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    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    /*!
     * This module implements the Hmac function - a Message Authentication Code using a Digest.
     */

    use std::iter::repeat;

    use cryptoutil;
    use digest::Digest;
    use mac::{Mac, MacResult};

    /**
     * The Hmac struct represents an Hmac function - a Message Authentication Code using a Digest.
     */
    pub struct Hmac<D> {
        digest: D,
        i_key: Vec<u8>,
        o_key: Vec<u8>,
        finished: bool
    }

    fn derive_key(key: &mut [u8], mask: u8) {
        for elem in key.iter_mut() {
            *elem ^= mask;
        }
    }

    // The key that Hmac processes must be the same as the block size of the underlying Digest. If the
    // provided key is smaller than that, we just pad it with zeros. If its larger, we hash it and then
    // pad it with zeros.
    fn expand_key<D: Digest>(digest: &mut D, key: &[u8]) -> Vec<u8> {
        let bs = digest.block_size();
        let mut expanded_key: Vec<u8> = repeat(0).take(bs).collect();

        if key.len() <= bs {
            cryptoutil::copy_memory(key, &mut expanded_key);
        } else {
            let output_size = digest.output_bytes();
            digest.input(key);
            digest.result(&mut expanded_key[..output_size]);
            digest.reset();
        }
        expanded_key
    }

    // Hmac uses two keys derived from the provided key - one by xoring every byte with 0x36 and another
    // with 0x5c.
    fn create_keys<D: Digest>(digest: &mut D, key: &[u8]) -> (Vec<u8>, Vec<u8>) {
        let mut i_key = expand_key(digest, key);
        let mut o_key = i_key.clone();
        derive_key(&mut i_key, 0x36);
        derive_key(&mut o_key, 0x5c);
        (i_key, o_key)
    }

    impl <D: Digest> Hmac<D> {
        /**
         * Create a new Hmac instance.
         *
         * # Arguments
         * * digest - The Digest to use.
         * * key - The key to use.
         *
         */
        pub fn new(mut digest: D, key: &[u8]) -> Hmac<D> {
            let (i_key, o_key) = create_keys(&mut digest, key);
            digest.input(&i_key[..]);
            Hmac {
                digest: digest,
                i_key: i_key,
                o_key: o_key,
                finished: false
            }
        }
    }

    impl <D: Digest> Mac for Hmac<D> {
        fn input(&mut self, data: &[u8]) {
            assert!(!self.finished);
            self.digest.input(data);
        }

        fn reset(&mut self) {
            self.digest.reset();
            self.digest.input(&self.i_key[..]);
            self.finished = false;
        }

        fn result(&mut self) -> MacResult {
            let output_size = self.digest.output_bytes();
            let mut code: Vec<u8> = repeat(0).take(output_size).collect();

            self.raw_result(&mut code);

            MacResult::new_from_owned(code)
        }

        fn raw_result(&mut self, output: &mut [u8]) {
            if !self.finished {
                self.digest.result(output);

                self.digest.reset();
                self.digest.input(&self.o_key[..]);
                self.digest.input(output);

                self.finished = true;
            }

            self.digest.result(output);
        }

        fn output_bytes(&self) -> usize { self.digest.output_bytes() }
    }

    #[cfg(test)]
    mod test {
        use std::iter::repeat;

        use mac::{Mac, MacResult};
        use hmac::Hmac;
        use digest::Digest;
        use md5::Md5;

        struct Test {
            key: Vec<u8>,
            data: Vec<u8>,
            expected: Vec<u8>
        }

        // Test vectors from: http://tools.ietf.org/html/rfc2104

        fn tests() -> Vec<Test> {
            vec![
                Test {
                    key: repeat(0x0bu8).take(16).collect(),
                    data: b"Hi There".to_vec(),
                    expected: vec![
                        0x92, 0x94, 0x72, 0x7a, 0x36, 0x38, 0xbb, 0x1c,
                        0x13, 0xf4, 0x8e, 0xf8, 0x15, 0x8b, 0xfc, 0x9d ]
                },
                Test {
                    key: b"Jefe".to_vec(),
                    data: b"what do ya want for nothing?".to_vec(),
                    expected: vec![
                        0x75, 0x0c, 0x78, 0x3e, 0x6a, 0xb0, 0xb5, 0x03,
                        0xea, 0xa8, 0x6e, 0x31, 0x0a, 0x5d, 0xb7, 0x38 ]
                },
                Test {
                    key: repeat(0xaau8).take(16).collect(),
                    data: repeat(0xddu8).take(50).collect(),
                    expected: vec![
                        0x56, 0xbe, 0x34, 0x52, 0x1d, 0x14, 0x4c, 0x88,
                        0xdb, 0xb8, 0xc7, 0x33, 0xf0, 0xe8, 0xb3, 0xf6 ]
                }
            ]
        }

        #[test]
        fn test_hmac_md5() {
            let tests = tests();
            for t in tests.iter() {
                let mut hmac = Hmac::new(Md5::new(), &t.key[..]);

                hmac.input(&t.data[..]);
                let result = hmac.result();
                let expected = MacResult::new(&t.expected[..]);
                assert!(result == expected);

                hmac.reset();

                hmac.input(&t.data[..]);
                let result2 = hmac.result();
                let expected2 = MacResult::new(&t.expected[..]);
                assert!(result2 == expected2);
            }
        }

        #[test]
        fn test_hmac_md5_incremental() {
            let tests = tests();
            for t in tests.iter() {
                let mut hmac = Hmac::new(Md5::new(), &t.key[..]);
                for i in 0..t.data.len() {
                    hmac.input(&t.data[i..i + 1]);
                }
                let result = hmac.result();
                let expected = MacResult::new(&t.expected[..]);
                assert!(result == expected);
            }
        }
    }

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