From e32b240ce2f4e573287744ecc5f41c547cbfce70 Mon Sep 17 00:00:00 2001
From: Kostas Chalkias <kostascrypto@fb.com>
Date: Thu, 4 Feb 2021 17:00:17 -0800
Subject: [PATCH] Reuse hashers by hasher.finalize_reset which is slightly
 faster than constructing the hasher object each time.

---
 src/key_exchange/tripledh.rs | 22 ++++++++++------------
 src/map_to_curve.rs          |  5 ++---
 2 files changed, 12 insertions(+), 15 deletions(-)

diff --git a/src/key_exchange/tripledh.rs b/src/key_exchange/tripledh.rs
index 61606b2..ce03567 100644
--- a/src/key_exchange/tripledh.rs
+++ b/src/key_exchange/tripledh.rs
@@ -129,19 +129,18 @@ impl<D: Hash, G: Group> KeyExchange<D, G> for TripleDH {
         ]
         .concat();
 
-        let mut hasher2 = D::new();
-        hasher2.update(&transcript2);
-        let hashed_transcript_without_mac = hasher2.finalize();
+        let mut hasher = D::new();
+        hasher.update(&transcript2);
+        let hashed_transcript_without_mac = hasher.finalize_reset();
 
         let mut mac_hasher =
             Hmac::<D>::new_varkey(&km2).map_err(|_| InternalPakeError::HmacError)?;
         mac_hasher.update(&hashed_transcript_without_mac);
         let mac = mac_hasher.finalize().into_bytes();
 
-        let mut hasher3 = D::new();
-        hasher3.update(&transcript2);
-        hasher3.update(&mac);
-        let hashed_transcript = hasher3.finalize();
+        hasher.update(&transcript2);
+        hasher.update(&mac);
+        let hashed_transcript = hasher.finalize();
 
         Ok((
             ke1_message.info,
@@ -194,7 +193,7 @@ impl<D: Hash, G: Group> KeyExchange<D, G> for TripleDH {
 
         let mut hasher = D::new();
         hasher.update(&transcript);
-        let hashed_transcript_without_mac = hasher.finalize();
+        let hashed_transcript_without_mac = hasher.finalize_reset();
 
         let mut server_mac =
             Hmac::<D>::new_varkey(&km2).map_err(|_| InternalPakeError::HmacError)?;
@@ -206,10 +205,9 @@ impl<D: Hash, G: Group> KeyExchange<D, G> for TripleDH {
             ));
         }
 
-        let mut hasher2 = D::new();
-        hasher2.update(transcript);
-        hasher2.update(ke2_message.mac.to_vec());
-        let hashed_transcript = hasher2.finalize();
+        hasher.update(transcript);
+        hasher.update(ke2_message.mac.to_vec());
+        let hashed_transcript = hasher.finalize();
 
         let mut client_mac =
             Hmac::<D>::new_varkey(&km3).map_err(|_| InternalPakeError::HmacError)?;
diff --git a/src/map_to_curve.rs b/src/map_to_curve.rs
index f3c5564..3875b88 100644
--- a/src/map_to_curve.rs
+++ b/src/map_to_curve.rs
@@ -84,17 +84,16 @@ pub(crate) fn expand_message_xmd<H: Hash>(
     h.update(&b[0]);
     h.update(&i2osp(1, 1));
     h.update(&dst_prime);
-    b.push(h.finalize().to_vec()); // b[1]
+    b.push(h.finalize_reset().to_vec()); // b[1]
 
     let mut uniform_bytes: Vec<u8> = Vec::new();
     uniform_bytes.extend_from_slice(&b[1]);
 
     for i in 2..(ell + 1) {
-        let mut h = H::new();
         h.update(xor(&b[0], &b[i - 1])?);
         h.update(&i2osp(i, 1));
         h.update(&dst_prime);
-        b.push(h.finalize().to_vec()); // b[i]
+        b.push(h.finalize_reset().to_vec()); // b[i]
         uniform_bytes.extend_from_slice(&b[i]);
     }