test_crypto.doctest - test various internal crypto helper functions

Copyright (C) 2016 Arthur de Jong

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>>> import random
>>> from binascii import a2b_hex, b2a_hex
>>> def tostr(x):
...     return str(x.decode())
>>> def decode(f):
...     return lambda x: tostr(f(x))
>>> b2a_hex = decode(b2a_hex)

>>> from pskc.crypto import pad, unpad


Tests for padding and unpadding. This should follow PKCS#7 as defined in RFC
5652 section 6.3.

>>> b2a_hex(pad(a2b_hex('1234'), 16))
'12340e0e0e0e0e0e0e0e0e0e0e0e0e0e'
>>> b2a_hex(pad(a2b_hex('abcdefabcdefabcdefabcdefabcdef'), 16))
'abcdefabcdefabcdefabcdefabcdef01'
>>> b2a_hex(pad(bytearray(16 * [65]), 16))
'4141414141414141414141414141414110101010101010101010101010101010'

>>> b2a_hex(unpad(a2b_hex('12340e0e0e0e0e0e0e0e0e0e0e0e0e0e'), 16))
'1234'
>>> b2a_hex(unpad(a2b_hex('abcdefabcdefabcdefabcdefabcdef01'), 16))
'abcdefabcdefabcdefabcdefabcdef'


Padding should always work but there are some conditions where unpadding does
not work. Specifically, the to be removed padding should all have the same
bytes:

>>> b2a_hex(unpad(a2b_hex('ffff0606060f0606'), 8))  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
DecryptionError: Invalid padding

The unpadding should also refuse to remove more than a block size worth of
data:

>>> b2a_hex(unpad(a2b_hex('ffff0e0e0e0e0e0e0e0e0e0e0e0e0e0e'), 8))  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
DecryptionError: Invalid padding


We generate a number of (non-cryptographically strong) random strings and
ensure that padding and unpadding works for them.

>>> for i in range(100):
...     l = random.randrange(1, 255)
...     r = bytearray(random.getrandbits(8) for x in range(l))
...     b = random.randrange(10, 100)
...     p = pad(r, b)
...     u = unpad(p, b)
...     if u != r:
...         raise ValueError('%s -> %s -> %s (%d)' % (
...             b2a_hex(r), b2a_hex(p), b2a_hex(u), b))