How it works…

To begin, we must import hashlib as shown in the following. For ease of use, we have defined a dictionary of algorithms that our script can use: MD5, SHA-1, SHA-256 and SHA-512. By updating this dictionary, we can support other hash functions that have update() and hexdigest() methods, including some from libraries other than hashlib:

from __future__ import print_function
import argparse
import hashlib
import os

__authors__ = ["Chapin Bryce", "Preston Miller"]
__date__ = 20170815
__description__ = "Script to hash a file's name and contents"

available_algorithms = {
    "md5": hashlib.md5,
    "sha1": hashlib.sha1,
    "sha256": hashlib.sha256,
    "sha512": hashlib.sha512
}

parser = argparse.ArgumentParser(
    description=__description__,
    epilog="Developed by {} on {}".format(", ".join(__authors__), __date__)
)
parser.add_argument("FILE_NAME", help="Path of file to hash")
parser.add_argument("ALGORITHM", help="Hash algorithm to use",
                    choices=sorted(available_algorithms.keys()))
args = parser.parse_args()

input_file = args.FILE_NAME
hash_alg = args.ALGORITHM

With our hash algorithms defined, we now can hash the file's absolute path, a similar method employed during file naming for iTunes backups of an iOS device, by passing the string into the update() method. When we are ready to display the hex value of the calculated hash, we can call the hexdigest() method on our file_name object:

file_name = available_algorithms[hash_alg]()
abs_path = os.path.abspath(input_file)
file_name.update(abs_path.encode())

print("The {} of the filename is: {}".format(
    hash_alg, file_name.hexdigest()))

Let's move onto opening the file and hashing its contents. While we can read the entire file and pass it to the hash function, not all files are small enough to fit in memory. To ensure our code works on larger files, we will use the technique in the following example to read a file in a piecemeal fashion and hash it in chunks.

By opening the file as rb, we will ensure that we are reading the binary contents of the file, not the string content that may exist. With the file open, we will define the buffer size to read in content and then read the first chunk of data in.

Entering a while loop, we will update our hashing object with the new content for as long as there is content in the file. This is possible as the read() method allows us to pass an integer of the number of bytes to read and, if the integer is larger than the number of bytes remaining in the file, will simply pass us the remaining bytes.

Once the entire file is read, we call the hexdigest() method of our object to display the file hash to the examiner:

file_content = available_algorithms[hash_alg]()
with open(input_file, 'rb') as open_file:
    buff_size = 1024
    buff = open_file.read(buff_size)

    while buff:
        file_content.update(buff)
        buff = open_file.read(buff_size)

print("The {} of the content is: {}".format(
    hash_alg, file_content.hexdigest()))

When we execute the code, we see the output from the two print statements revealing the hash value of the file's absolute path and content. We can generate additional hashes for the file by changing the algorithm at the command line: