Feeding could handle an empty buffer more gracefully

[teeberg]

The CBC encryption feeding example in the README currently reads something like:

encrypter = pyaes.Encrypter(pyaes.AESModeOfOperationCBC(key, iv))
ciphertext = encrypter.feed(plaintext)
# Make a final call to flush any remaining bytes and add paddin
ciphertext += encrypter.feed()

If you pass in an empty string as the plaintext, this results in:

.../.venv/lib/python2.7/site-packages/pyaes/blockfeeder.pyc in feed(self, data)
    163 
    164         if self._buffer is None:
--> 165             raise ValueError('already finished feeder')
    166 
    167         # Finalize; process the spare bytes we were keeping

ValueError: already finished feeder

This could be made ever so slightly more reliable by changing this line to:

if data is None:

This would make it easier to have a generic encrypt_aes function taking an arbitrary string, without having to handle an empty string as a special case.

Does that seem like a sensible approach?

[ricmoo]

That absolutely makes sense.

Sorry this has taken so long to get to. I will add a test case this weekend for that case (to reproduce the issue), make that change, and get it updated.

[ricmoo]

Fixed and updated in pypi.

Thanks!

[teeberg]

@ricmoo thank you for the fix!

for the record, fixed in 23a1b4c

[ricmoo]

Ah yes! Thank you for including that.