fix(webhooks): compare signatures in constant time

src/main/java/com/gr4vy/sdk/Webhooks.java

@@ -1,6 +1,7 @@
 package com.gr4vy.sdk;
 
 import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
 import java.util.Arrays;
 import java.util.List;
 
@@ -69,7 +70,11 @@ public static void verifyWebhook(String payload,
             throw new IllegalArgumentException("Invalid secret");
         }
 
-        if(!signatures.contains(expectedSignature)) {
+        // Compare in constant time, so the check doesn't leak how much of one matched.
+        byte[] expected = expectedSignature.getBytes(StandardCharsets.UTF_8);
+        boolean matched = signatures.stream()
+            .anyMatch(signature -> MessageDigest.isEqual(expected, signature.getBytes(StandardCharsets.UTF_8)));
+        if(!matched) {
             throw new IllegalArgumentException("No matching signature found");
         }
     }

src/test/java/com/gr4vy/sdk/WebhooksTest.java

@@ -78,4 +78,23 @@ void testVerifyWebhookMissingTimestampHeader() {
 
         assertEquals(thrown.getMessage(), "Missing header values");
     }
+
+    @Test
+    void testVerifyWebhookAcceptsAValidSignatureInAnyPosition() {
+        String header = "other,78aca0c78005107a654a957b8566fa6e0e5e06aea92d7da72a6da9e5a690d013";
+
+        assertDoesNotThrow(() -> Webhooks.verifyWebhook(payload, secret, header, 0, timestampHeader));
+    }
+
+    @Test
+    void testVerifyWebhookRejectsANearlyMatchingSignature() {
+        String header = "78aca0c78005107a654a957b8566fa6e0e5e06aea92d7da72a6da9e5a690d014";
+
+        IllegalArgumentException thrown = assertThrows(
+            IllegalArgumentException.class,
+            () -> Webhooks.verifyWebhook(payload, secret, header, 0, timestampHeader)
+        );
+
+        assertEquals(thrown.getMessage(), "No matching signature found");
+    }
 }