"""
Demonstrates implementation of SHA1 Hash function in a Python class and gives utilities
to find hash of string or hash of text from a file.
Usage: python sha1.py --string "Hello World!!"
 pyhton sha1.py --file "hello_world.txt"
 When run without any arguments, it prints the hash of the string "Hello World!! Welcome to Cryptography"
Also contains a Test class to verify that the generated Hash is same as that
returned by the hashlib library

SHA1 hash or SHA1 sum of a string is a crytpographic function which means it is easy
to calculate forwards but extemely difficult to calculate backwards. What this means
is, you can easily calculate the hash of a string, but it is extremely difficult to
know the original string if you have its hash. This property is useful to communicate
securely, send encrypted messages and is very useful in payment systems, blockchain
and cryptocurrency etc.
The Algorithm as described in the reference:
First we start with a message. The message is padded and the length of the message
is added to the end. It is then split into blocks of 512 bits or 64 bytes. The blocks
are then processed one at a time. Each block must be expanded and compressed.
The value after each compression is added to a 160bit buffer called the current hash
state. After the last block is processed the current hash state is returned as
the final hash.
Reference: https://deadhacker.com/2006/02/21/sha-1-illustrated/
"""

import argparse
import struct
import hashlib #hashlib is only used inside the Test class
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:
 with open(args.input_file, 'rb') as f:
 hash_input = f.read()
 else:
 hash_input = bytes(input_string, 'utf-8')
 print(SHA1Hash(hash_input).final_hash())


if __name__ == '__main__':
 main()