Encrypting-Decrypting multiple times

Question

I'm trying my hand at Cryptography using Python2.7. I'm able to encrypt-decrypt a text once. But when I try to encrypt-decrypt multiple times, in a loop, it doesn't give back the original string in variable p. Please have a look at my code, and suggest what can be done. Thanks!

import os, random
from Crypto.Cipher import AES
from Crypto.Hash import SHA256

'''Encryption'''

k = 'mykey'
hasher = SHA256.new(k)
k = hasher.digest()

p = 'enigmaticaura'
print p
IV = ''

lp = len(p)

for i in range(16):
    IV += chr(random.randint(0,0xFF))

if len(p)%16 != 0:
    p += ' '*(16 - len(p)%16)

enc = AES.new(k, AES.MODE_CBC, IV)
for i in range(2):
    p = enc.encrypt(p)
    print p

'''Decryption'''

dec = AES.new(k, AES.MODE_CBC, IV)
for i in range(2):
    p = dec.decrypt(p)

#p = p[:lp]
    print p

Your problem description is not helpful. What exactly are you expecting? What exactly are you getting? — David Schwartz, Feb 25 '18 at 06:34
The problem is that I'm not able to get the original string p (='enigmaticaura') after decryption. Have also edited the question accordingly. — TheKoolhatkar, Feb 26 '18 at 02:25
Please use the checkmark next to the answer to say the question is solved — OneCricketeer, Feb 26 '18 at 02:31

score 1 · Accepted Answer · answered Feb 25 '18 at 13:10

Disclaimer first: Python isn't exactly my strong point but hey, it's been a while since I answered anything so why not...

I guess your question should have been something like "why doesn't the decryption produce the original input".

For one thing - why? Using the same key isn't really going to make the encryption any better.

And for two - it's not meant to work like that. I'm pretty sure it is to do with the fact that the encrypt and decrypt objects are stateful and cannot be reused. This SO post describes it in more detail.

But, two small changes will make it (sort of) work, in as much as you'll get the original source back.

for i in range(2):
    enc = AES.new(k, AES.MODE_CBC, IV)   ## move this line inside the loop
    p = enc.encrypt(p)
    print p

and

for i in range(2):
    dec = AES.new(k, AES.MODE_CBC, IV)   ## and this line too
    p = dec.decrypt(p)

which may, or may not be what you were hoping for.

It worked. Thanks a lot! :) Although, I'm not really sure why it worked. The enc/dec variable was not altered, so redefining them again for the next iteration doesn't make sense. I guess using the .encrypt/.decrypt method was perhaps the reason. — TheKoolhatkar, Feb 26 '18 at 02:28
@harsh-kolhatkar That's the point - the enc/dec variables *are* altered, internally. They maintain internal state and can't be reused. By recreating them you are, in effect, resetting them. — shunty, Feb 26 '18 at 07:48

Encrypting-Decrypting multiple times

1 Answers1