Hashcash

hashcash.py

# https://en.wikipedia.org/wiki/Hashcash
# Author: https://twitter.com/ObjSal

import hashlib
import base64
from math import ceil
import re
import datetime

'''
    X-Hashcash: 1:20:2209300908:ObjSal@twitter::QE9ialNhbA:NP7f

    The header contains:

    ver:      Hashcash format version, 1 (which supersedes version 0).
    bits:     Number of "partial pre-image" (zero) bits in the hashed code.
    date:     The time that the message was sent, in the format YYMMDD[hhmm[ss]].
    resource: Resource data string being transmitted, e.g., an IP address or email address.
    ext:      Extension (optional; ignored in version 1).
    rand:     String of random characters, encoded in base-64 format.
    counter:  Binary counter, encoded in base-64 format.

'''

def validate_pattern(hashcash):
    # check if hashcash format is valid
    # \d+  -> one or more digits
    # [^:] -> any character that is not :
    pattern = re.compile(r'^(\d+:){3}[^:]+:[^:]*(:[^:]+){2}$')
    if not pattern.fullmatch(hashcash):
        raise Exception('hashcash format is invalid')

def get_pre_image(hashcash):
    # pre-image is the second number in the hashcash
    first_colon = hashcash.find(':') + 1
    second_colon = hashcash.find(':', first_colon)
    bits = hashcash[first_colon : second_colon]
    leading_zeros_count = int(bits) / 4
    pre_image_v = ['0'] * int(leading_zeros_count)
    return ''.join(pre_image_v)

def sha1(hashcash):
    hash = hashlib.sha1()
    hash.update(bytes(hashcash, 'utf-8'))
    hexdigest = hash.hexdigest()
    # print(hexdigest)
    return hexdigest

def verify(hashcash):
    validate_pattern(hashcash)

    # get the number of leading zeros expected from the hashcash
    pre_image = get_pre_image(hashcash)

    # re-create the hash to validate hashcash
    hashcash_hash = sha1(hashcash)
    
    # check if the leading zeros matches the specified number in the hashcash
    if not hashcash_hash.startswith(pre_image):
        raise Exception("Number of 'partial pre-image' (zero) bits in the hashed code is different than expected")
    
    # print(hashcash_hash)

    return True

'''
Create the hashtash header then computes the 160-bit SHA-1 hash.
If the first 20 bits (i.e. the 5 most significant hex digits) of the hash are all zeros, 
then this is an acceptable header. If not, then it increments the counter and tries the hash again.

Out of 2^160 possible hash values, there are 2^140 hash values that satisfy this criterion.
Thus the chance of randomly selecting a header that will have 20 zeros as the beginning of the hash 
is 1 in 2^20 (approx. 10^6, or about one in a million).
'''
def create_hashtash(recipient):
    # ver:      Hashcash format version, 1 (which supersedes version 0).
    # bits:     Number of "partial pre-image" (zero) bits in the hashed code.
    # date:     The time that the message was sent, in the format YYMMDD[hhmm[ss]].
    # resource: Resource data string being transmitted, e.g., an IP address or email address.
    # ext:      Extension (optional; ignored in version 1).
    # rand:     String of random characters, encoded in base-64 format.
    # counter:  Binary counter, encoded in base-64 format.
    ver = '1'
    bits = '20'
    date = datetime.datetime.utcnow().strftime('%y%m%d%H%M')
    resource = recipient
    ext = ''
    rand = base64.b64encode(b'@ObjSal').decode("utf-8").rstrip('=')
    counter = 0

    pre = [ver, bits, date, resource, ext, rand]
    pre_hashcash = ':'.join(pre) + ':'

    valid = False
    result = ''
    while not valid:
        try:
            length = max(1, ceil(counter.bit_length() / 8))
            counter_bytes = counter.to_bytes(int(length), byteorder='big', signed=False)
            counter_str = base64.b64encode(counter_bytes).decode("utf-8").rstrip('=')
            result = pre_hashcash + counter_str
            valid = verify(result)
        except:
            pass
        finally:
            counter += 1
    
    return result


def main():
    hashcash = create_hashtash("ObjSal@twitter")
    print(hashcash)
    if verify(hashcash):
        print("Valid")
    else:
        print("Invalid")


if __name__ == "__main__":
    main()