RE: How is the LESC OOB data generated?

raccoon — Wed, 29 Jul 2020 05:45:35 GMT

The content of the OOB data is spec defined,
so let's assume that you simply want to know how OOB data is generated according to the spec.

By changing 2 Bytes - even on both sides - it is quite natural that you fail the pairing.
Have a look at the spec and notice that the confirmation value is a computed value.
It is something that peers compute on their own and then simply compare to what they received in the OOB data.

Have a look a the following python snippet I used once upon a time to understand on my own the OOB data:

from Crypto.Random import get_random_bytes
from Crypto.Hash import CMAC
from Crypto.Cipher import AES
from Crypto.PublicKey import ECC

"""
From v5.1 Vol 3 - Part H - Section 2.2.6
The confirm value generation function makes use of the MAC function AES-CMAC_X, 
with a 128-bit key X.

The inputs to function f4 are:
- U is 256 bits
- V is 256 bits
- Z is 8 bits.
  For Numeric Comparison and OOB protocol - all zeros.
- X is 128 bits

U, V and Z are concatenated and used as input m to the function AES-CMAC.
X is used as the key k.

f4(U, V, X, Z) = AES-CMAC_X(U||V||Z)

The inputs to f4 are different depending on different association models.
For OOB f4(PKx, PKx, r, 0) where r is a random value.
"""

key = ECC.generate(curve='P-256')
PKx = key.public_key()._point.x.to_bytes()
Z = b'\x00\x00\x00\x00\x00\x00\x00\x00'
r = get_random_bytes(16)

cobj = CMAC.new(r, ciphermod=AES)
cobj.update(PKx + PKx + Z)


print(f'For your reference, this is the private key: {key.d.to_bytes().hex()}')
print(f'OOB: PKx: {PKx.hex()}')
print(f'OOB: PKy: {key.public_key()._point.y.to_bytes().hex()}')
print(f'OOB: r: {r.hex()}')
print(f'OOB: Confirmation: {cobj.hexdigest()}')

RE: How is the LESC OOB data generated?

Øyvind — Fri, 24 Jul 2020 11:54:37 GMT

Hello,

The technical support team is currently understaffed and have too many support tickets to handle. We expect a more normal function on Monday, and I will have a look at your case then.

I'm sorry for the inconvenience this may cause.

Kind regards,
Øyvind