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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.

    //! Authenticated Encryption with Associated Data (AEAD).
    //!
    //! See [Authenticated encryption: relations among notions and analysis of the
    //! generic composition paradigm][AEAD] for an introduction to the concept of
    //! AEADs.
    //!
    //! C analog: `GFp/aead.h`
    //!
    //! Go analog: [`crypto.cipher.AEAD`]
    //!
    //! [AEAD]: http://www-cse.ucsd.edu/~mihir/papers/oem.html
    //! [`crypto.cipher.AEAD`]: https://golang.org/pkg/crypto/cipher/#AEAD

    pub mod chacha20_poly1305_openssh;

    mod chacha20_poly1305;
    mod aes_gcm;

    use {constant_time, error, init, poly1305, polyfill};

    pub use self::chacha20_poly1305::CHACHA20_POLY1305;
    pub use self::aes_gcm::{AES_128_GCM, AES_256_GCM};

    /// A key for authenticating and decrypting (“opening”) AEAD-protected data.
    ///
    /// C analog: `EVP_AEAD_CTX` with direction `evp_aead_open`
    ///
    /// Go analog: [`crypto.cipher.AEAD`]
    pub struct OpeningKey {
        key: Key,
    }

    impl OpeningKey {
        /// Create a new opening key.
        ///
        /// `key_bytes` must be exactly `algorithm.key_len` bytes long.
        ///
        /// C analogs: `EVP_AEAD_CTX_init_with_direction` with direction
        ///            `evp_aead_open`, `EVP_AEAD_CTX_init`.
        ///
        /// Go analog:
        ///   [`crypto.aes.NewCipher`](https://golang.org/pkg/crypto/aes/#NewCipher)
        /// + [`crypto.cipher.NewGCM`](https://golang.org/pkg/crypto/cipher/#NewGCM)
        #[inline]
        pub fn new(algorithm: &'static Algorithm, key_bytes: &[u8])
                   -> Result<OpeningKey, error::Unspecified> {
            let mut key = OpeningKey {
                key: Key {
                    algorithm: algorithm,
                    ctx_buf: [0; KEY_CTX_BUF_ELEMS],
                },
            };
            key.key.init(key_bytes)?;
            Ok(key)
        }

        /// The key's AEAD algorithm.
        ///
        /// C analog: `EVP_AEAD_CTX.aead`
        #[inline(always)]
        pub fn algorithm(&self) -> &'static Algorithm { self.key.algorithm() }
    }

    /// Authenticates and decrypts (“opens”) data in place. When
    ///
    /// The input may have a prefix that is `in_prefix_len` bytes long; any such
    /// prefix is ignored on input and overwritten on output. The last
    /// `key.algorithm().tag_len()` bytes of `ciphertext_and_tag_modified_in_place`
    /// must be the tag. The part of `ciphertext_and_tag_modified_in_place` between
    /// the prefix and the tag is the input ciphertext.
    ///
    /// When `open_in_place()` returns `Ok(plaintext)`, the decrypted output is
    /// `plaintext`, which is
    /// `&mut ciphertext_and_tag_modified_in_place[..plaintext.len()]`. That is,
    /// the output plaintext overwrites some or all of the prefix and ciphertext.
    /// To put it another way, the ciphertext is shifted forward `in_prefix_len`
    /// bytes and then decrypted in place. To have the output overwrite the input
    /// without shifting, pass 0 as `in_prefix_len`.
    ///
    /// When `open_in_place()` returns `Err(..)`,
    /// `ciphertext_and_tag_modified_in_place` may have been overwritten in an
    /// unspecified way.
    ///
    /// The shifting feature is useful in the case where multiple packets are
    /// being reassembled in place. Consider this example where the peer has sent
    /// the message “Split stream reassembled in place” split into three sealed
    /// packets:
    ///
    /// ```ascii-art
    ///                 Packet 1                  Packet 2                 Packet 3
    /// Input:  [Header][Ciphertext][Tag][Header][Ciphertext][Tag][Header][Ciphertext][Tag]
    ///                      |         +--------------+                        |
    ///               +------+   +-----+    +----------------------------------+
    ///               v          v          v
    /// Output: [Plaintext][Plaintext][Plaintext]
    ///        “Split stream reassembled in place”
    /// ```
    ///
    /// Let's say the header is always 5 bytes (like TLS 1.2) and the tag is always
    /// 16 bytes (as for AES-GCM and ChaCha20-Poly1305). Then for this example,
    /// `in_prefix_len` would be `5` for the first packet, `(5 + 16) + 5` for the
    /// second packet, and `(2 * (5 + 16)) + 5` for the third packet.
    ///
    /// (The input/output buffer is expressed as combination of `in_prefix_len`
    /// and `ciphertext_and_tag_modified_in_place` because Rust's type system
    /// does not allow us to have two slices, one mutable and one immutable, that
    /// reference overlapping memory.)
    ///
    /// C analog: `EVP_AEAD_CTX_open`
    ///
    /// Go analog: [`AEAD.Open`](https://golang.org/pkg/crypto/cipher/#AEAD)
    pub fn open_in_place<'a>(key: &OpeningKey, nonce: &[u8], ad: &[u8],
                             in_prefix_len: usize,
                             ciphertext_and_tag_modified_in_place: &'a mut [u8])
                             -> Result<&'a mut [u8], error::Unspecified> {
        let nonce = slice_as_array_ref!(nonce, NONCE_LEN)?;
        let ciphertext_and_tag_len =
            ciphertext_and_tag_modified_in_place.len()
                    .checked_sub(in_prefix_len).ok_or(error::Unspecified)?;
        let ciphertext_len =
            ciphertext_and_tag_len.checked_sub(TAG_LEN).ok_or(error::Unspecified)?;
        check_per_nonce_max_bytes(ciphertext_len)?;
        let (in_out, received_tag) =
            ciphertext_and_tag_modified_in_place
                .split_at_mut(in_prefix_len + ciphertext_len);
        let mut calculated_tag = [0u8; TAG_LEN];
        (key.key.algorithm.open)(&key.key.ctx_buf, nonce, &ad, in_prefix_len,
                                 in_out, &mut calculated_tag)?;
        if constant_time::verify_slices_are_equal(&calculated_tag, received_tag)
                .is_err() {
            // Zero out the plaintext so that it isn't accidentally leaked or used
            // after verification fails. It would be safest if we could check the
            // tag before decrypting, but some `open` implementations interleave
            // authentication with decryption for performance.
            for b in &mut in_out[..ciphertext_len] {
                *b = 0;
            }
            return Err(error::Unspecified);
        }
        // `ciphertext_len` is also the plaintext length.
        Ok(&mut in_out[..ciphertext_len])
    }

