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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 PBKDF2 Key Derivation Function as specified by
     * http://tools.ietf.org/html/rfc2898.
     */

    use std::iter::repeat;
    use std::io;
    use cryptoutil::copy_memory;

    use rand::{OsRng, Rng};
    use serialize::base64;
    use serialize::base64::{FromBase64, ToBase64};

    use cryptoutil::{read_u32_be, write_u32_be};
    use hmac::Hmac;
    use mac::Mac;
    use sha2::Sha256;
    use util::fixed_time_eq;

    // Calculate a block of the output of size equal to the output_bytes of the underlying Mac function
    // mac - The Mac function to use
    // salt - the salt value to use
    // c - the iteration count
    // idx - the 1 based index of the block
    // scratch - a temporary variable the same length as the block
    // block - the block of the output to calculate
    fn calculate_block<M: Mac>(
            mac: &mut M,
            salt: &[u8],
            c: u32,
            idx: u32,
            scratch: &mut [u8],
            block: &mut [u8]) {
        // Perform the 1st iteration. The output goes directly into block
        mac.input(salt);
        let mut idx_buf = [0u8; 4];
        write_u32_be(&mut idx_buf, idx);
        mac.input(&idx_buf);
        mac.raw_result(block);
        mac.reset();

        // Perform the 2nd iteration. The input comes from block and is output into scratch. scratch is
        // then exclusive-or added into block. After all this, the input to the next step is now in
        // scratch and block is left to just accumulate the exclusive-of sum of remaining iterations.
        if c > 1 {
            mac.input(block);
            mac.raw_result(scratch);
            mac.reset();
            for (output, &input) in block.iter_mut().zip(scratch.iter()) {
                *output ^= input;
            }
        }

        // Perform all remaining iterations
        for _ in 2..c {
            mac.input(scratch);
            mac.raw_result(scratch);
            mac.reset();
            for (output, &input) in block.iter_mut().zip(scratch.iter()) {
                *output ^= input;
            }
        }
    }

    /**
     * Execute the PBKDF2 Key Derivation Function. The Scrypt Key Derivation Function generally provides
     * better security, so, applications that do not have a requirement to use PBKDF2 specifically
     * should consider using that function instead.
     *
     * # Arguments
     * * mac - The Pseudo Random Function to use.
     * * salt - The salt value to use.
     * * c - The iteration count. Users should carefully determine this value as it is the primary
     *       factor in determining the security of the derived key.
     * * output - The output buffer to fill with the derived key value.
     *
     */
    pub fn pbkdf2<M: Mac>(mac: &mut M, salt: &[u8], c: u32, output: &mut [u8]) {
        assert!(c > 0);

        let os = mac.output_bytes();

        // A temporary storage array needed by calculate_block. This is really only necessary if c > 1.
        // Most users of pbkdf2 should use a value much larger than 1, so, this allocation should almost
        // always be necessary. A big exception is Scrypt. However, this allocation is unlikely to be
        // the bottleneck in Scrypt performance.
        let mut scratch: Vec<u8> = repeat(0).take(os).collect();

        let mut idx: u32 = 0;

        for chunk in output.chunks_mut(os) {
            // The block index starts at 1. So, this is supposed to run on the first execution.
            idx = idx.checked_add(1).expect("PBKDF2 size limit exceeded.");

            if chunk.len() == os {
                calculate_block(mac, salt, c, idx, &mut scratch, chunk);
            } else {
                let mut tmp: Vec<u8> = repeat(0).take(os).collect();
                calculate_block(mac, salt, c, idx, &mut scratch[..], &mut tmp[..]);
                let chunk_len = chunk.len();
                copy_memory(&tmp[..chunk_len], chunk);
            }
        }
    }

