pbkdf2: remove the parallel crate feature (#702)

Author: newpavlov

Cargo.lock

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 0.13.0 (UNRELEASED)
+### Removed
+- The `parallel` crate feature  ([#702])
+
+[#702]: https://github.com/RustCrypto/password-hashes/pull/702
+
 ## 0.12.2 (2023-07-08)
 ### Fixed
 - Use `RECOMMENDED_ROUNDS` in `Default` impl for `Params` ([#442])

pbkdf2/CHANGELOG.md

@@ -17,7 +17,6 @@ rust-version = "1.85"
 digest = { version = "0.11.0-rc.1", features = ["mac"] }
 
 # optional dependencies
-rayon = { version = "1.7", optional = true }
 password-hash = { version = "0.6.0-rc.0", default-features = false, optional = true, features = ["rand_core"] }
 hmac = { version = "0.13.0-rc.1", default-features = false, optional = true }
 sha1 = { version = "0.11.0-rc.2", default-features = false, optional = true }
@@ -34,8 +33,6 @@ belt-hash = "0.2.0-rc.1"
 [features]
 default = ["hmac"]
 std = ["password-hash/os_rng"]
-
-parallel = ["rayon", "std"]
 simple = ["hmac", "password-hash", "sha2"]
 
 [package.metadata.docs.rs]

pbkdf2/Cargo.toml

@@ -3,11 +3,18 @@
 //!
 //! # Examples
 //!
-//! PBKDF2 is defined in terms of a keyed pseudo-random function (PRF). Most
-//! commonly HMAC is used as this PRF. In such cases you can use [`pbkdf2_hmac`]
-//! and [`pbkdf2_hmac_array`] functions. The former accepts a byte slice which
-//! gets filled with generated key, while the latter returns an array with
-//! generated key of requested length.
+//! PBKDF2 is defined in terms of a keyed pseudo-random function (PRF).
+//! The most commonly used PRF for this purpose is HMAC. In such cases
+//! you can use [`pbkdf2_hmac`] and [`pbkdf2_hmac_array`] functions.
+//! The former accepts a byte slice which gets filled with generated key,
+//! while the latter returns an array with generated key of requested length.
+//!
+//! Note that it is not recommended to generate keys using PBKDF2 that exceed
+//! the output size of the PRF (equal to the hash size in the case of HMAC).
+//! If you need to generate a large amount of cryptographic material,
+//! consider using a separate [key derivation function][KDF].
+//!
+//! [KDF]: https://github.com/RustCrypto/KDFs
 //!
 //! ```
 //! # #[cfg(feature = "hmac")] {
@@ -96,9 +103,6 @@ pub use hmac;
 #[cfg(feature = "simple")]
 pub use crate::simple::{Algorithm, Params, Pbkdf2};
 
-#[cfg(feature = "parallel")]
-use rayon::prelude::*;
-
 use digest::{FixedOutput, InvalidLength, KeyInit, Update, typenum::Unsigned};
 
 #[cfg(feature = "hmac")]
@@ -162,21 +166,10 @@ where
     PRF: KeyInit + Update + FixedOutput + Clone + Sync,
 {
     let n = PRF::OutputSize::to_usize();
-    // note: HMAC can be initialized with keys of any size,
-    // so this panic never happens with it
     let prf = PRF::new_from_slice(password)?;
 
-    #[cfg(not(feature = "parallel"))]
-    {
-        for (i, chunk) in res.chunks_mut(n).enumerate() {
-            pbkdf2_body(i as u32, chunk, &prf, salt, rounds);
-        }
-    }
-    #[cfg(feature = "parallel")]
-    {
-        res.par_chunks_mut(n).enumerate().for_each(|(i, chunk)| {
-            pbkdf2_body(i as u32, chunk, &prf, salt, rounds);
-        });
+    for (i, chunk) in res.chunks_mut(n).enumerate() {
+        pbkdf2_body(i as u32, chunk, &prf, salt, rounds);
     }
 
     Ok(())