    /// A key for encrypting and signing (“sealing”) data.
    ///
    /// C analog: `EVP_AEAD_CTX` with direction `evp_aead_seal`.
    ///
    /// Go analog: [`AEAD`](https://golang.org/pkg/crypto/cipher/#AEAD)
    pub struct SealingKey {
        key: Key,
    }

    impl SealingKey {
        /// C analogs: `EVP_AEAD_CTX_init_with_direction` with direction
        ///            `evp_aead_seal`, `EVP_AEAD_CTX_init`.
        ///
        /// Go analog:
        ///   [`crypto.aes.NewCipher`](https://golang.org/pkg/crypto/aes/#NewCipher)
        /// + [`crypto.cipher.NewGCM`](https://golang.org/pkg/crypto/cipher/#NewGCM)
        #[inline]
        pub fn new(algorithm: &'static Algorithm, key_bytes: &[u8])
                   -> Result<SealingKey, error::Unspecified> {
            let mut key = SealingKey {
                key: Key {
                    algorithm: algorithm,
                    ctx_buf: [0; KEY_CTX_BUF_ELEMS],
                },
            };
            key.key.init(key_bytes)?;
            Ok(key)
        }

        /// The key's AEAD algorithm.
        ///
        /// C analog: `EVP_AEAD_CTX.aead`
        #[inline(always)]
        pub fn algorithm(&self) -> &'static Algorithm { self.key.algorithm() }
    }

    /// Encrypts and signs (“seals”) data in place.
    ///
    /// `nonce` must be unique for every use of the key to seal data.
    ///
    /// The input is `in_out[..(in_out.len() - out_suffix_capacity)]`; i.e. the
    /// input is the part of `in_out` that precedes the suffix. When
    /// `seal_in_place()` returns `Ok(out_len)`, the encrypted and signed output is
    /// `in_out[..out_len]`; i.e.  the output has been written over input and at
    /// least part of the data reserved for the suffix. (The input/output buffer
    /// is expressed this way because Rust's type system does not allow us to have
    /// two slices, one mutable and one immutable, that reference overlapping
    /// memory at the same time.)
    ///
    /// `out_suffix_capacity` must be at least `key.algorithm().tag_len()`. See
    /// also `MAX_TAG_LEN`.
    ///
    /// `ad` is the additional authenticated data, if any.
    ///
    /// C analog: `EVP_AEAD_CTX_seal`.
    ///
    /// Go analog: [`AEAD.Seal`](https://golang.org/pkg/crypto/cipher/#AEAD)
    pub fn seal_in_place(key: &SealingKey, nonce: &[u8], ad: &[u8],
                         in_out: &mut [u8], out_suffix_capacity: usize)
                         -> Result<usize, error::Unspecified> {
        if out_suffix_capacity < key.key.algorithm.tag_len() {
            return Err(error::Unspecified);
        }
        let nonce = slice_as_array_ref!(nonce, NONCE_LEN)?;
        let in_out_len =
            in_out.len().checked_sub(out_suffix_capacity).ok_or(error::Unspecified)?;
        check_per_nonce_max_bytes(in_out_len)?;
        let (in_out, tag_out) = in_out.split_at_mut(in_out_len);
        let tag_out = slice_as_array_ref_mut!(tag_out, TAG_LEN)?;
        (key.key.algorithm.seal)(&key.key.ctx_buf, nonce, ad, in_out, tag_out)?;
        Ok(in_out_len + TAG_LEN)
    }

