When padding is enabled $des->decrypt() will return false with ciphertext's
of invalid length. Which makes sense. Null padding doesn't work when padding
is enabled because 0 isn't a valid PKCS7 padding character (has to be between
1 and the block_length, inclusive).
aside from the addition of OpenSSL support a few other changes have been included:
- setEngine(), as added by petrich, is depricated (not that it was ever in trunk to begin with)
it has been replaced with isValidEngine() and setPreferredEngine().
- replace _generate_xor() with increment_str()
_increment_str() had extra functionality that wasn't being used. ie. it could concatenate
multiple successive string increments to one another automatically. but not only was that
functionality not used - it also made the function less versatile. _increment_str() can be
used more easily for iterative brute forcing (for example)
- rename Crypt_Base::_stringShift to Crypt_Base::_string_shift (for consistency)
- more expansive unit test coverage
