xep-0285: Add verification classes

domain/src/main/java/org/jivesoftware/smackx/signed/xep0285/verification/StrictlyMonotoneTimestampValidator.java

@@ -0,0 +1,29 @@
+package org.jivesoftware.smackx.signed.xep0285.verification;
+
+import org.jivesoftware.smackx.signed.xep0285.element.PlainElement;
+
+import java.util.Date;
+
+/**
+ * So boring...
+ */
+public class StrictlyMonotoneTimestampValidator implements TimestampValidator {
+
+    private Date lastTimestamp;
+
+    public StrictlyMonotoneTimestampValidator(Date lastTimestamp) {
+        this.lastTimestamp = lastTimestamp;
+    }
+
+    @Override
+    public boolean isValid(PlainElement plainElement) {
+        if (plainElement.getTimestamp() == null) {
+            return false;
+        }
+        return lastTimestamp == null || plainElement.getTimestamp().after(lastTimestamp);
+    }
+
+    public void set(PlainElement plainElement) {
+        this.lastTimestamp = plainElement.getTimestamp();
+    }
+}

domain/src/main/java/org/jivesoftware/smackx/signed/xep0285/verification/TimestampValidator.java

@@ -0,0 +1,18 @@
+package org.jivesoftware.smackx.signed.xep0285.verification;
+
+import org.jivesoftware.smackx.signed.xep0285.element.PlainElement;
+
+public interface TimestampValidator {
+
+    boolean isValid(PlainElement plainElement);
+
+    void set(PlainElement plainElement);
+
+    default boolean compareAndSet(PlainElement plainElement) {
+        if (isValid(plainElement)) {
+            set(plainElement);
+            return true;
+        }
+        return false;
+    }
+}