misc(core): simplify encoding logic (#781)

* perf(core): simplify encoding logic

* make constant-time

Author: themighty1

crates/core/src/transcript/encoding/encoder.rs

@@ -108,18 +108,16 @@ impl Encoder for ChaChaEncoder {
 
     fn encode_subsequence(&self, direction: Direction, seq: &Subsequence) -> Vec<u8> {
         const ZERO: [u8; 16] = [0; BIT_ENCODING_SIZE];
+
         let mut encoding = self.encode_idx(direction, seq.index());
-        for (byte_idx, &byte) in seq.data().iter().enumerate() {
-            let start = byte_idx * BYTE_ENCODING_SIZE;
-            for (bit_idx, bit) in byte.iter_lsb0().enumerate() {
-                let pos = start + (bit_idx * BIT_ENCODING_SIZE);
-                let delta = if bit { &self.delta } else { &ZERO };
-
-                encoding[pos..pos + BIT_ENCODING_SIZE]
-                    .iter_mut()
-                    .zip(delta)
-                    .for_each(|(a, b)| *a ^= *b);
-            }
+        for (pos, bit) in seq.data().iter_lsb0().enumerate() {
+            // Add the delta to the encoding whenever the encoded bit is 1,
+            // otherwise add a zero.
+            let summand = if bit { &self.delta } else { &ZERO };
+            encoding[pos * BIT_ENCODING_SIZE..(pos + 1) * BIT_ENCODING_SIZE]
+                .iter_mut()
+                .zip(summand)
+                .for_each(|(a, b)| *a ^= *b);
         }
 
         encoding