From 2d9e240fb6ae13e6139713f9bb8ccb51e5dc0bff Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Fri, 28 Mar 2025 13:42:46 +0000
Subject: [PATCH] [hashes] Use `fixed_time_eq` for `Hmac::eq`

When someone is checking if an `Hmac` is equal to some other
`Hmac`, its fairly common for them to be doing a
constant-time-ness-sensitive operation. Thus, here we default to
our existing `fixed_time_eq` method for `PartialEq` on `Hamc`,
rather than the naive Rust slice comparison.

While we should consider doing the same for all hash types, we do
not yet do so here.
---
 hashes/src/hmac/mod.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/hashes/src/hmac/mod.rs b/hashes/src/hmac/mod.rs
index 2d70253fc..d69c3ccc2 100644
--- a/hashes/src/hmac/mod.rs
+++ b/hashes/src/hmac/mod.rs
@@ -15,8 +15,9 @@ use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use crate::{Hash, HashEngine};
 
 /// A hash computed from a RFC 2104 HMAC. Parameterized by the underlying hash function.
-#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialOrd, Ord, Hash)]
 #[repr(transparent)]
+#[allow(clippy::derived_hash_with_manual_eq)]
 pub struct Hmac<T: Hash>(T);
 
 impl<T: Hash + str::FromStr> str::FromStr for Hmac<T> {
@@ -24,6 +25,14 @@ impl<T: Hash + str::FromStr> str::FromStr for Hmac<T> {
     fn from_str(s: &str) -> Result<Self, Self::Err> { Ok(Hmac(str::FromStr::from_str(s)?)) }
 }
 
+impl<T: Hash> PartialEq for Hmac<T> {
+    fn eq(&self, other: &Self) -> bool {
+        crate::cmp::fixed_time_eq(self.as_ref(), other.as_ref())
+    }
+}
+
+impl<T: Hash> Eq for Hmac<T> {}
+
 /// Pair of underlying hash engines, used for the inner and outer hash of HMAC.
 #[derive(Debug, Clone)]
 pub struct HmacEngine<T: HashEngine> {