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    // Copyright 2015 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.

    //! PBKDF2 derivation and verification.
    //!
    //! Use `derive` to derive PBKDF2 outputs. Use `verify` to verify secret
    //! against previously-derived outputs.
    //!
    //! PBKDF2 is specified in [RFC 2898 Section 5.2] with test vectors given in
    //! [RFC 6070]. See also [NIST Special Publication 800-132].
    //!
    //! [RFC 2898 Section 5.2]: https://tools.ietf.org/html/rfc2898#section-5.2
    //! [RFC 6070]: https://tools.ietf.org/html/rfc6070
    //! [NIST Special Publication 800-132]:
    //!    http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
    //!
    //! # Examples
    //!
    //! ## Password Database Example
    //!
    //! ```
    //! use ring::{digest, pbkdf2};
    //! use std::collections::HashMap;
    //!
    //! static DIGEST_ALG: &'static digest::Algorithm = &digest::SHA256;
    //! const CREDENTIAL_LEN: usize = digest::SHA256_OUTPUT_LEN;
    //! pub type Credential = [u8; CREDENTIAL_LEN];
    //!
    //! enum Error {
    //!     WrongUsernameOrPassword
    //! }
    //!
    //! struct PasswordDatabase {
    //!     pbkdf2_iterations: u32,
    //!     db_salt_component: [u8; 16],
    //!
    //!     // Normally this would be a persistent database.
    //!     storage: HashMap<String, Credential>,
    //! }
    //!
    //! impl PasswordDatabase {
    //!     pub fn store_password(&mut self, username: &str, password: &str) {
    //!         let salt = self.salt(username);
    //!         let mut to_store: Credential = [0u8; CREDENTIAL_LEN];
    //!         pbkdf2::derive(DIGEST_ALG, self.pbkdf2_iterations, &salt,
    //!                        password.as_bytes(), &mut to_store);
    //!         self.storage.insert(String::from(username), to_store);
    //!     }
    //!
    //!     pub fn verify_password(&self, username: &str, attempted_password: &str)
    //!                            -> Result<(), Error> {
    //!         match self.storage.get(username) {
    //!            Some(actual_password) => {
    //!                let salt = self.salt(username);
    //!                pbkdf2::verify(DIGEST_ALG, self.pbkdf2_iterations, &salt,
    //!                               attempted_password.as_bytes(),
    //!                               actual_password)
    //!                     .map_err(|_| Error::WrongUsernameOrPassword)
    //!            },
    //!
    //!            None => Err(Error::WrongUsernameOrPassword)
    //!         }
    //!     }
    //!
    //!     // The salt should have a user-specific component so that an attacker
    //!     // cannot crack one password for multiple users in the database. It
    //!     // should have a database-unique component so that an attacker cannot
    //!     // crack the same user's password across databases in the unfortunate
    //!     // but common case that the user has used the same password for
    //!     // multiple systems.
    //!     fn salt(&self, username: &str) -> Vec<u8> {
    //!         let mut salt = Vec::with_capacity(self.db_salt_component.len() +
    //!                                           username.as_bytes().len());
    //!         salt.extend(self.db_salt_component.as_ref());
    //!         salt.extend(username.as_bytes());
    //!         salt
    //!     }
    //! }
    //!
    //! fn main() {
    //!     // Normally these parameters would be loaded from a configuration file.
    //!     let mut db = PasswordDatabase {
    //!         pbkdf2_iterations: 100_000,
    //!         db_salt_component: [
    //!             // This value was generated from a secure PRNG.
    //!             0xd6, 0x26, 0x98, 0xda, 0xf4, 0xdc, 0x50, 0x52,
    //!             0x24, 0xf2, 0x27, 0xd1, 0xfe, 0x39, 0x01, 0x8a
    //!         ],
    //!         storage: HashMap::new(),
    //!     };
    //!
    //!     db.store_password("alice", "@74d7]404j|W}6u");
    //!
    //!     // An attempt to log in with the wrong password fails.
    //!     assert!(db.verify_password("alice", "wrong password").is_err());
    //!
    //!     // Normally there should be an expoentially-increasing delay between
    //!     // attempts to further protect against online attacks.
    //!
    //!     // An attempt to log in with the right password succeeds.
    //!     assert!(db.verify_password("alice", "@74d7]404j|W}6u").is_ok());
    //! }

    use {constant_time, digest, error, hmac, polyfill};

