Make Passphrase comparison constant time

pgpainless-core/src/main/java/org/pgpainless/util/BCUtil.java

@@ -47,4 +47,41 @@ public final class BCUtil {
         return bitStrength;
     }
 
+
+    /**
+     * A constant time equals comparison - does not terminate early if
+     * test will fail. For best results always pass the expected value
+     * as the first parameter.
+     *
+     * @param expected first array
+     * @param supplied second array
+     * @return true if arrays equal, false otherwise.
+     */
+    public static boolean constantTimeAreEqual(
+            char[]  expected,
+            char[]  supplied) {
+        if (expected == null || supplied == null) {
+            return false;
+        }
+
+        if (expected == supplied) {
+            return true;
+        }
+
+        int len = (expected.length < supplied.length) ? expected.length : supplied.length;
+
+        int nonEqual = expected.length ^ supplied.length;
+
+        // do the char-wise comparison
+        for (int i = 0; i != len; i++) {
+            nonEqual |= (expected[i] ^ supplied[i]);
+        }
+        // If supplied is longer than expected, iterate over rest of supplied with NOPs
+        for (int i = len; i < supplied.length; i++) {
+            nonEqual |= ((byte) supplied[i] ^ (byte) ~supplied[i]);
+        }
+
+        return nonEqual == 0;
+    }
+
 }

pgpainless-core/src/main/java/org/pgpainless/util/Passphrase.java

@@ -8,6 +8,8 @@ import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.Arrays;
 
+import static org.pgpainless.util.BCUtil.constantTimeAreEqual;
+
 public class Passphrase {
 
     public final Object lock = new Object();
@@ -162,6 +164,7 @@ public class Passphrase {
             return false;
         }
         Passphrase other = (Passphrase) obj;
-        return Arrays.equals(getChars(), other.getChars());
+        return (getChars() == null && other.getChars() == null) ||
+                constantTimeAreEqual(getChars(), other.getChars());
     }
 }