kms: use constant-time comparison for admin token verification

Replace timing-vulnerable `!=` with `subtle::ConstantTimeEq` to
prevent timing side-channel attacks on admin token hash comparison.

Author: kvinwang

Cargo.lock

@@ -31,6 +31,7 @@ load_config.workspace = true
 serde-human-bytes.workspace = true
 reqwest = { workspace = true, features = ["json"] }
 sha2.workspace = true
+subtle = "2"
 sha3.workspace = true
 k256.workspace = true
 rand.workspace = true

kms/Cargo.toml

@@ -138,7 +138,8 @@ impl RpcHandler {
 
     fn ensure_admin(&self, token: &str) -> Result<()> {
         let token_hash = sha2::Sha256::new_with_prefix(token).finalize();
-        if token_hash.as_slice() != self.state.config.admin_token_hash.as_slice() {
+        use subtle::ConstantTimeEq;
+        if !bool::from(token_hash.as_slice().ct_eq(self.state.config.admin_token_hash.as_slice())) {
             bail!("Invalid token");
         }
         Ok(())