known_attack.sh

#!/bin/bash
#
# Implementation of a Known Plaintext Attack
#
# Author: Calum Gilchrist
# email: [email protected]

tmp_file=temp-$RANDOM.txt

if [[ ! $# -eq 2 ]]; then
    echo "Usage: known_attack.sh encrypted_file plaintext_file"
    exit 1
fi

in_file=$1
orig_file=$2

sha1=$(shasum $orig_file | grep -Eo "^([0-9]|[a-Z])*")

while read line; do

    # Decrypt the file without a salt and use a word from words.txt as pass
    openssl enc -aes-128-cbc -d \
        -nosalt \
        -in $in_file \
        -out $tmp_file \
        -pass pass:$line \
        2>/dev/null

    if [[ $? -eq 0 ]]; then 

        sha2=$(shasum $tmp_file | grep -Eo "^([0-9]|[a-Z])*")

        # Compare shas, if password found, stop looking
        if [[ $sha1 == $sha2 ]]; then
            echo Password: $line
            break
        fi
    fi
done < words.txt

rm -v $tmp_file