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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.

    use {error, init, rand};
    use untrusted;

    /// A key agreement algorithm.
    // XXX: This doesn't seem like the best place for this.
    pub struct AgreementAlgorithmImpl {
        pub curve: &'static Curve,
        pub ecdh: fn(out: &mut [u8], private_key: &PrivateKey,
                     peer_public_key: untrusted::Input)
                     -> Result<(), error::Unspecified>,
    }

    pub struct Curve {
        pub public_key_len: usize,
        pub elem_and_scalar_len: usize,

        pub id: CurveID,

        // Precondition: `bytes` is the correct length.
        check_private_key_bytes: fn(bytes: &[u8]) -> Result<(), error::Unspecified>,

        generate_private_key: fn(rng: &rand::SecureRandom)
                                 -> Result<PrivateKey, error::Unspecified>,

        public_from_private: fn(public_out: &mut [u8], private_key: &PrivateKey)
                                -> Result<(), error::Unspecified>,
    }

    #[derive(Clone, Copy, PartialEq)]
    pub enum CurveID {
        Curve25519,
        P256,
        P384,
    }

    pub struct KeyPair {
        pub private_key: PrivateKey,
        pub public_key: [u8; PUBLIC_KEY_MAX_LEN],
    }

    pub struct PrivateKey {
        bytes: [u8; SCALAR_MAX_BYTES],
    }

    impl<'a> PrivateKey {
        pub fn generate(curve: &Curve, rng: &rand::SecureRandom)
                        -> Result<PrivateKey, error::Unspecified> {
            init::init_once();
            (curve.generate_private_key)(rng)
        }

        pub fn from_bytes(curve: &Curve, bytes: untrusted::Input)
                          -> Result<PrivateKey, error::Unspecified> {
            init::init_once();
            let bytes = bytes.as_slice_less_safe();
            if curve.elem_and_scalar_len != bytes.len() {
                return Err(error::Unspecified);
            }
            (curve.check_private_key_bytes)(bytes)?;
            let mut r = PrivateKey {
                bytes: [0; SCALAR_MAX_BYTES],
            };
            r.bytes[..curve.elem_and_scalar_len].copy_from_slice(bytes);
            Ok(r)
        }

        pub fn bytes(&'a self, curve: &Curve) -> &'a [u8] {
            &self.bytes[..curve.elem_and_scalar_len]
        }

        #[inline(always)]
        pub fn compute_public_key(&self, curve: &Curve, out: &mut [u8])
                                  -> Result<(), error::Unspecified> {
            if out.len() != curve.public_key_len {
                return Err(error::Unspecified);
            }
            (curve.public_from_private)(out, self)
        }
    }

    const ELEM_MAX_BITS: usize = 384;
    pub const ELEM_MAX_BYTES: usize = (ELEM_MAX_BITS + 7) / 8;

    pub const SCALAR_MAX_BYTES: usize = ELEM_MAX_BYTES;

    /// The maximum length, in bytes, of an encoded public key.
    pub const PUBLIC_KEY_MAX_LEN: usize = 1 + (2 * ELEM_MAX_BYTES);

    /// The maximum length of a PKCS#8 documents generated by *ring* for ECC keys.
    ///
    /// This is NOT the maximum length of a PKCS#8 document that can be consumed by
    /// `pkcs8::unwrap_key()`.
    ///
    /// `40` is the length of the P-384 template. It is actually one byte shorter
    /// than the P-256 template, but the private key and the public key are much
    /// longer.
    pub const PKCS8_DOCUMENT_MAX_LEN: usize =
        40 + SCALAR_MAX_BYTES + PUBLIC_KEY_MAX_LEN;

    #[path = "curve25519/curve25519.rs"]
    pub mod curve25519;

    #[path = "suite_b/suite_b.rs"]
    pub mod suite_b;

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