# from https://github.com/janglin/crypto-pkcs7-example/blob/master/pkcs7.py
import binascii
try:
import StringIO
except ImportError:
from io import StringIO
class PKCS7Encoder(object):
'''
RFC 2315: PKCS#7 page 21
Some content-encryption algorithms assume the
input length is a multiple of k octets, where k > 1, and
let the application define a method for handling inputs
whose lengths are not a multiple of k octets. For such
algorithms, the method shall be to pad the input at the
trailing end with k - (l mod k) octets all having value k -
(l mod k), where l is the length of the input. In other
words, the input is padded at the trailing end with one of
the following strings:
01 -- if l mod k = k-1
02 02 -- if l mod k = k-2
.
.
.
k k ... k k -- if l mod k = 0
The padding can be removed unambiguously since all input is
padded and no padding string is a suffix of another. This
padding method is well-defined if and only if k < 256;
methods for larger k are an open issue for further study.
'''
def __init__(self, k=16):
self.k = k
# @param text The padded text for which the padding is to be removed.
# @exception ValueError Raised when the input padding is missing or corrupt.
def decode(self, text):
'''
Remove the PKCS#7 padding from a text string
'''
nl = len(text)
try:
val = int(binascii.hexlify(text[-1]), 16)
except TypeError:
val = text[-1]
if val > self.k:
raise ValueError('Input is not padded or padding is corrupt')
l = nl - val
return text[:l]
# @param text The text to encode.
def encode(self, text):
'''
Pad an input string according to PKCS#7
'''
l = len(text)
output = StringIO.StringIO()
val = self.k - (l % self.k)
for _ in xrange(val):
output.write('%02x' % val)
return text + binascii.unhexlify(output.getvalue())
