computer science python

# John B. Schneider

# gen_amp_seed() function

#######################################################################

# Function to calculate an "amplitude" and a "seed" appropriate for

# starting the encryption/decryption process.  These values are based

# on the password given as the argument (which is assumed to be a

# string).  This function uses the SHA1 hash function from the hashlib

# module.  See http://en.wikipedia.org/wiki/SHA-1 for details

# concerning this function.  The hash function creates a unique

# 160-bit 'digest' for a given string.  This digest is split into two

# parts.  One part is used to set the amplitude and the other is used

# to set the seed value.

import hashlib

def gen_amp_seed(password):
    h = hashlib.sha1()          # Create a SHA1 hash object.
    h.update(password.encode()) # Update object with password.
    # Create a 160-bit hexadecimal digest which is returned as a
    # 40-character string.
    d = h.hexdigest()
    # Split the digest into two parts, each of 20 hexadecimal
    # characters/digits.
    d1 = d[ : 20]     
    d2 = d[20 : ]
    # max_possible is the maximum possible value that 20 hexadecimal
    # digits can have.
    max_possible = 0xFFFFFFFFFFFFFFFFFFFF
    # Set the amp and seed values.  amp is 3.99 plus a number that is
    # between 0.0 and 0.01 while seed is between 0.0 and 1.0.  In the
    # unlikely event that d2 equals max_possible, then seed will be
    # 1.0 which will cause the log_map() function to return zero.
    # Although simple to account for this, we will not do so here.
    amp = 3.99 + eval('0x' + d1) / max_possible / 100
    seed = eval('0x' + d2) / max_possible
    return amp, seed