pkcs7.py

def pad_pkcs7(x):
    ''' Pad the byte array according to the PKCS7 standard.
    If the length is 15, pad with 0x1 = 16 - 15
    If the length is 14, pad with 0x2 = 16 - 14
    ...
    '''
    length = len(x)
    delta = 16 - length
    result = x[:]
    for i in range(length, 16):
        result.append(delta)
    assert len(result) == 16
    return result

def is_valid_pad_pkcs7(x):
    ''' Check that the byte array has a valid padding.
    The padding value must be a value between 1 and 16.
    '''
    pad_value = x[-1]
    if pad_value == 0:
        return False
    for i in range(len(x) - 1, len(x) - pad_value - 1, -1):
        if x[i] != pad_value:
            return False
    return True