    /// `OpeningKey` and `SealingKey` are type-safety wrappers around `Key`, which
    /// does all the actual work via the C AEAD interface.
    ///
    /// C analog: `EVP_AEAD_CTX`
    struct Key {
        ctx_buf: [u64; KEY_CTX_BUF_ELEMS],
        algorithm: &'static Algorithm,
    }

    const KEY_CTX_BUF_ELEMS: usize = (KEY_CTX_BUF_LEN + 7) / 8;

    // Keep this in sync with `aead_aes_gcm_ctx` in e_aes.c.
    const KEY_CTX_BUF_LEN: usize = self::aes_gcm::AES_KEY_CTX_BUF_LEN;

    impl Key {
        /// XXX: Assumes self.algorithm is already filled in.
        ///
        /// C analogs: `EVP_AEAD_CTX_init`, `EVP_AEAD_CTX_init_with_direction`
        fn init(&mut self, key_bytes: &[u8]) -> Result<(), error::Unspecified> {
            init::init_once();

            if key_bytes.len() != self.algorithm.key_len() {
                return Err(error::Unspecified);
            }

            let ctx_buf_bytes = polyfill::slice::u64_as_u8_mut(&mut self.ctx_buf);
            (self.algorithm.init)(ctx_buf_bytes, key_bytes)
        }

        /// The key's AEAD algorithm.
        #[inline(always)]
        fn algorithm(&self) -> &'static Algorithm { self.algorithm }
    }

    /// An AEAD Algorithm.
    ///
    /// C analog: `EVP_AEAD`
    ///
    /// Go analog:
    ///     [`crypto.cipher.AEAD`](https://golang.org/pkg/crypto/cipher/#AEAD)
    pub struct Algorithm {
        init: fn(ctx_buf: &mut [u8], key: &[u8]) -> Result<(), error::Unspecified>,

        seal: fn(ctx: &[u64; KEY_CTX_BUF_ELEMS], nonce: &[u8; NONCE_LEN], ad: &[u8],
                 in_out: &mut [u8], tag_out: &mut [u8; TAG_LEN])
                 -> Result<(), error::Unspecified>,
        open: fn(ctx: &[u64; KEY_CTX_BUF_ELEMS], nonce: &[u8; NONCE_LEN],
                 ad: &[u8], in_prefix_len: usize, in_out: &mut [u8],
                 tag_out: &mut [u8; TAG_LEN]) -> Result<(), error::Unspecified>,

        key_len: usize,
    }

    impl Algorithm {
        /// The length of the key.
        ///
        /// C analog: `EVP_AEAD_key_length`
        #[inline(always)]
        pub fn key_len(&self) -> usize { self.key_len }

        /// The length of a tag.
        ///
        /// See also `MAX_TAG_LEN`.
        ///
        /// C analog: `EVP_AEAD_max_overhead`
        ///
        /// Go analog:
        ///   [`crypto.cipher.AEAD.Overhead`](https://golang.org/pkg/crypto/cipher/#AEAD)
        #[inline(always)]
        pub fn tag_len(&self) -> usize { TAG_LEN }

        /// The length of the nonces.
        ///
        /// C analog: `EVP_AEAD_nonce_length`
        ///
        /// Go analog:
        ///   [`crypto.cipher.AEAD.NonceSize`](https://golang.org/pkg/crypto/cipher/#AEAD)
        #[inline(always)]
        pub fn nonce_len(&self) -> usize { NONCE_LEN }
    }

    /// The maximum length of a tag for the algorithms in this module.
    pub const MAX_TAG_LEN: usize = TAG_LEN;

    // All the AEADs we support use 128-bit tags.
    const TAG_LEN: usize = poly1305::TAG_LEN;

    // All the AEADs we support use 96-bit nonces.
    const NONCE_LEN: usize = 96 / 8;

    /// |GFp_chacha_20| uses a 32-bit block counter, so we disallow individual
    /// operations that work on more than 256GB at a time, for all AEADs.
    fn check_per_nonce_max_bytes(in_out_len: usize)
                                 -> Result<(), error::Unspecified> {
        if polyfill::u64_from_usize(in_out_len) >= (1u64 << 32) * 64 - 64 {
            return Err(error::Unspecified);
        }
        Ok(())
    }

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