    /**
     * pbkdf2_simple is a helper function that should be sufficient for the majority of cases where
     * an application needs to use PBKDF2 to hash a password for storage. The result is a String that
     * contains the parameters used as part of its encoding. The pbkdf2_check function may be used on
     * a password to check if it is equal to a hashed value.
     *
     * # Format
     *
     * The format of the output is a modified version of the Modular Crypt Format that encodes algorithm
     * used and iteration count. The format is indicated as "rpbkdf2" which is short for "Rust PBKF2
     * format."
     *
     * $rpbkdf2$0$<base64(c)>$<base64(salt)>$<based64(hash)>$
     *
     * # Arguments
     *
     * * password - The password to process as a str
     * * c - The iteration count
     *
     */
    pub fn pbkdf2_simple(password: &str, c: u32) -> io::Result<String> {
        let mut rng = try!(OsRng::new());

        // 128-bit salt
        let salt: Vec<u8> = rng.gen_iter::<u8>().take(16).collect();

        // 256-bit derived key
        let mut dk = [0u8; 32];

        let mut mac = Hmac::new(Sha256::new(), password.as_bytes());

        pbkdf2(&mut mac, &salt[..], c, &mut dk);

        let mut result = "$rpbkdf2$0$".to_string();
        let mut tmp = [0u8; 4];
        write_u32_be(&mut tmp, c);
        result.push_str(&tmp.to_base64(base64::STANDARD)[..]);
        result.push('$');
        result.push_str(&salt.to_base64(base64::STANDARD)[..]);
        result.push('$');
        result.push_str(&dk.to_base64(base64::STANDARD)[..]);
        result.push('$');

        Ok(result)
    }

    /**
     * pbkdf2_check compares a password against the result of a previous call to pbkdf2_simple and
     * returns true if the passed in password hashes to the same value.
     *
     * # Arguments
     *
     * * password - The password to process as a str
     * * hashed_value - A string representing a hashed password returned by pbkdf2_simple()
     *
     */
    pub fn pbkdf2_check(password: &str, hashed_value: &str) -> Result<bool, &'static str> {
        static ERR_STR: &'static str = "Hash is not in Rust PBKDF2 format.";

        let mut iter = hashed_value.split('$');

        // Check that there are no characters before the first "$"
        match iter.next() {
            Some(x) => if x != "" { return Err(ERR_STR); },
            None => return Err(ERR_STR)
        }

        // Check the name
        match iter.next() {
            Some(t) => if t != "rpbkdf2" { return Err(ERR_STR); },
            None => return Err(ERR_STR)
        }

        // Parse format - currenlty only version 0 is supported
        match iter.next() {
            Some(fstr) => {
                match fstr {
                    "0" => { }
                    _ => return Err(ERR_STR)
                }
            }
            None => return Err(ERR_STR)
        }

        // Parse the iteration count
        let c = match iter.next() {
            Some(pstr) => match pstr.from_base64() {
                Ok(pvec) => {
                    if pvec.len() != 4 { return Err(ERR_STR); }
                    read_u32_be(&pvec[..])
                }
                Err(_) => return Err(ERR_STR)
            },
            None => return Err(ERR_STR)
        };

        // Salt
        let salt = match iter.next() {
            Some(sstr) => match sstr.from_base64() {
                Ok(salt) => salt,
                Err(_) => return Err(ERR_STR)
            },
            None => return Err(ERR_STR)
        };

        // Hashed value
        let hash = match iter.next() {
            Some(hstr) => match hstr.from_base64() {
                Ok(hash) => hash,
                Err(_) => return Err(ERR_STR)
            },
            None => return Err(ERR_STR)
        };

        // Make sure that the input ends with a "$"
        match iter.next() {
            Some(x) => if x != "" { return Err(ERR_STR); },
            None => return Err(ERR_STR)
        }

        // Make sure there is no trailing data after the final "$"
        match iter.next() {
            Some(_) => return Err(ERR_STR),
            None => { }
        }

        let mut mac = Hmac::new(Sha256::new(), password.as_bytes());

        let mut output: Vec<u8> = repeat(0).take(hash.len()).collect();
        pbkdf2(&mut mac, &salt[..], c, &mut output[..]);

        // Be careful here - its important that the comparison be done using a fixed time equality
        // check. Otherwise an adversary that can measure how long this step takes can learn about the
        // hashed value which would allow them to mount an offline brute force attack against the
        // hashed password.
        Ok(fixed_time_eq(&output[..], &hash[..]))
    }

