1// Copyright 2015 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15//! PBKDF2 derivation and verification.
16//!
17//! Use `derive` to derive PBKDF2 outputs. Use `verify` to verify secret
18//! against previously-derived outputs.
19//!
20//! PBKDF2 is specified in [RFC 2898 Section 5.2] with test vectors given in
21//! [RFC 6070]. See also [NIST Special Publication 800-132].
22//!
23//! [RFC 2898 Section 5.2]: https://tools.ietf.org/html/rfc2898#section-5.2
24//! [RFC 6070]: https://tools.ietf.org/html/rfc6070
25//! [NIST Special Publication 800-132]:
26//! http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
27//!
28//! # Examples
29//!
30//! ## Password Database Example
31//!
32//! ```
33//! use ring::{digest, pbkdf2};
34//! use std::{collections::HashMap, num::NonZeroU32};
35//!
36//! static PBKDF2_ALG: pbkdf2::Algorithm = pbkdf2::PBKDF2_HMAC_SHA256;
37//! const CREDENTIAL_LEN: usize = digest::SHA256_OUTPUT_LEN;
38//! pub type Credential = [u8; CREDENTIAL_LEN];
39//!
40//! enum Error {
41//! WrongUsernameOrPassword
42//! }
43//!
44//! struct PasswordDatabase {
45//! pbkdf2_iterations: NonZeroU32,
46//! db_salt_component: [u8; 16],
47//!
48//! // Normally this would be a persistent database.
49//! storage: HashMap<String, Credential>,
50//! }
51//!
52//! impl PasswordDatabase {
53//! pub fn store_password(&mut self, username: &str, password: &str) {
54//! let salt = self.salt(username);
55//! let mut to_store: Credential = [0u8; CREDENTIAL_LEN];
56//! pbkdf2::derive(PBKDF2_ALG, self.pbkdf2_iterations, &salt,
57//! password.as_bytes(), &mut to_store);
58//! self.storage.insert(String::from(username), to_store);
59//! }
60//!
61//! pub fn verify_password(&self, username: &str, attempted_password: &str)
62//! -> Result<(), Error> {
63//! match self.storage.get(username) {
64//! Some(actual_password) => {
65//! let salt = self.salt(username);
66//! pbkdf2::verify(PBKDF2_ALG, self.pbkdf2_iterations, &salt,
67//! attempted_password.as_bytes(),
68//! actual_password)
69//! .map_err(|_| Error::WrongUsernameOrPassword)
70//! },
71//!
72//! None => Err(Error::WrongUsernameOrPassword)
73//! }
74//! }
75//!
76//! // The salt should have a user-specific component so that an attacker
77//! // cannot crack one password for multiple users in the database. It
78//! // should have a database-unique component so that an attacker cannot
79//! // crack the same user's password across databases in the unfortunate
80//! // but common case that the user has used the same password for
81//! // multiple systems.
82//! fn salt(&self, username: &str) -> Vec<u8> {
83//! let mut salt = Vec::with_capacity(self.db_salt_component.len() +
84//! username.as_bytes().len());
85//! salt.extend(self.db_salt_component.as_ref());
86//! salt.extend(username.as_bytes());
87//! salt
88//! }
89//! }
90//!
91//! fn main() {
92//! // Normally these parameters would be loaded from a configuration file.
93//! let mut db = PasswordDatabase {
94//! pbkdf2_iterations: NonZeroU32::new(100_000).unwrap(),
95//! db_salt_component: [
96//! // This value was generated from a secure PRNG.
97//! 0xd6, 0x26, 0x98, 0xda, 0xf4, 0xdc, 0x50, 0x52,
98//! 0x24, 0xf2, 0x27, 0xd1, 0xfe, 0x39, 0x01, 0x8a
99//! ],
100//! storage: HashMap::new(),
101//! };
102//!
103//! db.store_password("alice", "@74d7]404j|W}6u");
104//!
105//! // An attempt to log in with the wrong password fails.
106//! assert!(db.verify_password("alice", "wrong password").is_err());
107//!
108//! // Normally there should be an expoentially-increasing delay between
109//! // attempts to further protect against online attacks.
110//!
111//! // An attempt to log in with the right password succeeds.
112//! assert!(db.verify_password("alice", "@74d7]404j|W}6u").is_ok());
113//! }
114
115use crate::{constant_time, digest, error, hmac};
116use core::num::NonZeroU32;
117
118/// A PBKDF2 algorithm.
119#[derive(Clone, Copy, PartialEq, Eq)]
120pub struct Algorithm(hmac::Algorithm);
121
122/// PBKDF2 using HMAC-SHA1.
123pub static PBKDF2_HMAC_SHA1: Algorithm = Algorithm(hmac::HMAC_SHA1_FOR_LEGACY_USE_ONLY);
124
125/// PBKDF2 using HMAC-SHA256.
126pub static PBKDF2_HMAC_SHA256: Algorithm = Algorithm(hmac::HMAC_SHA256);
127
128/// PBKDF2 using HMAC-SHA384.
129pub static PBKDF2_HMAC_SHA384: Algorithm = Algorithm(hmac::HMAC_SHA384);
130
131/// PBKDF2 using HMAC-SHA512.
