What is wrong with this CMAC computation? - cmac

Question

I have an example of a CMAC computation, which I want to reproduce in Python, however I am failing. The example looks like this:
key = 3ED0920E5E6A0320D823D5987FEAFBB1
msg = CEE9A53E3E463EF1F459635736738962&cmac=
The expected (truncated) CMAC looks like this (note: truncated means that every second byte is dropped)
ECC1E7F6C6C73BF6
So I tried to reenact this example with the following code:
from Crypto.Hash import CMAC
from Crypto.Cipher import AES
from binascii import hexlify, unhexlify
def generate_cmac(key, msg):
"""generate a truncated cmac message.
Inputs:
key: 1-dimensional bytearray of arbitrary length
msg: 1-dimensional bytearray of arbitrary length
Outputs:
CMAC: The cmac number
CMAC_t: Trunacted CMAC"""
# Generate CMAC via the CMAC algorithm
cobj = CMAC.new(key=key, ciphermod=AES)
cobj.update(msg)
mac_raw = cobj.digest()
# Truncate by initializing an empty array and assigning every second byte
mac_truncated = bytearray(8 * b'\x00')
it2 = 0
for it in range(len(mac_raw)):
if it % 2:
mac_truncated[it2:it2+1] = mac_raw[it:it+1]
it2 += 1
return mac_raw, mac_truncated
key = unhexlify('3ED0920E5E6A0320D823D5987FEAFBB1') # The key as in the example
msg = 'CEE9A53E3E463EF1F459635736738962&cmac=' # The msg as in the example
msg_utf = msg.encode('utf-8')
msg_input = hexlify(msg_utf) # Trying to get the bytearray
mac, mact_calc = generate_cmac(key, msg_input) # Calculate the CMAC and truncated CMAC
# However the calculated CMAC does not match the cmac of the example
My function generate_cmac() works perfectly for other cases, why not for this example?
(If anybody is curious, the example stems from this document Page 18/Table 6)
Edit: An example for a successful cmac computation is the following:
mact_expected = unhexlify('94EED9EE65337086') # as stated in the application note
key = unhexlify('3FB5F6E3A807A03D5E3570ACE393776F') # called K_SesSDMFileReadMAC
msg = [] # zero length input
mac, mact_calc = generate_cmac(key, msg) # mact_expected and mact_calc are the same
assert mact_expected == mact_calc, "Example 1 failed" # This assertion passes

Answer 1

TLDR: overhexlification
Much to my stupefaction, the linked example indeed seems to mean CEE9A53E3E463EF1F459635736738962&cmac=when it writes that, since the box below contains 76 hex characters for the the 38 bytes coding that in ASCII, that is 434545394135334533453436334546314634353936333537333637333839363226636d61633d.
However I'm positive that this does not need to be further hexlified on the tune of 76 bytes as the code does. In other words, my bets are on
key = unhexlify('3ED0920E5E6A0320D823D5987FEAFBB1')
msg = 'CEE9A53E3E463EF1F459635736738962&cmac='.encode()
mac, mact_calc = generate_cmac(key, msg)