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

        use pbkdf2::{pbkdf2, pbkdf2_simple, pbkdf2_check};
        use hmac::Hmac;
        use sha1::Sha1;

        struct Test {
            password: Vec<u8>,
            salt: Vec<u8>,
            c: u32,
            expected: Vec<u8>
        }

        // Test vectors from http://tools.ietf.org/html/rfc6070. The 4th test vector is omitted because
        // it takes too long to run.

        fn tests() -> Vec<Test> {
            vec![
                Test {
                    password: b"password".to_vec(),
                    salt: b"salt".to_vec(),
                    c: 1,
                    expected: vec![
                        0x0c, 0x60, 0xc8, 0x0f, 0x96, 0x1f, 0x0e, 0x71,
                        0xf3, 0xa9, 0xb5, 0x24, 0xaf, 0x60, 0x12, 0x06,
                        0x2f, 0xe0, 0x37, 0xa6 ]
                },
                Test {
                    password: b"password".to_vec(),
                    salt: b"salt".to_vec(),
                    c: 2,
                    expected: vec![
                        0xea, 0x6c, 0x01, 0x4d, 0xc7, 0x2d, 0x6f, 0x8c,
                        0xcd, 0x1e, 0xd9, 0x2a, 0xce, 0x1d, 0x41, 0xf0,
                        0xd8, 0xde, 0x89, 0x57 ]
                },
                Test {
                    password: b"password".to_vec(),
                    salt: b"salt".to_vec(),
                    c: 4096,
                    expected: vec![
                        0x4b, 0x00, 0x79, 0x01, 0xb7, 0x65, 0x48, 0x9a,
                        0xbe, 0xad, 0x49, 0xd9, 0x26, 0xf7, 0x21, 0xd0,
                        0x65, 0xa4, 0x29, 0xc1 ]
                },
                Test {
                    password: b"passwordPASSWORDpassword".to_vec(),
                    salt: b"saltSALTsaltSALTsaltSALTsaltSALTsalt".to_vec(),
                    c: 4096,
                    expected: vec![
                        0x3d, 0x2e, 0xec, 0x4f, 0xe4, 0x1c, 0x84, 0x9b,
                        0x80, 0xc8, 0xd8, 0x36, 0x62, 0xc0, 0xe4, 0x4a,
                        0x8b, 0x29, 0x1a, 0x96, 0x4c, 0xf2, 0xf0, 0x70, 0x38 ]
                },
                Test {
                    password: vec![112, 97, 115, 115, 0, 119, 111, 114, 100],
                    salt: vec![115, 97, 0, 108, 116],
                    c: 4096,
                    expected: vec![
                        0x56, 0xfa, 0x6a, 0xa7, 0x55, 0x48, 0x09, 0x9d,
                        0xcc, 0x37, 0xd7, 0xf0, 0x34, 0x25, 0xe0, 0xc3 ]
                }
            ]
        }

        #[test]
        fn test_pbkdf2() {
            let tests = tests();
            for t in tests.iter() {
                let mut mac = Hmac::new(Sha1::new(), &t.password[..]);
                let mut result: Vec<u8> = repeat(0).take(t.expected.len()).collect();
                pbkdf2(&mut mac, &t.salt[..], t.c, &mut result);
                assert!(result == t.expected);
            }
        }

        #[test]
        fn test_pbkdf2_simple() {
            let password = "password";

            let out1 = pbkdf2_simple(password, 1024).unwrap();
            let out2 = pbkdf2_simple(password, 1024).unwrap();

            // This just makes sure that a salt is being applied. It doesn't verify that that salt is
            // cryptographically strong, however.
            assert!(out1 != out2);

            match pbkdf2_check(password, &out1[..]) {
                Ok(r) => assert!(r),
                Err(_) => panic!()
            }
            match pbkdf2_check(password, &out2[..]) {
                Ok(r) => assert!(r),
                Err(_) => panic!()
            }

            match pbkdf2_check("wrong", &out1[..]) {
                Ok(r) => assert!(!r),
                Err(_) => panic!()
            }
            match pbkdf2_check("wrong", &out2[..]) {
                Ok(r) => assert!(!r),
                Err(_) => panic!()
            }
        }
    }

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