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    // Copyright 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.

    //! The [chacha20-poly1305@openssh.com] AEAD-ish construct.
    //!
    //! This should only be used by SSH implementations. It has a similar, but
    //! different API from `ring::aead` because the construct cannot use the same
    //! API as `ring::aead` due to the way the construct handles the encrypted
    //! packet length.
    //!
    //! The concatenation of a and b is denoted `a||b`. `K_1` and `K_2` are defined
    //! in the [chacha20-poly1305@openssh.com] specification. `packet_length`,
    //! `padding_length`, `payload`, and `random padding` are defined in
    //! [RFC 4253]. The term `plaintext` is used as a shorthand for
    //! `padding_length||payload||random padding`.
    //!
    //! [chacha20-poly1305@openssh.com]:
    //!    http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.chacha20poly1305?annotate=HEAD
    //! [RFC 4253]: https://tools.ietf.org/html/rfc4253

    use {chacha, error, poly1305};

    /// A key for sealing packets.
    pub struct SealingKey {
        key: Key,
    }

    impl SealingKey {
        /// Constructs a new `SealingKey`.
        pub fn new(key_material: &[u8; KEY_LEN]) -> SealingKey {
            SealingKey { key: Key::new(key_material) }
        }

        /// Seals (encrypts and signs) a packet.
        ///
        /// On input, `plaintext_in_ciphertext_out` must contain the unencrypted
        /// `packet_length||plaintext` where `plaintext` is the
        /// `padding_length||payload||random padding`. It will be overwritten by
        /// `encrypted_packet_length||ciphertext`, where `encrypted_packet_length`
        /// is encrypted with `K_1` and `ciphertext` is encrypted by `K_2`.
        pub fn seal_in_place(&self, sequence_number: u32,
                             plaintext_in_ciphertext_out: &mut [u8],
                             tag_out: &mut [u8; TAG_LEN]) {
            let mut counter = make_counter(sequence_number);

            {
                let (len_in_out, data_and_padding_in_out) =
                    plaintext_in_ciphertext_out.split_at_mut(PACKET_LENGTH_LEN);

                chacha::chacha20_xor_in_place(&self.key.k_1, &counter, len_in_out);

                counter[0] = 1;
                chacha::chacha20_xor_in_place(&self.key.k_2, &counter,
                                              data_and_padding_in_out);
            }

            counter[0] = 0;
            let poly_key = poly1305::Key::derive_using_chacha(&self.key.k_2,
                                                              &counter);
            poly1305::sign(poly_key, plaintext_in_ciphertext_out, tag_out);
        }
    }

    /// A key for opening packets.
    pub struct OpeningKey {
        key: Key,
    }

    impl OpeningKey {
        /// Constructs a new `OpeningKey`.
        pub fn new(key_material: &[u8; KEY_LEN]) -> OpeningKey {
            OpeningKey { key: Key::new(key_material) }
        }

        /// Returns the decrypted, but unauthenticated, packet length.
        ///
        /// Importantly, the result won't be authenticated until `open_in_place` is
        /// called.
        pub fn decrypt_packet_length(
                &self, sequence_number: u32,
                encrypted_packet_length: [u8; PACKET_LENGTH_LEN])
                -> [u8; PACKET_LENGTH_LEN] {
            let mut packet_length = encrypted_packet_length;
            let counter = make_counter(sequence_number);
            chacha::chacha20_xor_in_place(&self.key.k_1, &counter,
                                          &mut packet_length);
            packet_length
        }

        /// Opens (authenticates and decrypts) a packet.
        ///
        /// `ciphertext_in_plaintext_out` must be of the form
        /// `encrypted_packet_length||ciphertext` where `ciphertext` is the
        /// encrypted `plaintext`. When the function succeeds the ciphertext is
        /// replaced by the plaintext and the result is `Ok(plaintext)`, where
        /// `plaintext` is `&ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..]`;
        /// otherwise the contents of `ciphertext_in_plaintext_out` are unspecified
        /// and must not be used.
        pub fn open_in_place<'a>(&self, sequence_number: u32,
                                 ciphertext_in_plaintext_out: &'a mut [u8],
                                 tag: &[u8; TAG_LEN])
                                 -> Result<&'a [u8], error::Unspecified> {
            let mut counter = make_counter(sequence_number);

            // We must verify the tag before decrypting so that
            // `ciphertext_in_plaintext_out` is unmodified if verification fails.
            // This is beyond what we guarantee.
            let poly_key = poly1305::Key::derive_using_chacha(&self.key.k_2,
                                                              &counter);
            poly1305::verify(poly_key, ciphertext_in_plaintext_out, tag)?;

            let plaintext_in_ciphertext_out =
                &mut ciphertext_in_plaintext_out[PACKET_LENGTH_LEN..];
            counter[0] = 1;
            chacha::chacha20_xor_in_place(&self.key.k_2, &counter,
                                          plaintext_in_ciphertext_out);

            Ok(plaintext_in_ciphertext_out)
        }
    }

    struct Key {
        k_1: chacha::Key,
        k_2: chacha::Key,
    }

    impl Key {
        pub fn new(key_material: &[u8; KEY_LEN]) -> Key {
            // The first half becomes K_2 and the second half becomes K_1.
            Key {
                k_1: chacha::key_from_bytes(
                        slice_as_array_ref!(
                            &key_material[chacha::KEY_LEN_IN_BYTES..],
                            chacha::KEY_LEN_IN_BYTES).unwrap()),
                k_2: chacha::key_from_bytes(
                        slice_as_array_ref!(
                            &key_material[..chacha::KEY_LEN_IN_BYTES],
                            chacha::KEY_LEN_IN_BYTES).unwrap()),
            }
        }
    }

    fn make_counter(sequence_number: u32) -> chacha::Counter {
        let mut sequence_number = sequence_number;
        let mut nonce = [0; chacha::NONCE_LEN];
        for i in 0..4 {
            nonce[chacha::NONCE_LEN - 1 - i] = (sequence_number % 0x100) as u8;
            sequence_number /= 0x100;
        }
        chacha::make_counter(&nonce, 0)
    }

    /// The length of key.
    pub const KEY_LEN: usize = chacha::KEY_LEN_IN_BYTES * 2;

    /// The length of a tag.
    pub const TAG_LEN: usize = poly1305::TAG_LEN;

    /// The length in bytes of the `packet_length` field in a SSH packet.
    pub const PACKET_LENGTH_LEN: usize = 4; // 32 bits

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