    /// Fills `out` with the key derived using PBKDF2 with the given inputs.
    ///
    /// Do not use `derive` as part of verifying a secret; use `verify` instead, to
    /// minimize the effectiveness of timing attacks.
    ///
    /// `out.len()` must be no larger than the digest length * (2**32 - 1), per the
    /// PBKDF2 specification.
    ///
    /// | Parameter   | RFC 2898 Section 5.2 Term
    /// |-------------|-------------------------------------------
    /// | digest_alg  | PRF (HMAC with the given digest algorithm)
    /// | iterations  | c (iteration count)
    /// | salt        | S (salt)
    /// | secret      | P (password)
    /// | out         | dk (derived key)
    /// | out.len()   | dkLen (derived key length)
    ///
    /// C analog: `PKCS5_PBKDF2_HMAC`
    ///
    /// # Panics
    ///
    /// `derive` panics if `iterations < 1`.
    ///
    /// `derive` panics if `out.len()` is larger than (2**32 - 1) * the digest
    /// algorithm's output length, per the PBKDF2 specification.
    pub fn derive(digest_alg: &'static digest::Algorithm, iterations: u32,
                  salt: &[u8], secret: &[u8], out: &mut [u8]) {
        assert!(iterations >= 1);

        let output_len = digest_alg.output_len;

        // This implementation's performance is asymptotically optimal as described
        // in https://jbp.io/2015/08/11/pbkdf2-performance-matters/. However, it
        // hasn't been optimized to the same extent as fastpbkdf2. In particular,
        // this implementation is probably doing a lot of unnecessary copying.

        let secret = hmac::SigningKey::new(digest_alg, secret);

        // Clear |out|.
        polyfill::slice::fill(out, 0);

        let mut idx: u32 = 0;

        for chunk in out.chunks_mut(output_len) {
            idx = idx.checked_add(1).expect("derived key too long");
            derive_block(&secret, iterations, salt, idx, chunk);
        }
    }

    fn derive_block(secret: &hmac::SigningKey, iterations: u32, salt: &[u8],
                    idx: u32, out: &mut [u8]) {
        let mut ctx = hmac::SigningContext::with_key(secret);
        ctx.update(salt);
        ctx.update(&polyfill::slice::be_u8_from_u32(idx));

        let mut u = ctx.sign();

        let mut remaining = iterations;
        loop {
            for i in 0..out.len() {
                out[i] ^= u.as_ref()[i];
            }

            if remaining == 1 {
                break;
            }
            remaining -= 1;

            u = hmac::sign(secret, u.as_ref());
        }
    }

    /// Verifies that a previously-derived (e.g., using `derive`) PBKDF2 value
    /// matches the PBKDF2 value derived from the other inputs.
    ///
    /// The comparison is done in constant time to prevent timing attacks. The
    /// comparison will fail if `previously_derived` is empty (has a length of
    /// zero).
    ///
    /// | Parameter                  | RFC 2898 Section 5.2 Term
    /// |----------------------------|--------------------------------------------
    /// | digest_alg                 | PRF (HMAC with the given digest algorithm).
    /// | `iterations`               | c (iteration count)
    /// | `salt`                     | S (salt)
    /// | `secret`                   | P (password)
    /// | `previously_derived`       | dk (derived key)
    /// | `previously_derived.len()` | dkLen (derived key length)
    ///
    /// C analog: `PKCS5_PBKDF2_HMAC` + `CRYPTO_memcmp`
    ///
    /// # Panics
    ///
    /// `verify` panics if `iterations < 1`.
    ///
    /// `verify` panics if `out.len()` is larger than (2**32 - 1) * the digest
    /// algorithm's output length, per the PBKDF2 specification.
    pub fn verify(digest_alg: &'static digest::Algorithm, iterations: u32,
                  salt: &[u8], secret: &[u8], previously_derived: &[u8])
                  -> Result<(), error::Unspecified> {
        if previously_derived.is_empty() {
            return Err(error::Unspecified);
        }

        let mut derived_buf = [0u8; digest::MAX_OUTPUT_LEN];

        let output_len = digest_alg.output_len;
        let secret = hmac::SigningKey::new(digest_alg, secret);
        let mut idx: u32 = 0;

        let mut matches = 1;

        for previously_derived_chunk in previously_derived.chunks(output_len) {
            idx = idx.checked_add(1).expect("derived key too long");

            let derived_chunk = &mut derived_buf[..previously_derived_chunk.len()];
            polyfill::slice::fill(derived_chunk, 0);

            derive_block(&secret, iterations, salt, idx, derived_chunk);

            // XXX: This isn't fully constant-time-safe. TODO: Fix that.
            let current_block_matches =
                if constant_time::verify_slices_are_equal(
                        derived_chunk, previously_derived_chunk).is_ok() {
                    1
                } else {
                    0
                };

            matches &= current_block_matches;
        }

        if matches == 0 {
            return Err(error::Unspecified);
        }

        Ok(())
    }

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