In February 2017, CWI Amsterdam and Google announced they had performed a collision attack against SHA-1, printing two dissimilar PDF files which produced the same SHA-1 hash. replace SHA-1 is urgent where it is used for signatures.
""" 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!!" python 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 extremely 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 >>> SHA1Hash(bytes('Allan', 'utf-8')).final_hash() '872af2d8ac3d8695387e7c804bf0e02c18df9e6e' """ 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. >>> SHA1Hash('').rotate(12,2) 48 """ 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 after some bit operations """ w = list(struct.unpack(">16L", block)) +  * 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, b to h 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 + a & 0xFFFFFFFF, self.h + b & 0xFFFFFFFF, self.h + c & 0xFFFFFFFF, self.h + d & 0xFFFFFFFF, self.h + 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 don't 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() import doctest doctest.testmod()