132pub static PBKDF2_HMAC_SHA512: Algorithm = Algorithm(hmac::HMAC_SHA512);
133
134/// Fills `out` with the key derived using PBKDF2 with the given inputs.
135///
136/// Do not use `derive` as part of verifying a secret; use `verify` instead, to
137/// minimize the effectiveness of timing attacks.
138///
139/// `out.len()` must be no larger than the digest length * (2**32 - 1), per the
140/// PBKDF2 specification.
141///
142/// | Parameter | RFC 2898 Section 5.2 Term
143/// |-------------|-------------------------------------------
144/// | digest_alg | PRF (HMAC with the given digest algorithm)
145/// | iterations | c (iteration count)
146/// | salt | S (salt)
147/// | secret | P (password)
148/// | out | dk (derived key)
149/// | out.len() | dkLen (derived key length)
150///
151/// # Panics
152///
153/// `derive` panics if `out.len()` is larger than (2**32 - 1) * the digest
154/// algorithm's output length, per the PBKDF2 specification.
155pub fn derive(
156 algorithm: Algorithm,
157 iterations: NonZeroU32,
158 salt: &[u8],
159 secret: &[u8],
160 out: &mut [u8],
161) {
162 let digest_alg: &Algorithm = algorithm.0.digest_algorithm();
163 let output_len: usize = digest_alg.output_len();
164
165 // This implementation's performance is asymptotically optimal as described
166 // in https://jbp.io/2015/08/11/pbkdf2-performance-matters/. However, it
167 // hasn't been optimized to the same extent as fastpbkdf2. In particular,
168 // this implementation is probably doing a lot of unnecessary copying.
169
170 let secret: Key = hmac::Key::new(algorithm:algorithm.0, key_value:secret);
171
172 // Clear |out|.
173 out.fill(0);
174
175 let mut idx: u32 = 0;
176
177 for chunk: &mut [u8] in out.chunks_mut(chunk_size:output_len) {
178 idx = idx.checked_add(1).expect(msg:"derived key too long");
179 derive_block(&secret, iterations, salt, idx, out:chunk);
180 }
181}
182
183fn derive_block(secret: &hmac::Key, iterations: NonZeroU32, salt: &[u8], idx: u32, out: &mut [u8]) {
184 let mut ctx: Context = hmac::Context::with_key(signing_key:secret);
185 ctx.update(data:salt);
186 ctx.update(&u32::to_be_bytes(self:idx));
187
188 let mut u: Tag = ctx.sign();
189
190 let mut remaining: u32 = iterations.into();
191 loop {
192 out.iter_mut().zip(u.as_ref()).for_each(|(o: &mut u8, u: &u8)| *o ^= *u);
193
194 if remaining == 1 {
195 break;
196 }
197 remaining -= 1;
198
199 u = hmac::sign(key:secret, data:u.as_ref());
200 }
201}
202
203/// Verifies that a previously-derived (e.g., using `derive`) PBKDF2 value
204/// matches the PBKDF2 value derived from the other inputs.
205///
206/// The comparison is done in constant time to prevent timing attacks. The
207/// comparison will fail if `previously_derived` is empty (has a length of
208/// zero).
209///
210/// | Parameter | RFC 2898 Section 5.2 Term
211/// |----------------------------|--------------------------------------------
212/// | digest_alg | PRF (HMAC with the given digest algorithm).
213/// | `iterations` | c (iteration count)
214/// | `salt` | S (salt)
215/// | `secret` | P (password)
216/// | `previously_derived` | dk (derived key)
217/// | `previously_derived.len()` | dkLen (derived key length)
218///
219/// # Panics
220///
221/// `verify` panics if `out.len()` is larger than (2**32 - 1) * the digest
222/// algorithm's output length, per the PBKDF2 specification.
223pub fn verify(
224 algorithm: Algorithm,
225 iterations: NonZeroU32,
226 salt: &[u8],
227 secret: &[u8],
228 previously_derived: &[u8],
229) -> Result<(), error::Unspecified> {
230 let digest_alg = algorithm.0.digest_algorithm();
231
232 if previously_derived.is_empty() {
233 return Err(error::Unspecified);
234 }
235
236 let mut derived_buf = [0u8; digest::MAX_OUTPUT_LEN];
237
238 let output_len = digest_alg.output_len();
239 let secret = hmac::Key::new(algorithm.0, secret);
240 let mut idx: u32 = 0;
241
242 let mut matches = 1;
243
244 for previously_derived_chunk in previously_derived.chunks(output_len) {
245 idx = idx.checked_add(1).expect("derived key too long");
246
247 let derived_chunk = &mut derived_buf[..previously_derived_chunk.len()];
248 derived_chunk.fill(0);
249
250 derive_block(&secret, iterations, salt, idx, derived_chunk);
251
252 // XXX: This isn't fully constant-time-safe. TODO: Fix that.
253 #[allow(clippy::bool_to_int_with_if)]
254 let current_block_matches =
255 if constant_time::verify_slices_are_equal(derived_chunk, previously_derived_chunk)
256 .is_ok()
257 {
258 1
259 } else {
260 0
261 };
262
263 matches &= current_block_matches;
264 }
265
266 if matches == 0 {
267 return Err(error::Unspecified);
268 }
269
270 Ok(())
271}
272