java - Input length must be multiple of 16 when decrypting with padded cipher

Question

I have a server and client socket program, a server send a encrypted message to client, i.e. the server side code:

cipher2 = Cipher.getInstance("AES"); 
secretKeySpec = new SecretKeySpec(decryptedText, "AES");
cipher2.init(Cipher.ENCRYPT_MODE, secretKeySpec);
feedback = "Your answer is wrong".getBytes();
cipher2.doFinal(feedback);
dos.writeInt(feedback.length);
dos.write(feedback);

the client side code:

int result_len = 0;
result_len = din.readInt();            
byte[] result_Bytes = new byte[result_len];
din.readFully(result_Bytes);
cipher2 = Cipher.getInstance("AES");
cipher2.init(Cipher.DECRYPT_MODE, aesKey);             
byte[] encrypt = cipher2.doFinal(result_Bytes);

Exception throw at byte[] encrypt = cipher2.doFinal(result_Bytes);

javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:750)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:676)
    at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:313)
    at javax.crypto.Cipher.doFinal(Cipher.java:2086)

what is the problem?

Answer 1

This is a very old question, but my answer may help someone.

• In the encrypt method, don't forget to encode your string to Base64
• In the decrypt method, don't forget to decode your string to Base64

Below is the working code

    import java.util.Arrays;
    import java.util.Base64;

    import javax.crypto.Cipher;
    import javax.crypto.SecretKey;
    import javax.crypto.spec.SecretKeySpec;

    public class EncryptionDecryptionUtil {

    public static String encrypt(final String secret, final String data) {


        byte[] decodedKey = Base64.getDecoder().decode(secret);

        try {
            Cipher cipher = Cipher.getInstance("AES");
            // rebuild key using SecretKeySpec
            SecretKey originalKey = new SecretKeySpec(Arrays.copyOf(decodedKey, 16), "AES");
            cipher.init(Cipher.ENCRYPT_MODE, originalKey);
            byte[] cipherText = cipher.doFinal(data.getBytes("UTF-8"));
            return Base64.getEncoder().encodeToString(cipherText);
        } catch (Exception e) {
            throw new RuntimeException(
                    "Error occured while encrypting data", e);
        }

    }

    public static String decrypt(final String secret,
            final String encryptedString) {


        byte[] decodedKey = Base64.getDecoder().decode(secret);

        try {
            Cipher cipher = Cipher.getInstance("AES");
            // rebuild key using SecretKeySpec
            SecretKey originalKey = new SecretKeySpec(Arrays.copyOf(decodedKey, 16), "AES");
            cipher.init(Cipher.DECRYPT_MODE, originalKey);
            byte[] cipherText = cipher.doFinal(Base64.getDecoder().decode(encryptedString));
            return new String(cipherText);
        } catch (Exception e) {
            throw new RuntimeException(
                    "Error occured while decrypting data", e);
        }
    }


    public static void main(String[] args) {

        String data = "This is not easy as you think";
        String key = "---------------------------------";
        String encrypted = encrypt(key, data);
        System.out.println(encrypted);
        System.out.println(decrypt(key, encrypted));
      }
  }

For Generating Key you can use below class

    import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Base64;

import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;

public class SecretKeyGenerator {

    public static void main(String[] args) throws NoSuchAlgorithmException {

        KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");

        SecureRandom secureRandom = new SecureRandom();
        int keyBitSize = 256;
        keyGenerator.init(keyBitSize, secureRandom);

        SecretKey secretKey = keyGenerator.generateKey();

 System.out.println(Base64.getEncoder().encodeToString(secretKey.getEncoded()));
    }

}