serpent: add serpent_no_unroll configuration flag (#476)

RustCrypto · Feb 28, 2025 · cbdb9f0 · cbdb9f0
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Showing 7 changed files with 77 additions and 71 deletions.
diff --git a/.github/workflows/serpent.yml b/.github/workflows/serpent.yml
@@ -36,6 +36,9 @@ jobs:
           toolchain: ${{ matrix.rust }}
           targets: ${{ matrix.target }}
       - run: cargo build --no-default-features --release --target ${{ matrix.target }}
+      - env:
+          RUSTFLAGS: "-Dwarnings --cfg serpent_no_unroll"
+        run: cargo build --no-default-features --release --target ${{ matrix.target }}
 
   minimal-versions:
     if: false # TODO: temp disabled due to unpublished prerelease dependencies
@@ -60,3 +63,6 @@ jobs:
       - run: cargo test --no-default-features
       - run: cargo test
       - run: cargo test --all-features
+      - env:
+          RUSTFLAGS: "-Dwarnings --cfg serpent_no_unroll"
+        run: cargo test --all-features
diff --git a/serpent/CHANGELOG.md b/serpent/CHANGELOG.md
@@ -5,6 +5,16 @@ 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).
 
+## 0.6.0 (UNRELEASED)
+### Added
+- `serpent_no_unroll` configuration flag ([#476])
+
+### Changed
+- Improve bitslicing implementation ([#474])
+
+[#474]: https://github.com/RustCrypto/block-ciphers/pull/474
+[#476]: https://github.com/RustCrypto/block-ciphers/pull/476
+
 ## 0.5.1 (2022-02-17)
 ### Fixed
 - Minimal versions build ([#303])

diff --git a/serpent/Cargo.toml b/serpent/Cargo.toml
@@ -24,3 +24,6 @@ zeroize = ["cipher/zeroize"]
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(serpent_no_unroll)'] }
diff --git a/serpent/README.md b/serpent/README.md
@@ -26,6 +26,14 @@ architectures.
 
 USE AT YOUR OWN RISK!
 
+## Configuration flags
+
+You can modify crate using the following configuration flags:
+
+- `serpent_no_unroll`: do not unroll rounds loop. Reduces binary size at the cost of slightly lower performance.
+
+The flag can be enabled using RUSTFLAGS environmental variable (e.g. RUSTFLAGS="--cfg serpent_no_unroll") or by modifying .cargo/config.
+
 ## Minimum Supported Rust Version
 
 Rust **1.65** or higher.

diff --git a/serpent/src/bitslice.rs b/serpent/src/bitslice.rs
@@ -1,4 +1,3 @@
-//!
 //! Serpent uses 8 4-bit Sboxes which were designed to be implemented using small
 //! circuits. For each block these Sboxes are applied in a bitsliced fashion.
 //!
@@ -11,7 +10,6 @@
 //!
 //! "Speeding Up Serpent", Osvik
 //! <https://www.ii.uib.no/~osvik/pub/aes3.pdf>
-//!
 
 use crate::Words;
 

diff --git a/serpent/src/lib.rs b/serpent/src/lib.rs
@@ -31,6 +31,8 @@ use cipher::{
 use core::fmt;
 
 mod bitslice;
+#[macro_use]
+mod unroll;
 
 #[cfg(feature = "zeroize")]
 use cipher::zeroize::{Zeroize, ZeroizeOnDrop};
@@ -56,73 +58,6 @@ fn xor(b1: Words, k: Words) -> Words {
     res
 }
 
-macro_rules! repeat31 {
-    ($i:ident, $body:block) => {
-        let $i = 0;
-        $body;
-        let $i = 1;
-        $body;
-        let $i = 2;
-        $body;
-        let $i = 3;
-        $body;
-        let $i = 4;
-        $body;
-        let $i = 5;
-        $body;
-        let $i = 6;
-        $body;
-        let $i = 7;
-        $body;
-        let $i = 8;
-        $body;
-        let $i = 9;
-        $body;
-        let $i = 10;
-        $body;
-        let $i = 11;
-        $body;
-        let $i = 12;
-        $body;
-        let $i = 13;
-        $body;
-        let $i = 14;
-        $body;
-        let $i = 15;
-        $body;
-        let $i = 16;
-        $body;
-        let $i = 17;
-        $body;
-        let $i = 18;
-        $body;
-        let $i = 19;
-        $body;
-        let $i = 20;
-        $body;
-        let $i = 21;
-        $body;
-        let $i = 22;
-        $body;
-        let $i = 23;
-        $body;
-        let $i = 24;
-        $body;
-        let $i = 25;
-        $body;
-        let $i = 26;
-        $body;
-        let $i = 27;
-        $body;
-        let $i = 28;
-        $body;
-        let $i = 29;
-        $body;
-        let $i = 30;
-        $body;
-    };
-}
-
 fn expand_key(source: &[u8], len_bits: usize) -> [u8; 32] {
     let mut key = [0u8; 32];
     key[..source.len()].copy_from_slice(source);
@@ -208,7 +143,7 @@ impl BlockCipherEncBackend for Serpent {
     fn encrypt_block(&self, mut block: InOut<'_, '_, Block<Self>>) {
         let mut b: [u32; 4] = read_words(block.get_in().into());
 
-        repeat31!(i, {
+        unroll31!(i, {
             let xb = xor(b, self.round_keys[i]);
             let s = bitslice::apply_s(i, xb);
             b = bitslice::linear_transform(s);
@@ -238,7 +173,7 @@ impl BlockCipherDecBackend for Serpent {
         let xb = bitslice::apply_s_inv(ROUNDS - 1, s);
         b = xor(xb, self.round_keys[ROUNDS - 1]);
 
-        repeat31!(i, {
+        unroll31!(i, {
             let i = 30 - i;
             let s = bitslice::linear_transform_inv(b);
             let xb = bitslice::apply_s_inv(i, s);

diff --git a/serpent/src/unroll.rs b/serpent/src/unroll.rs
@@ -0,0 +1,46 @@
+#[cfg(not(serpent_no_unroll))]
+#[rustfmt::skip]
+macro_rules! unroll31 {
+    ($i:ident, $body:block) => {
+        let $i = 0; $body;
+        let $i = 1; $body;
+        let $i = 2; $body;
+        let $i = 3; $body;
+        let $i = 4; $body;
+        let $i = 5; $body;
+        let $i = 6; $body;
+        let $i = 7; $body;
+        let $i = 8; $body;
+        let $i = 9; $body;
+        let $i = 10; $body;
+        let $i = 11; $body;
+        let $i = 12; $body;
+        let $i = 13; $body;
+        let $i = 14; $body;
+        let $i = 15; $body;
+        let $i = 16; $body;
+        let $i = 17; $body;
+        let $i = 18; $body;
+        let $i = 19; $body;
+        let $i = 20; $body;
+        let $i = 21; $body;
+        let $i = 22; $body;
+        let $i = 23; $body;
+        let $i = 24; $body;
+        let $i = 25; $body;
+        let $i = 26; $body;
+        let $i = 27; $body;
+        let $i = 28; $body;
+        let $i = 29; $body;
+        let $i = 30; $body;
+    };
+}
+
+#[cfg(serpent_no_unroll)]
+macro_rules! unroll31 {
+    ($i:ident, $body:block) => {
+        for $i in 0..31 {
+            $body;
+        }
+    };
+}