Initial bytes incorrect after Java AES/CBC decryption

Question

What's wrong with the following example?

The problem is that the first part of the decrypted string is nonsense. However, the rest is fine, I get...

Result: `£eB6O�geS��i are you? Have a nice day.

@Test
public void testEncrypt() {
  try {
    String s = "Hello there. How are you? Have a nice day.";

    // Generate key
    KeyGenerator kgen = KeyGenerator.getInstance("AES");
    kgen.init(128);
    SecretKey aesKey = kgen.generateKey();

    // Encrypt cipher
    Cipher encryptCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    encryptCipher.init(Cipher.ENCRYPT_MODE, aesKey);

    // Encrypt
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    CipherOutputStream cipherOutputStream = new CipherOutputStream(outputStream, encryptCipher);
    cipherOutputStream.write(s.getBytes());
    cipherOutputStream.flush();
    cipherOutputStream.close();
    byte[] encryptedBytes = outputStream.toByteArray();

    // Decrypt cipher
    Cipher decryptCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
    IvParameterSpec ivParameterSpec = new IvParameterSpec(aesKey.getEncoded());
    decryptCipher.init(Cipher.DECRYPT_MODE, aesKey, ivParameterSpec);

    // Decrypt
    outputStream = new ByteArrayOutputStream();
    ByteArrayInputStream inStream = new ByteArrayInputStream(encryptedBytes);
    CipherInputStream cipherInputStream = new CipherInputStream(inStream, decryptCipher);
    byte[] buf = new byte[1024];
    int bytesRead;
    while ((bytesRead = cipherInputStream.read(buf)) >= 0) {
        outputStream.write(buf, 0, bytesRead);
    }

    System.out.println("Result: " + new String(outputStream.toByteArray()));

  } 
  catch (Exception ex) {
    ex.printStackTrace();
  }
}

Answer 1

Lot of people including myself face lot of issues in making this work due to missing some information like, forgetting to convert to Base64, initialization vectors, character set, etc. So I thought of making a fully functional code.

Hope this will be useful to you all: To compile you need additional Apache Commons Codec jar, which is available here: http://commons.apache.org/proper/commons-codec/download_codec.cgi

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

import org.apache.commons.codec.binary.Base64;

public class Encryptor {
    public static String encrypt(String key, String initVector, String value) {
        try {
            IvParameterSpec iv = new IvParameterSpec(initVector.getBytes("UTF-8"));
            SecretKeySpec skeySpec = new SecretKeySpec(key.getBytes("UTF-8"), "AES");

            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");
            cipher.init(Cipher.ENCRYPT_MODE, skeySpec, iv);

            byte[] encrypted = cipher.doFinal(value.getBytes());
            System.out.println("encrypted string: "
                    + Base64.encodeBase64String(encrypted));

            return Base64.encodeBase64String(encrypted);
        } catch (Exception ex) {
            ex.printStackTrace();
        }

        return null;
    }

    public static String decrypt(String key, String initVector, String encrypted) {
        try {
            IvParameterSpec iv = new IvParameterSpec(initVector.getBytes("UTF-8"));
            SecretKeySpec skeySpec = new SecretKeySpec(key.getBytes("UTF-8"), "AES");

            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING");
            cipher.init(Cipher.DECRYPT_MODE, skeySpec, iv);

            byte[] original = cipher.doFinal(Base64.decodeBase64(encrypted));

            return new String(original);
        } catch (Exception ex) {
            ex.printStackTrace();
        }

        return null;
    }

    public static void main(String[] args) {
        String key = "Bar12345Bar12345"; // 128 bit key
        String initVector = "RandomInitVector"; // 16 bytes IV

        System.out.println(decrypt(key, initVector,
                encrypt(key, initVector, "Hello World")));
    }
}