Phonetic password schema, a la iCloud

phoneticpassword.py

'''
Small tool that generates a phonetic password with a schema
identical to that used in iCloud's password generators.
Pronouncable-ish with 20 characters and ~70 bits of entropy.
Useful for password managers or sysadmins.
'''

# Example password from schema: fivhAp-riznux-0mizhi
# Let V be the set of all lowercase vowels "aiueoy".
# Let C be the set of all lowercase consonants in the Latin (English) alphabet.
# A password is three blocks separated by a "-" character.
# Two blocks are CVCCVC;
# one (randomly positioned) is CVCCV with a digit (0-9) prepended or appended.
# One letter, randomly selected, is capitalised.

# I have my own personal criticisms of the schema.
# semivowels Y and W are a bit awkward sometimes in English
# and there is a chance of capitalising I which is sometimes ambiguous.
# The random capital placement also makes it annoying to pronounce.
# The ability to more easily communicate over a clear line / store in short term memory
# is worth losing 4 bits of entropy, in my mind.

# That said, it's close enough to the phonotactics of most languages
# that I must admit it is very charming.
# It's no xkcd 936 but it's a lot nicer than a random string.

from secrets import choice as r, randbelow
from string import ascii_lowercase, digits as D

__all__ = ('password', )

V = tuple('aeiouy')
C = tuple(set(ascii_lowercase) - set(V))

def _block(digit: bool) -> str:
    block = r(C)+r(V)+r(C)+r(C)+r(V)
    if digit:
        if randbelow(2):  # <- half and half
            return r(D) + block
        else:
            return block + r(D)
    else:
        return block + r(C)

def password() -> str:
    '''Generate an iCloud-like password.'''
    blocks = [_block(False), _block(False)]
    blocks.insert(randbelow(3), _block(True))
    password = list('-'.join(blocks))

    # this is a slightly silly but relatively simple method.
    # password generating isn't subject to timing attacks
    # or at least I'm quite confident they really shouldn't be...
    i = 6  # "-"
    while password[i] not in ascii_lowercase:
        i = randbelow(len(password))
    password[i] = password[i].upper()

    return ''.join(password)

def _bits_of_entropy():
    from math import log2
    N_vowels = 6
    N_consonants = 11
    return(
        log2(len(V)) * N_vowels
        + log2(len(C)) * N_consonants
        + log2(3)  # places to insert numeric block
        + log2(2)  # places to insert number
        + log2(17)  # letters, one of which is capitalised
    )

if __name__ == '__main__':
    from argparse import ArgumentParser
    parser = ArgumentParser(description=__doc__)
    parser.add_argument('-n', '-N', type=int, nargs='?', default=1, help='Number of passwords to generate (default 1)')
    args = parser.parse_args()

    for i in range(args.n):
        print(password())