86 lines
2 KiB
Python
86 lines
2 KiB
Python
# common crypto functions (mostly specific to XEP-0116, but useful elsewhere)
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import os
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import math
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from Crypto.Hash import SHA256
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# convert a large integer to a big-endian bitstring
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def encode_mpi(n):
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if n >= 256:
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return encode_mpi(n / 256) + chr(n % 256)
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else:
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return chr(n)
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# convert a large integer to a big-endian bitstring, padded with \x00s to
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# a multiple of 16 bytes
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def encode_mpi_with_padding(n):
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return pad_to_multiple(encode_mpi(n), 16, '\x00', True)
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# pad 'string' to a multiple of 'multiple_of' with 'char'.
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# pad on the left if 'left', otherwise pad on the right.
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def pad_to_multiple(string, multiple_of, char, left):
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mod = len(string) % multiple_of
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if mod == 0:
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return string
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else:
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padding = (multiple_of - mod) * char
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if left:
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return padding + string
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else:
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return string + padding
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# convert a big-endian bitstring to an integer
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def decode_mpi(s):
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if len(s) == 0:
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return 0
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else:
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return 256 * decode_mpi(s[:-1]) + ord(s[-1])
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def sha256(string):
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sh = SHA256.new()
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sh.update(string)
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return sh.digest()
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base28_chr = "acdefghikmopqruvwxy123456789"
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def sas_28x5(m_a, form_b):
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sha = sha256(m_a + form_b + 'Short Authentication String')
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lsb24 = decode_mpi(sha[-3:])
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return base28(lsb24)
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def base28(n):
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if n >= 28:
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return base28(n / 28) + base28_chr[n % 28]
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else:
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return base28_chr[n]
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def random_bytes(bytes):
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return os.urandom(bytes)
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def generate_nonce():
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return random_bytes(8)
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# generate a random number between 'bottom' and 'top'
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def srand(bottom, top):
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# minimum number of bytes needed to represent that range
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bytes = int(math.ceil(math.log(top - bottom, 256)))
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# in retrospect, this is horribly inadequate.
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return (decode_mpi(random_bytes(bytes)) % (top - bottom)) + bottom
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# a faster version of (base ** exp) % mod
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# taken from <http://lists.danga.com/pipermail/yadis/2005-September/001445.html>
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def powmod(base, exp, mod):
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square = base % mod
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result = 1
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while exp > 0:
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if exp & 1: # exponent is odd
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result = (result * square) % mod
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square = (square * square) % mod
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exp /= 2
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return result
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