使用 NoPadding 解密 DESede 密码

Question

For a Crypto class we've been given a DESede ciphertext using ECB with no padding => "6t8Z5bKl5ybJL+MiFerNBmiTDS7wlTEUdWNwJJApWmQ==" which is in base64 form. We were given clues about the key and so I constructed all possible keys (where all are in an ASCII format).

            String strToDecrypt="6t8Z5bKl5ybJL+MiFerNBmiTDS7wlTEUdWNwJJApWmQ==";
            byte[] input = strToDecrypt.getBytes();

            //Decrypt
            Cipher b = Cipher.getInstance("DESede/ECB/NoPadding");
            b.init(Cipher.DECRYPT_MODE, keySpec);
            byte output[] = b.doFinal(input);
            String out = new String(output);
            System.out.println(new String(out));

When I run this code using my keys, I get an IllegalBlockSizeException as my input isn't a multiple of 8 bytes. I'm confused as to which "bases" to use. As I've said above, the ciphertext is in base64 so When running Cipher.DECRYPT should I be giving keys in a certain "base" or the string I want to decrypt in a certain base.

Answer 1

Don't make the key generation harder than it needs to be. You want to try each possible value for the first byte of the variable section. But what if that value is 0xFB? Subsequent values would be 0xFC, 0xFD, 0xFE, and 0xFF. But what about the last value? You can assume that they wrap around, to 0x00.

If that's the case, something like this should work to find the correct key:

static byte[] search(byte[] ciphertext) throws GeneralSecurityException {
  byte[] key = template(); /* Initialize the fixed bytes of "key" */
  Cipher nopadding = ... ; /* Create the correct cipher */
  for (long xy = 0; xy < 256; ++xy) { /* Iterate over possible values */
    for (long wz = 0; wz < 256; ++wz) { /* Is there another range? */
      for (int off = 0; off < 6; ++off) {
        key[8 + off] = (byte) (xy + off); /* Set the first range */
      }
      for (int off = 0; off < 6; ++off) {
        key[16 + off] = (byte) (wz + off); /* Set the second range */
      }
      nopadding.init(/* Initialize the cipher with your key */);
      byte[] plaintext = nopadding.doFinal(ciphertext);
      String str = new String(plaintext, StandardCharsets.US_ASCII);
      /* Return the key if it produced valid text */
      if (str.indexOf('\uFFFD') < 0) return key;
    }
  }
  throw new IllegalArgumentException(); /* No key found */
}