sha1.py

import argparse
import hashlib  # hashlib is only used inside the Test class
import struct
import unittest


class SHA1Hash:
    """
    Class to contain the entire pipeline for SHA1 Hashing Algorithm
    """

    def __init__(self, data):
        """
        Inititates the variables data and h. h is a list of 5 8-digit Hexadecimal
        numbers corresponding to (1732584193, 4023233417, 2562383102, 271733878, 3285377520)
        respectively. We will start with this as a message digest. 0x is how you write
        Hexadecimal numbers in Python
        """
        self.data = data
        self.h = [0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0]

    @staticmethod
    def rotate(n, b):
        """
        Static method to be used inside other methods. Left rotates n by b.
        """
        return ((n << b) | (n >> (32 - b))) & 0xFFFFFFFF

    def padding(self):
        """
        Pads the input message with zeros so that padded_data has 64 bytes or 512 bits
        """
        padding = b"\x80" + b"\x00" * (63 - (len(self.data) + 8) % 64)
        padded_data = self.data + padding + struct.pack(">Q", 8 * len(self.data))
        return padded_data

    def split_blocks(self):
        """
        Returns a list of bytestrings each of length 64
        """
        return [
            self.padded_data[i : i + 64] for i in range(0, len(self.padded_data), 64)
        ]

    # @staticmethod
    def expand_block(self, block):
        """
        Takes a bytestring-block of length 64, unpacks it to a list of integers and returns a
        list of 80 integers pafter some bit operations
        """
        w = list(struct.unpack(">16L", block)) + [0] * 64
        for i in range(16, 80):
            w[i] = self.rotate((w[i - 3] ^ w[i - 8] ^ w[i - 14] ^ w[i - 16]), 1)
        return w

    def final_hash(self):
        """
        Calls all the other methods to process the input. Pads the data, then splits into
        blocks and then does a series of operations for each block (including expansion).
        For each block, the variable h that was initialized is copied to a,b,c,d,e
        and these 5 variables a,b,c,d,e undergo several changes. After all the blocks are
        processed, these 5 variables are pairwise added to h ie a to h[0], b to h[1] and so on.
        This h becomes our final hash which is returned.
        """
        self.padded_data = self.padding()
        self.blocks = self.split_blocks()
        for block in self.blocks:
            expanded_block = self.expand_block(block)
            a, b, c, d, e = self.h
            for i in range(0, 80):
                if 0 <= i < 20:
                    f = (b & c) | ((~b) & d)
                    k = 0x5A827999
                elif 20 <= i < 40:
                    f = b ^ c ^ d
                    k = 0x6ED9EBA1
                elif 40 <= i < 60:
                    f = (b & c) | (b & d) | (c & d)
                    k = 0x8F1BBCDC
                elif 60 <= i < 80:
                    f = b ^ c ^ d
                    k = 0xCA62C1D6
                a, b, c, d, e = (
                    self.rotate(a, 5) + f + e + k + expanded_block[i] & 0xFFFFFFFF,
                    a,
                    self.rotate(b, 30),
                    c,
                    d,
                )
        self.h = (
            self.h[0] + a & 0xFFFFFFFF,
            self.h[1] + b & 0xFFFFFFFF,
            self.h[2] + c & 0xFFFFFFFF,
            self.h[3] + d & 0xFFFFFFFF,
            self.h[4] + e & 0xFFFFFFFF,
        )
        return "%08x%08x%08x%08x%08x" % tuple(self.h)


class SHA1HashTest(unittest.TestCase):
    """
    Test class for the SHA1Hash class. Inherits the TestCase class from unittest
    """

    def testMatchHashes(self):
        msg = bytes("Test String", "utf-8")
        self.assertEqual(SHA1Hash(msg).final_hash(), hashlib.sha1(msg).hexdigest())


def main():
    """
    Provides option 'string' or 'file' to take input and prints the calculated SHA1 hash.
    unittest.main() has been commented because we probably dont want to run
    the test each time.
    """
    # unittest.main()
    parser = argparse.ArgumentParser(description="Process some strings or files")
    parser.add_argument(
        "--string",
        dest="input_string",
        default="Hello World!! Welcome to Cryptography",
        help="Hash the string",
    )
    parser.add_argument("--file", dest="input_file", help="Hash contents of a file")
    args = parser.parse_args()
    input_string = args.input_string
    # In any case hash input should be a bytestring
    if args.input_file:
        hash_input = open(args.input_file, "rb").read()
    else:
        hash_input = bytes(input_string, "utf-8")
    print(SHA1Hash(hash_input).final_hash())


if __name__ == "__main__":
    main()