chacha20: unify quarter_round in soft backend and xchacha (#349)

Deduplicates `quarter_round` in xchacha using soft backend version

chacha20/src/backends/soft.rs

@@ -1,7 +1,7 @@
 //! Portable implementation which does not rely on architecture-specific
 //! intrinsics.
 
-use crate::{ChaChaCore, Rounds, Variant, STATE_WORDS};
+use crate::{quarter_round, ChaChaCore, Rounds, Variant, STATE_WORDS};
 
 #[cfg(feature = "cipher")]
 use crate::chacha::Block;
@@ -74,22 +74,3 @@ fn run_rounds<R: Rounds>(state: &[u32; STATE_WORDS]) -> [u32; STATE_WORDS] {
     }
     res
 }
-
-/// The ChaCha20 quarter round function
-fn quarter_round(a: usize, b: usize, c: usize, d: usize, state: &mut [u32; STATE_WORDS]) {
-    state[a] = state[a].wrapping_add(state[b]);
-    state[d] ^= state[a];
-    state[d] = state[d].rotate_left(16);
-
-    state[c] = state[c].wrapping_add(state[d]);
-    state[b] ^= state[c];
-    state[b] = state[b].rotate_left(12);
-
-    state[a] = state[a].wrapping_add(state[b]);
-    state[d] ^= state[a];
-    state[d] = state[d].rotate_left(8);
-
-    state[c] = state[c].wrapping_add(state[d]);
-    state[b] ^= state[c];
-    state[b] = state[b].rotate_left(7);
-}

chacha20/src/lib.rs

@@ -343,3 +343,32 @@ impl<R: Rounds, V: Variant> Drop for ChaChaCore<R, V> {
 #[cfg(feature = "zeroize")]
 #[cfg_attr(docsrs, doc(cfg(feature = "zeroize")))]
 impl<R: Rounds, V: Variant> ZeroizeOnDrop for ChaChaCore<R, V> {}
+
+/// The ChaCha20 quarter round function
+///
+/// We located this function in the root of the crate as we want it to be available
+/// for the soft backend and for xchacha.
+#[allow(dead_code)]
+pub(crate) fn quarter_round(
+    a: usize,
+    b: usize,
+    c: usize,
+    d: usize,
+    state: &mut [u32; STATE_WORDS],
+) {
+    state[a] = state[a].wrapping_add(state[b]);
+    state[d] ^= state[a];
+    state[d] = state[d].rotate_left(16);
+
+    state[c] = state[c].wrapping_add(state[d]);
+    state[b] ^= state[c];
+    state[b] = state[b].rotate_left(12);
+
+    state[a] = state[a].wrapping_add(state[b]);
+    state[d] ^= state[a];
+    state[d] = state[d].rotate_left(8);
+
+    state[c] = state[c].wrapping_add(state[d]);
+    state[b] ^= state[c];
+    state[b] = state[b].rotate_left(7);
+}

chacha20/src/xchacha.rs

@@ -7,7 +7,9 @@ use cipher::{
     StreamCipherSeekCore, StreamClosure,
 };
 
-use crate::{variants::Ietf, ChaChaCore, Rounds, CONSTANTS, R12, R20, R8, STATE_WORDS};
+use crate::{
+    quarter_round, variants::Ietf, ChaChaCore, Rounds, CONSTANTS, R12, R20, R8, STATE_WORDS,
+};
 
 #[cfg(feature = "zeroize")]
 use zeroize::ZeroizeOnDrop;
@@ -151,26 +153,6 @@ pub fn hchacha<R: Rounds>(key: &Key, input: &Array<u8, U16>) -> Array<u8, U32> {
     output
 }
 
-/// The ChaCha20 quarter round function
-// for simplicity this function is copied from the software backend
-fn quarter_round(a: usize, b: usize, c: usize, d: usize, state: &mut [u32; STATE_WORDS]) {
-    state[a] = state[a].wrapping_add(state[b]);
-    state[d] ^= state[a];
-    state[d] = state[d].rotate_left(16);
-
-    state[c] = state[c].wrapping_add(state[d]);
-    state[b] ^= state[c];
-    state[b] = state[b].rotate_left(12);
-
-    state[a] = state[a].wrapping_add(state[b]);
-    state[d] ^= state[a];
-    state[d] = state[d].rotate_left(8);
-
-    state[c] = state[c].wrapping_add(state[d]);
-    state[b] ^= state[c];
-    state[b] = state[b].rotate_left(7);
-}
-
 #[cfg(test)]
 mod hchacha20_tests {
     use super::*;