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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 crate::{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(())
}