CODEWITHSUNDEEP
python-3.xencryptioncounterctr-modepycryptodome
ArekBulski
ArekBulski

Reputation: 5118

pycryptodome: OverflowError: The counter has wrapped around in CTR mode

I am having difficulty with AES-CTR encryption using pycryptodome on Python 3. Data can be ~1000 bytes but when it gets long enough it breaks. I do not understand what this error supposedly means or how to get around it. 

from os import urandom
from Crypto.Cipher import AES

cipher = AES.new(urandom(16), AES.MODE_CTR, nonce=urandom(15))
cipher.encrypt(urandom(10000))

---------------------------------------------------------------------------
OverflowError                             Traceback (most recent call last)
<ipython-input-116-a48990362615> in <module>()
      3 
      4 cipher = AES.new(urandom(16), AES.MODE_CTR, nonce=urandom(15))
----> 5 cipher.encrypt(urandom(10000))
      6 

/usr/local/lib/python3.5/dist-packages/Crypto/Cipher/_mode_ctr.py in encrypt(self, plaintext)
    188         if result:
    189             if result == 0x60002:
--> 190                 raise OverflowError("The counter has wrapped around in"
    191                                     " CTR mode")
    192             raise ValueError("Error %X while encrypting in CTR mode" % result)

OverflowError: The counter has wrapped around in CTR mode

Upvotes: 1

Views: 492

Answers (1)

ArekBulski
ArekBulski

Reputation: 5118

I figured it out. Nonce is only 1 byte short of block size so counter mode can only produce 256 blocks which would allow to encrypt 4096 bytes. If nonce is few bytes shorter the problem goes away.

Upvotes: 1

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