafe {
                GFp_x25519_ge_scalarmult_base(&mut a, &scalar);
            }

            Ed25519KeyPair {
                private_scalar: scalar,
                private_prefix: prefix,
                public_key: a.into_encoded_point(),
            }
        }

        /// Returns a reference to the little-endian-encoded public key bytes.
        pub fn public_key_bytes(&'a self) -> &'a [u8] {
            &self.public_key
        }

        /// Returns the signature of the message `msg`.
        pub fn sign(&self, msg: &[u8]) -> signature::Signature {
            let mut signature_bytes = [0u8; SIGNATURE_LEN];
            { // Borrow `signature_bytes`.
                let (signature_r, signature_s) =
                    signature_bytes.split_at_mut(ELEM_LEN);
                let signature_r =
                    slice_as_array_ref_mut!(signature_r, ELEM_LEN).unwrap();
                let signature_s =
                    slice_as_array_ref_mut!(signature_s, SCALAR_LEN).unwrap();

                let nonce = {
                    let mut ctx = digest::Context::new(&digest::SHA512);
                    ctx.update(&self.private_prefix);
                    ctx.update(msg);
                    ctx.finish()
                };
                let nonce = digest_scalar(nonce);

                let mut r = ExtPoint::new_at_infinity();
                unsafe {
                    GFp_x25519_ge_scalarmult_base(&mut r, &nonce);
                }
                *signature_r = r.into_encoded_point();
                let hram_digest = eddsa_digest(signature_r, &self.public_key, msg);
                let hram = digest_scalar(hram_digest);
                unsafe {
                    GFp_x25519_sc_muladd(signature_s, &hram, &self.private_scalar,
                                         &nonce);
                }
            }
            signature_impl::signature_from_bytes(&signature_bytes)
        }
    }

    fn unwrap_pkcs8(version: pkcs8::Version, input: untrusted::Input)
            -> Result<(untrusted::Input, Option<untrusted::Input>),
                      error::Unspecified> {
        let (private_key, public_key) =
            pkcs8::unwrap_key(&PKCS8_TEMPLATE, version, input)?;
        let private_key = private_key.read_all(error::Unspecified, |input| {
            der::expect_tag_and_get_value(input, der::Tag::OctetString)
        })?;
        Ok((private_key, public_key))
    }

    /// Verification of [Ed25519] signatures.
    ///
    /// Ed25519 uses SHA-512 as the digest algorithm.
    ///
    /// [Ed25519]: https://ed25519.cr.yp.to/
    pub static ED25519: EdDSAParameters = EdDSAParameters {};

    impl signature::VerificationAlgorithm for EdDSAParameters {
        fn verify(&self, public_key: untrusted::Input, msg: untrusted::Input,
                  signature: untrusted::Input) -> Result<(), error::Unspecified> {
            let public_key = public_key.as_slice_less_safe();
            let public_key = slice_as_array_ref!(public_key, ELEM_LEN)?;

            let (signature_r, signature_s) =
                    signature.read_all(error::Unspecified, |input| {
                let r = input.skip_and_get_input(ELEM_LEN)?;
                let r = r.as_slice_less_safe();
                // `r` is only used as a slice, so don't convert it to an array ref.

                let s = input.skip_and_get_input(SCALAR_LEN)?;
                let s = s.as_slice_less_safe();
                let s = slice_as_array_ref!(s, SCALAR_LEN).unwrap();

                Ok((r, s))
            })?;

            // Ensure `s` is not too large.
            if (signature_s[SCALAR_LEN - 1] & 0b11100000) != 0 {
                return Err(error::Unspecified);
            }

            let mut a = ExtPoint::from_encoded_point_vartime(public_key)?;
            a.invert_vartime();

            let h_digest =
                eddsa_digest(signature_r, public_key, msg.as_slice_less_safe());
            let h = digest_scalar(h_digest);

            let mut r = Point::new_at_infinity();
            unsafe {
                GFp_ge_double_scalarmult_vartime(&mut r, &h, &a, &signature_s)
            };
            let r_check = r.into_encoded_point();
            if signature_r != r_check {
                return Err(error::Unspecified);
            }
            Ok(())
        }
    }

    impl private::Private for EdDSAParameters {}

    fn eddsa_digest(signature_r: &[u8], public_key: &[u8], msg: &[u8])
                    -> digest::Digest {
        let mut ctx = digest::Context::new(&digest::SHA512);
        ctx.update(signature_r);
        ctx.update(public_key);
        ctx.update(msg);
        ctx.finish()
    }

    fn digest_scalar(digest: digest::Digest) -> Scalar {
        let mut unreduced = [0u8; digest::SHA512_OUTPUT_LEN];
        unreduced.copy_from_slice(digest.as_ref());
        unsafe { GFp_x25519_sc_reduce(&mut unreduced) };
        let mut scalar = [0u8; SCALAR_LEN];
        scalar.copy_from_slice(&unreduced[..SCALAR_LEN]);
        scalar
    }

    extern  {
        fn GFp_curve25519_scalar_mask(a: &mut Scalar);
        fn GFp_ge_double_scalarmult_vartime(r: &mut Point, a_coeff: &Scalar,
                                            a: &ExtPoint, b_coeff: &Scalar);
        fn GFp_x25519_ge_scalarmult_base(h: &mut ExtPoint, a: &Seed);
        fn GFp_x25519_sc_muladd(s: &mut Scalar, a: &Scalar, b: &Scalar, c: &Scalar);
        fn GFp_x25519_sc_reduce(s: &mut UnreducedScalar);
    }

    type PublicKey = [u8; PUBLIC_KEY_LEN];
    const PUBLIC_KEY_LEN: usize = ELEM_LEN;

    type Prefix = [u8; PREFIX_LEN];
    const PREFIX_LEN: usize = digest::SHA512_OUTPUT_LEN - SCALAR_LEN;

    const SIGNATURE_LEN: usize = ELEM_LEN + SCALAR_LEN;

    type Seed = [u8; SEED_LEN];
    const SEED_LEN: usize = 32;

    static PKCS8_TEMPLATE: pkcs8::Template = pkcs8::Template {
        bytes: include_bytes!("ed25519_pkcs8_v2_template.der"),
        alg_id_range: core::ops::Range { start: 7, end: 12 },
        curve_id_index: 0,
        private_key_index: 0x10,
    };

    /// The length of an Ed25519 public key.
    pub const ED25519_PUBLIC_KEY_LEN: usize = PUBLIC_KEY_LEN;

    /// The length of a Ed25519 PKCS#8 (v2) private key generated by
    /// `Ed25519KeyPair::generate_pkcs8()`. Ed25519 PKCS#8 files generated by other
    /// software may have different lengths, and `Ed25519KeyPair::generate_pkcs8()`
    /// may generate files of a different length in the future.
    pub const ED25519_PKCS8_V2_LEN: usize = 0x55;

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