feat(sheet2): task 2 solution

sheet02/a2/VerifyECDSA.java

@@ -1,5 +1,28 @@
 package sheet02.a2;
 
+import java.security.KeyFactory;
+import java.security.Signature;
+import java.security.spec.X509EncodedKeySpec;
+import java.util.Base64;
+
 public class VerifyECDSA {
-    
+    public static void main(String[] args) throws Exception {
+        if (args.length < 3) {
+            System.out.println(
+                    "Please specify in the following order: Public Key (X509), Signature (Base64), Message (plain text)");
+            return;
+        }
+
+        final String PUBLIC_KEY = args[0];
+        final String SIGNATURE = args[1];
+        final String MESSAGE = args[2];
+
+        final var kf = KeyFactory.getInstance("EC");
+        final var pub = kf.generatePublic(new X509EncodedKeySpec(Base64.getDecoder().decode(PUBLIC_KEY)));
+
+        final var sig = Signature.getInstance("SHA256withECDSA");
+        sig.initVerify(pub);
+        sig.update(MESSAGE.getBytes());
+        System.out.println("Correct: " + sig.verify(Base64.getDecoder().decode(SIGNATURE)));
+    }
 }