Question

MCrypt:

    import java.security.NoSuchAlgorithmException;

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

    public class MCrypt {

            private String iv = "fedcba9876543210";
            private IvParameterSpec ivspec;
            private SecretKeySpec keyspec;
            private Cipher cipher;

            private String SecretKey = "0123456789abcdef";

            public MCrypt()
            {
                    ivspec = new IvParameterSpec(iv.getBytes());

                    keyspec = new SecretKeySpec(SecretKey.getBytes(), "AES");

                    try {
                            cipher = Cipher.getInstance("AES/CBC/NoPadding");
                    } catch (NoSuchAlgorithmException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                    } catch (NoSuchPaddingException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                    }
            }

            public byte[] encrypt(String text) throws Exception
            {
                    if(text == null || text.length() == 0)
                            throw new Exception("Empty string");

                    byte[] encrypted = null;

                    try {
                            cipher.init(Cipher.ENCRYPT_MODE, keyspec, ivspec);

                            encrypted = cipher.doFinal(padString(text).getBytes());
                    } catch (Exception e)
                    {                       
                            throw new Exception("[encrypt] " + e.getMessage());
                    }

                    return encrypted;
            }

            public byte[] decrypt(String code) throws Exception
            {
                    if(code == null || code.length() == 0)
                            throw new Exception("Empty string");

                    byte[] decrypted = null;

                    try {
                            cipher.init(Cipher.DECRYPT_MODE, keyspec, ivspec);

                            decrypted = cipher.doFinal(hexToBytes(code));
                    } catch (Exception e)
                    {
                            throw new Exception("[decrypt] " + e.getMessage());
                    }
                    return decrypted;
            }



            public static String bytesToHex(byte[] data)
            {
                    if (data==null)
                    {
                            return null;
                    }

                    int len = data.length;
                    String str = "";
                    for (int i=0; i<len; i++) {
                            if ((data[i]&0xFF)<16)
                                    str = str + "0" + java.lang.Integer.toHexString(data[i]&0xFF);
                            else
                                    str = str + java.lang.Integer.toHexString(data[i]&0xFF);
                    }
                    return str;
            }


            public static byte[] hexToBytes(String str) {
                    if (str==null) {
                            return null;
                    } else if (str.length() < 2) {
                            return null;
                    } else {
                            int len = str.length() / 2;
                            byte[] buffer = new byte[len];
                            for (int i=0; i<len; i++) {
                                    buffer[i] = (byte) Integer.parseInt(str.substring(i*2,i*2+2),16);
                            }
                            return buffer;
                    }
            }



            private static String padString(String source)
            {
              char paddingChar = ' ';
              int size = 16;
              int x = source.length() % size;
              int padLength = size - x;

              for (int i = 0; i < padLength; i++)
              {
                      source += paddingChar;
              }

              return source;
            }
    }

Main:

mcrypt = new MCrypt();
/* Encrypt */
String encrypted = MCrypt.bytesToHex( mcrypt.encrypt("Text to Encrypt") );
//Returns 9975e28df055c336a9b7090b03f88689
/* Decrypt */
String decrypted = new String( mcrypt.decrypt( encrypted ) );
//Returns "Text to Encrypt "

The problem:

String encrypted = MCrypt.bytesToHex( mcrypt.encrypt("Text to Encrypt") );
Encrypt returns: 9975e28df055c336a9b7090b03f88689 (which is incorrect)
String decrypted = new String( mcrypt.decrypt( encrypted ) );
Decrypt returns: “Text to Encrypt ” (which properly reflects what the encryption came up with, there is a ” ” after Encrypt)

I’ve narrowed it down to this line:
encrypted = cipher.doFinal(padString(text).getBytes());

I tried changing the padString function so that char paddingChar = 0; instead of char paddingChar = ' '; with no luck…

When properly encrypted “Text to Encrypt” should turn into “cb4b4ca864213684070465b38783a6c8”

Answer 1

AES is a block cypher. It encrypts one 256 bit block (=16 bytes) into a different 256 bit block. Your plaintext, “Text to Encrypt” is 15 characters, or 248 bits. AES cannot encrypt it as-is, but must add some padding to make it up to a full block.

If you explicitly add a padding character, then you must explicitly remove it. Each different padding character will have a large impact on the decryption. On average, changing one bit in the input plaintext block will change 50% of the bits in the output cyphertext block.

Your easiest solution is to use the buit in padding facilities in Java. You specify your cipher as: "AES/CBC/NoPadding". Change this to "AES/CBC/PKCS5Padding" for both encryption and decryption. Don’t worry about what the cyphertext looks like, just check that the plaintext matches the decyphered cyphertext, character for character.

A common error is to convert a text string to a byte array using getBytes(). Don’t do that, because it is error-prone. You should specify precisely what mapping you are using between characters and bytes. Use something like:

byte[] plainBytes = plaintextString.getBytes("UTF-8");

and similar at the other side. Do not rely on system defaults always being the same.