feat(sheet2): task 2 solution

sheet02/a2/SignECDSA.java

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+package sheet02.a2;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.security.KeyPairGenerator;
+import java.security.Signature;
+import java.util.Base64;
+
+public class SignECDSA {
+    public static void main(String[] args) throws Exception {
+        final var gen = KeyPairGenerator.getInstance("EC");
+        final var kp = gen.genKeyPair();
+
+        final var bytes = Files.readAllBytes(Path.of("plaintext.txt"));
+        final var sig = Signature.getInstance("SHA256withECDSA");
+        sig.initVerify(kp.getPublic());
+        sig.initSign(kp.getPrivate());
+        sig.update(bytes);
+        final var signature = sig.sign();
+
+        Files.write(Path.of("signature.txt"), Base64.getEncoder().encode(signature));
+        Files.write(Path.of("sign-key.pub"), Base64.getEncoder().encode(kp.getPublic().getEncoded()));
+        Files.write(Path.of("sign-key"), Base64.getEncoder().encode(kp.getPrivate().getEncoded()));
+    }
+}

sheet02/a2/VerifyECDSA.java

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+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)));
+    }
+}

sheet02/a2/plaintext.txt

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+Julian
+Leonard
+Viktoria

sheet02/a2/verify-example.sh

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+java VerifyECDSA.java "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEnSgO/uuSq9QHH/rrnSgXeQTKPMI8tjLhyJzevl9fLQZW/zXuHk9iThiKY/c2k52quUby3czAwL2l2HaRqcBXEg==" "MEUCIAzRvK3CNFqMTZxIgMd1uQ/XybtRKXFht9S4q8qUy/IbAiEA7EcFYuRbViVC/zzIRW78HekjRHZVn3DjHVUcKyJ5sXw=" "hello world"