def decrypt(self, cipher_line):
if not util.is_encrypted(cipher_line):
raise KarnError('"%s" is not encrypted!' % cipher_line)
# convert from base 32 to bytes literal
cipherbytes = bytearray.fromhex(unicode(util.inttohex(int(cipher_line.strip(), 32))))
output = bytearray()
# start at byte 1 because byte 0 is the guard byte
for i in xrange(1, len(cipherbytes), BLOCK_SIZE):
cipher = cipherbytes[i:i+BLOCK_SIZE]
# + is the concatenation operator in python
# ^ is the xor operator
leftmd = bytearray(hashlib.sha1(str(util.right(cipher)) + str(self.rightkey)).digest())
leftcipher = util.left(cipher)
plaintext = bytearray(leftmd[i] ^ leftcipher[i] for i in xrange(len(leftmd)))
rightmd = bytearray(hashlib.sha1(str(plaintext) + str(self.leftkey)).digest())
rightcipher = util.right(cipher)
plaintext.extend(bytearray(rightmd[i] ^ rightcipher[i] for i in xrange(len(rightmd))))
# break if we decrypted a null byte
output.extend(plaintext.partition('\x00')[0])
if '\x00' in plaintext:
break
# we are expecting an ascii string, so print debug info
# if any decrypted character is out of ascii range
if any(i > 127 for i in output):
print 'couldn\'t decrypt ciphertext: %s' % cipher_line
print 'with key: %d' % self.key
print 'output: %s' % output
raise DecryptionError(cipher_line, self.key, output)
return str(output)
def __init__(self, key):
self.key = key
# need to convert integer to hex str before getting the byte array
keybytes = bytearray.fromhex(unicode(util.inttohex(key)))
# the monitor uses Java's BigInteger, which uses 2s complement.
# that means that it will add on a 0 byte in front to preserve the
# sign of the key, if the most significant bit of the key is set
if keybytes[0] & (1 << 7):
keybytes.insert(0,0)
# even if the key ends up being an odd number of bytes, the monitor breaks it
# into two equal halves, and the least significant byte gets discarded.
# probably a bug in the monitor, but that's how it is
halfkeylen = len(keybytes) // 2
self.leftkey = keybytes[:halfkeylen]
self.rightkey = keybytes[halfkeylen: 2*halfkeylen]
