move quarter_round function to the root

chacha20/src/backends/soft.rs

@@ -1,7 +1,7 @@
 //! Portable implementation which does not rely on architecture-specific
 //! intrinsics.
 
-use crate::{Block, ChaChaCore, Unsigned, STATE_WORDS};
+use crate::{quarter_round, Block, ChaChaCore, Unsigned, STATE_WORDS};
 use cipher::{
     consts::{U1, U64},
     BlockSizeUser, ParBlocksSizeUser, StreamBackend,
@@ -52,28 +52,3 @@ fn run_rounds<R: Unsigned>(state: &[u32; STATE_WORDS]) -> [u32; STATE_WORDS] {
     }
     res
 }
-
-/// The ChaCha20 quarter round function
-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/lib.rs

@@ -315,3 +315,31 @@ impl<R: Unsigned> Drop for ChaChaCore<R> {
 #[cfg(feature = "zeroize")]
 #[cfg_attr(docsrs, doc(cfg(feature = "zeroize")))]
 impl<R: Unsigned> ZeroizeOnDrop for ChaChaCore<R> {}
+
+/// 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.
+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

@@ -1,15 +1,13 @@
 //! XChaCha is an extended nonce variant of ChaCha
 
-use super::{ChaChaCore, Key, Nonce, CONSTANTS, STATE_WORDS};
+use super::{quarter_round, ChaChaCore, Key, Nonce, CONSTANTS, STATE_WORDS};
 use cipher::{
     consts::{U10, U16, U24, U32, U4, U6, U64},
     generic_array::{typenum::Unsigned, GenericArray},
     BlockSizeUser, IvSizeUser, KeyIvInit, KeySizeUser, StreamCipherCore, StreamCipherCoreWrapper,
     StreamCipherSeekCore, StreamClosure,
 };
 
-use crate::backends::soft::quarter_round;
-
 #[cfg(feature = "zeroize")]
 use cipher::zeroize::ZeroizeOnDrop;