Decrypttion.txt

import java.nio.charset.StandardCharsets;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Security;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.time.Instant;
import java.time.format.DateTimeFormatter;
import java.util.Base64;

import javax.crypto.Cipher;
import javax.crypto.KeyAgreement;
import javax.crypto.SecretKey;

import org.bouncycastle.jce.provider.BouncyCastleProvider;
 

public class Decrypt {


 public String decrypt(String clientPrivateKey, String proteanPublicKey, String challenge) {

		try {
			byte[] dataBytes = Base64.getDecoder().decode(proteanPublicKey);
			PublicKey publicKey = getPublicKey("X25519", dataBytes);

			dataBytes = Base64.getDecoder().decode(clientPrivateKey);
			PrivateKey privateKey = getPrivateKey("X25519", dataBytes);

			KeyAgreement atServer1 = KeyAgreement.getInstance("X25519", BouncyCastleProvider.PROVIDER_NAME);
			atServer1.init(privateKey); // Server1 uses its private key to initialize the aggreement object
			atServer1.doPhase(publicKey, true); // Uses Server2's ppublic Key
			SecretKey key1 = atServer1.generateSecret(secretKey); // derive secret at server 1.
			// "TlsPremasterSecret" is the algorithm for

			Cipher cipher2 = Cipher.getInstance("AES", BouncyCastleProvider.PROVIDER_NAME);
			cipher2.init(Cipher.DECRYPT_MODE, key1); // Same derived key in server 2same as key1
			byte[] decrypted2 = cipher2.doFinal(Base64.getDecoder().decode(challenge)); // b64 decode the
																					
			

			return new String(decrypted2);
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return "";
	}


public PublicKey getPublicKey(String algo, byte[] jceBytes) throws Exception {
		X509EncodedKeySpec x509EncodedKeySpec = new X509EncodedKeySpec(jceBytes);
		PublicKey key = KeyFactory.getInstance(algo, BouncyCastleProvider.PROVIDER_NAME)
				.generatePublic(x509EncodedKeySpec);
		return key;
	}

	public PrivateKey getPrivateKey(String algo, byte[] jceBytes) throws Exception {
		PrivateKey key = KeyFactory.getInstance(algo, BouncyCastleProvider.PROVIDER_NAME)
				.generatePrivate(new PKCS8EncodedKeySpec(jceBytes));
		return key;
	}

}

//the answer should be like
//{
//"answer":"Decode challenge"
//}