Authenticate a message

[mthethwansm]

Hi everyone

How I can authenticate that a message is coming from the right source using micropy module or any pycom library?

[g0hww]

I'm not sure if this answer is really pertinent to your question, but I use HMAC-MD5 authentication on an AES-128 encrypted payload on my SOCK_RAW LoRa links. Both MD5 and AES-128 are supported by the hardware acceleration. Both use the same pre-shared 128-bit key. I rolled my own HMAC-MD5 implementation though.

[mthethwansm]

Thanks @g0hww. I looked at HMAC it seems fine for me. Can please share your HMAC-MD5 implementation.

[g0hww]

@mthethwansm Yeah, here it is. Running that file does a quick test against the example result shown on wikipedia for HMAC-MD5. I have also tested it for interoperability with desktop python implementations and it seems fine.

#! /usr/bin/env python3

# Copyright 2017 Darren M Long, darren.long@mac.com
# GPLv3 applies.

import gc
import uhashlib

def hmac_md5(key, message):
    blocksize = 64
    # keys longer than blocksize are shortened
    if len(key) > blocksize:
        key = md5hash(key)
    # keys shorter than blocksize are zero-padded
    elif len(key) < blocksize:
        key = key + b'\x00'*(blocksize - len(key))
    assert len(key)==blocksize
    o_key_pad = sxor(b'\x5c'*blocksize, key)
    i_key_pad = sxor(b'\x36'*blocksize, key)
    inner_hash =  md5hash(i_key_pad+message)
    outer_hash =  md5hash(o_key_pad+inner_hash)
    gc.collect()
    return outer_hash

def md5hash(data):
    m = uhashlib.md5()
    m.update(data)
    return m.digest()

def sxor(s1,s2):
    return bytes(x ^ y for x, y in zip(s1, s2))


if __name__ == '__main__':
    # main tests against example here: https://en.wikipedia.org/wiki/HMAC
    import binascii
    from machine import Timer

    chrono = Timer.Chrono()
    print('testing uhmac ... ',end='')

    chrono.start()
    result = hmac_md5(b"key", b"The quick brown fox jumps over the lazy dog")
    chrono.stop()

    assert binascii.hexlify(result) == b'80070713463e7749b90c2dc24911e275'
    assert len(result) == 16

    print('ok')

    duration = chrono.read()
    print('time taken to test code: %f secs' % duration)
    gc.collect()

[jcaron]

@g0hww @mthethwansm

Note that MD5 is considered "cryptographically broken and unsuitable for further use" since 2008.

SHA-1 does not fare much better.

Please refrain from using MD5 (or SHA-1) and switch to more secure hashes like SHA-256 and up. Thankfully this should be a trivial change in the above code.

[g0hww]

@jcaron Well, I need stuff that fits in a LoRa payload. Furthermore, the wikipedia page says:

In 2011 an informational RFC 6151[13] was published to summarize security considerations in MD5 and HMAC-MD5. For HMAC-MD5 the RFC summarizes that – although the security of the MD5 hash function itself is severely compromised – the currently known "attacks on HMAC-MD5 do not seem to indicate a practical vulnerability when used as a message authentication code", but it also adds that "for a new protocol design, a ciphersuite with HMAC-MD5 should not be included".

Obviously YMMV.

[mthethwansm]

@g0hww and @jcaron thank you so much for sharing.