Add timing-resistant-secret-traits feature for PartialEq/Hash (#232)

Derived implementations of `PartialEq` and `Hash` are susceptible to
timing side channels that make them unsuitable for secret types. This
change introduces timing-safe implementations of `PartialEq` and `Hash`
to all secret types that compare/hash, respectively, the SHA-256 hash
of the secret rather than the secret itself. Because these
implementations are significantly more computationally expensive than
ordinary `PartialEq` and `Hash` implementations, they're behind a
non-default `timing-resistant-secret-traits` feature flag.

Cargo.toml

@@ -19,6 +19,7 @@ default = ["reqwest", "rustls-tls"]
 pkce-plain = []
 native-tls = ["reqwest/native-tls"]
 rustls-tls = ["reqwest/rustls-tls"]
+timing-resistant-secret-traits = []
 
 [dependencies]
 base64 = "0.13"

src/types.rs

@@ -1,6 +1,8 @@
 use std::convert::Into;
 use std::fmt::Error as FormatterError;
 use std::fmt::{Debug, Formatter};
+#[cfg(feature = "timing-resistant-secret-traits")]
+use std::hash::{Hash, Hasher};
 use std::ops::Deref;
 
 use rand::{thread_rng, Rng};
@@ -148,6 +150,7 @@ macro_rules! new_secret_type {
         $(
             #[$attr]
         )*
+        #[cfg_attr(feature = "timing-resistant-secret-traits", derive(Eq))]
         pub struct $name($type);
         impl $name {
             $($item)*
@@ -170,6 +173,21 @@ macro_rules! new_secret_type {
                 write!(f, concat!(stringify!($name), "([redacted])"))
             }
         }
+
+        #[cfg(feature = "timing-resistant-secret-traits")]
+        impl PartialEq for $name {
+            fn eq(&self, other: &Self) -> bool {
+                Sha256::digest(&self.0) == Sha256::digest(&other.0)
+            }
+        }
+
+        #[cfg(feature = "timing-resistant-secret-traits")]
+        impl Hash for $name {
+            fn hash<H: Hasher>(&self, state: &mut H) {
+                Sha256::digest(&self.0).hash(state)
+            }
+        }
+
     };
 }