Encryption key size DES Java

Question

I want to encrypt a challenge (like 162236fe0bec620827958c8fdf7e4bc7 ) using this key C6864E7696C686 with the DES algorithm.

Here is my code :

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.xml.bind.DatatypeConverter;
import javax.crypto.spec.DESKeySpec;


def data = prev.getResponseData();
String challenge = javax.xml.bind.DatatypeConverter.printHexBinary(data);
final String strPassPhrase = "C6864E7696C686";

String param = challenge;
System.out.println("Text : " + param);

SecretKeyFactory factory = SecretKeyFactory.getInstance("DES");
SecretKey key = factory.generateSecret(new   DESKeySpec(hexStringToByteArray(strPassPhrase)));
Cipher cipher = Cipher.getInstance("DES");

cipher.init(Cipher.ENCRYPT_MODE, key);
String str = DatatypeConverter.printBase64Binary(cipher.doFinal(param.getBytes()));
System.out.println("Text Encryted : " + str);
cipher.init(Cipher.DECRYPT_MODE, key);
String str2 = new String(cipher.doFinal(DatatypeConverter.parseBase64Binary(str)));
System.out.println("Text Decryted : " + str2);

But i get this exception :

java.security.InvalidKeyException: Wrong key size

Edit : I have copy this function to convert my hex string to bytes :

public static byte[] hexStringToByteArray(String s) {
int len = s.length();
byte[] data = new byte[len / 2];
for (int i = 0; i < len; i += 2) {
    data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                         + Character.digit(s.charAt(i+1), 16));
}
return data;
}

But I get the same exception ...

Answer 1

Your DES key should be 8 bytes (56 bits + 8 parity bits).

The string you're using as a key looks like a hexadecimal representation of 7 bytes, but instead of decoding it as hex, you're getting the bytes for the characters in the hex string.

Since there are 14 characters, you will most likely (depending on your encoding) end up with 14 bytes, which is too long for DES.

There are a couple of approaches described in this question that explain how to convert the hex string to a byte array.

That will however only get you so far, because you're still one byte short. A traditional approach seems to be to take the 56 bits you have and spread them out over 8 bytes, adding one parity bit to each byte. A Java example of how to do that is described in this answer. Another approach could be to just add a null byte at the end of the key. Which approach you should take depends on the key's intended usage, especially the way it is used by the other parties you're exchanging information with.