move each primitive into its own module; add rough documentation

This commit does not change anything about the implementations.

constant-time/src/xor.rs

@@ -0,0 +1,34 @@
+use core::hint::black_box;
+use rosenpass_to::{with_destination, To};
+
+/// Xors the source into the destination
+///
+/// # Panics
+/// If source and destination are of different sizes.
+///
+/// # Leaks
+/// TODO: mention here if this function leaks any information, see
+/// <https://github.com/rosenpass/rosenpass/issues/232>
+///
+/// ## Tests
+/// For discussion on how to ensure the constant-time execution of this function, see
+/// <https://github.com/rosenpass/rosenpass/issues/232>
+///
+/// # Examples
+///
+/// ```
+/// use rosenpass_constant_time::xor;
+/// use rosenpass_to::To;
+/// assert_eq!(
+///     xor(b"world").to_this(|| b"hello".to_vec()),
+///     b"\x1f\n\x1e\x00\x0b");
+/// ```
+#[inline]
+pub fn xor(src: &[u8]) -> impl To<[u8], ()> + '_ {
+    with_destination(|dst: &mut [u8]| {
+        assert!(black_box(src.len()) == black_box(dst.len()));
+        for (dv, sv) in dst.iter_mut().zip(src.iter()) {
+            *black_box(dv) ^= black_box(*sv);
+        }
+    })
+}