Bruteforcing a file encrypted with Openssl using bash

Question

I'm working on a school project where we are given a cipher file encrypted using openssl with AES-128 cipher and CBC mode. It is indicated that the file is encrypted with no salt and IV and has less than 16 characters in password which is appended by a number from 0-9 at the end. We are given a dictionary (words.txt) and the cipher file and I have to write a script to find the key and decrypt the content of the file.

For testing purposes, I encrypted a plaintext file using the following openssl command:

openssl enc -e -aes-128-cbc -in plain.txt -out test.txt -nosalt -pass pass:Zoroastrian6

And I am decrypting it using the following command:

openssl enc -d -aes-128-cbc -in test.txt -out test2.txt -nosalt -pass pass:Zoroastrian6

Which works fine. The password I have used for it is the last entry from the words.txt provided by my lecturer. This is what I have written so far:

#!/bin/bash

filename="words.txt"
let flag=0

while read line
do
    # Second loop to append 0-9 at the end of each line read from
    # the text file
    let count=0
    while [ $count -lt 10 ];
    do
        comm=$(openssl enc -d -aes-128-cbc -in test.txt -out test2.txt -nosalt -pass pass:$line$count 2>/dev/null)
        # If the program exits without an error, we print 
        # the password and exit the loop before setting the
        # exit flag to be checked in the first loop to break
        # out of it.
        if [ $? -eq 0 ]; then
            echo "$line$count"
            flag=1
            break
        fi
        ((count++))
    done
    # If flag is set here, it means the password was found
    # and we can exit the loop.
    if [ $flag -eq 1 ]; then
        break
    fi
    
done < "$filename"

The problem is that instead of reaching till the end of the file where the word "Zoroastrian" is located, it exits with the password "abstinent8". I don't know how to debug this as I'm new to bash scripting. Is there a way to check if openssl exited without any errors or a way to check if the decryption gave an error?

EDIT: words.txt can be found here.

that other guy that other guy · Accepted Answer · 2020-10-27T02:00:26

Is there a way to check if openssl exited without any errors or a way to check if the decryption gave an error?

Yes, that's what you're doing.

Unfortunately, using the wrong key is not an error when using low level primitives like this. It just gives you the wrong plaintext.

In your case you use PKCS#5 padding (compare -nopad), so some plaintexts (~99.7%) happen to fail the padding check and cause an error, but that's nowhere near enough to ensure that the password was correct.

Professional systems use Authenticated Encryption, but for a toy example, you can e.g. check that all the characters in the message are printable, or add and check for a "Secret: " header to make the correct plaintext recognizable.

Bruteforcing a file encrypted with Openssl using bash

1 Answers