RFE: add a reinit method to block ciphers

[Respawned]

Could you consider adding a reinit() method to block ciphers? Most crypto libraries provide one.
This would save creating a new object instead of resetting the cipher to the original IV. Below is a usage
comparison with python-mcrypt.

import base64
from mcrypt import MCRYPT
from Crypto.Cipher import AES

def beh64(data):
    return base64.encodestring(data).strip().replace("\n", "")

CZ_KEY = "f52412c4ff1dacd2111f4951f3db1260"
CZ_IV = "0e32f4c96203f892"

aes = MCRYPT("rijndael-128", "cbc")
aes.init(CZ_KEY, CZ_IV)

def cz_arg(strn):
    aes.reinit()
    return beh64(aes.encrypt(strn))

def cz_arg2(strn):
    aes2 = AES.new(CZ_KEY, AES.MODE_CBC, CZ_IV) # no reinit
    pad = len(strn) % 16
    if pad != 0:
        strn = strn + ('\x00' * (16 - pad))
    return beh64(aes2.encrypt(strn))

print cz_arg('abc')
print cz_arg2('abc')
print cz_arg('def')
print cz_arg2('def')

[Darsey Litzenberger (dlitz)]

What would be the purpose of this? In general, (key, IV) pairs should never be reused for encryption, so I'm hesitant to add an interface that would encourage doing so.

[Ryan Kelly (rfkelly)]

I've wanted a similar functionality myself in the past, but mostly to re-run a decryption rather than an encryption as in this example. I have a library for transparently accessing encrypted files as a file-like objects, that allows random access by resetting the cipher whenever seek() is called.

In this case I simply keep a copy of the original IV, and reset the cipher object by assigning this to its "IV" property; this seems to work correctly but I suspect that's an accident, and the block-cipher PEP specifically forbids setting the IV property.

[Darsey Litzenberger (dlitz)]

There has been another request for similar functionality: https://bugs.launchpad.net/pycrypto/+bug/1014715

As I mentioned there, this could be useful for implementing disk encryption modes of operation (e.g. ESSIV, XTS, etc.). I'm not sure what the API should look like yet, but I think it would be good if we had an API that allowed a cipher to be reinitialized with the same key without necessarily running the key-scheduling algorithm again.

[Richard Mitchell (mitchellrj)]

This would be useful for parsing & writing OpenPGP version 3 secret keys.

"With V3 keys [...] the CFB state is resynchronized at the beginning of each new MPI value, so that the CFB block boundary is aligned with the start of the MPI data."

~ https://tools.ietf.org/html/rfc4880#page-44