AES: Input strings must be a multiple of 16 in length

Question

I want to make a script to decrypt my filess, but when I try to run my script then show me this message , how can I fix it?

Traceback (most recent call last): File "F:\bug_bounty\decrypt.py", line 46, in File "F:\bug_bounty\decrypt.py", line 24, in decrypt File "C:\Python27\lib\site-packages\Crypto\Cipher\blockalgo.py", line 295, in decrypt return self._cipher.decrypt(ciphertext) ValueError: Input strings must be a multiple of 16 in length

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


def decrypt(key, filename):
    outFile = os.path.join(os.path.dirname(filename),
                           os.path.basename(filename[11:]))
    chunksize = 64 * 1024
    with open(filename, 'rb') as infile:
        filesize = infile.read(16)
        IV = infile.read(16)

        decryptor = AES.new(key, AES.MODE_CBC, IV)

        with open(outFile, 'wb') as outfile:
            while True:
                chunk = infile.read(chunksize)
                if len(chunk) == 0:
                    break

                outfile.write(decryptor.decrypt(chunk))

            outfile.truncate(int(filesize))


def allfiles():
    allFiles = []
    for (root, subfiles, files) in os.walk(os.getcwd()):
        for names in files:
            allFiles.append(os.path.join(root, names))

    return allFiles


password = 'M4st3rRul3zs'
files = allfiles();
for filename in files:
    if os.path.basename(filename).startswith("(encrypted)"):
       print "%s is already encrypted" %filename
       pass

    else:  
        decrypt(SHA256.new(password).digest(), filename)
        print "Done decrypting %s" %filename
        """os.remove(filename)"""

http://stackoverflow.com/a/14205319/3527520 may be helpful – Simon Fraser May 08 '17 at 07:06 — Simon Fraser, May 08 '17 at 07:06

score 7 · Answer 1 · edited Jan 02 '20 at 01:25

7

Here is the small trick you could use while encrypting the data if your data size is not large.

plaintext = "some text"
encryptor = AES.new(key, AES.MODE_CBC, iv)
ciphertext = encryptor.encrypt(plaintext*16)

This will ensure that your input data is a multiple of 16. And of course, you would like to get the original data back when decrypting.

cipher = AES.new(key, AES.MODE_CBC, iv)
decrypttext = cipher.decrypt(ciphertext)
decrypttext = decrypttext[0:len(plaintext)]

Now, decrpyttext has your original plaintext.

edited Jan 02 '20 at 01:25

B. Go

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answered Jan 01 '20 at 18:55

Mayank Pathela

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Or, you could also use `decrpyttext = decrpyttext[0: len(decrpyttext)//16]` – Mayank Pathela Jan 02 '20 at 11:00

McGrady · Answer 2 · 2017-05-08T07:22:09.363

From Crypto++ wiki.

The block size is determined by AES::BLOCKSIZE. For AES, this is always 16 bytes

AES is a block cipher, it works on 16-byte (128-bit) blocks. It can't work with data smaller or bigger than 16 bytes. Smaller data needs to be padded until they're 16 bytes, and larger data needs to be split into 16-byte blocks.

Also there are algorithms that help you achieve just that (work on data larger than the cipher's block size), they're called block cipher modes of operation.

Have a look at this How to encrypt more than 16 bytes using AES?

score 2 · Answer 3 · answered May 08 '17 at 07:15

2

ValueError: Input strings must be a multiple of 16 in length

That is because AES works with blocks of 128 bits (16 chars). You can consider adding padding to fix this.

answered May 08 '17 at 07:15

Chankey Pathak

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in code how should be? filesize = infile.read(16) IV = infile.read(16) decryptor = AES.new(key, AES.MODE_CBC, IV) – Aron Imperial May 08 '17 at 07:20
@Aron Imperial google 'pkcs7 padding' or 'zero padding' they're both quite easy to implement – t.m.adam May 08 '17 at 10:03

Sawan Chauhan · Answer 4 · 2020-10-03T06:31:40.630

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AES works with blocks of 16 chars. This how you can add extra padding

import random
import string

plaintext = "Encrypt me"
encryptor = AES.new(key, AES.MODE_CBC, iv)

while len(bytes(plaintext, encoding='utf-8')) % 16 != 0:
     plaintext = plaintext + random.choice(string.ascii_letters)

ciphertext = encryptor.encrypt(plaintext)

edited Oct 03 '20 at 06:31

answered Oct 02 '20 at 18:04

Sawan Chauhan

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While this code may solve the question, [including an explanation](//meta.stackexchange.com/q/114762) of how and why this solves the problem would really help to improve the quality of your post, and probably result in more up-votes. Remember that you are answering the question for readers in the future, not just the person asking now. Please [edit] your answer to add explanations and give an indication of what limitations and assumptions apply. – Dharman Oct 02 '20 at 21:58

AES: Input strings must be a multiple of 16 in length

4 Answers4