Re: [radext] I-D Action: draft-ietf-radext-tls-psk-09.txt

[Alexander Clouter <[e-mail]>]

On Thu, 29 Feb 2024, at 20:55, [e-mail] wrote:
> Internet-Draft draft-ietf-radext-tls-psk-09.txt is now available. It is a work
> item of the RADIUS EXTensions (RADEXT) WG of the IETF.
>
>    Title:   RADIUS and TLS-PSK
>    Author:  Alan DeKok
>    Name:    draft-ietf-radext-tls-psk-09.txt
>    Pages:   20
>    Dates:   2024-02-29

Got to get that aspell integration working again... :)

s/transistion/transition/

Since going hipster^WPython, we now have an off-by-one error[1] and ends up discarding ~24 bits of entropy.

---- implementation0.py
#!/usr/bin/env python3
import base64, secrets
str = base64.b32encode(secrets.token_bytes(16)).decode().lower()
print(str)
print("-".join([str[i:i + 4] for i in range(0, len(str), 3)][0:7]))
----
alex@hanzawa:~$ python3 implementation0.py 
nl4mzes5asqkxd4pklkwagltjq======   <---- raw base32 output of key
nl4m-mzes-s5as-sqkx-xd4p-pklk-kwag <---- processed
----

So you can see the trailing 'ltjq' is missing, and the repetition between tops'n'tails of each block.

It is though *technically* an adhering implementation as it follows "which SHOULD be derived from a source with at least 128 bits"...but likewise so does: :)

{ printf 0123456789abcde; dd if=/dev/urandom bs=1 count=16 2>/dev/null | cut -b1; }

Maybe we should amend that state to "which SHOULD be derived, whilst retaining, from a source with at least 128 bits"

Compounding the confusion is the Python implementation provides no hint on the purpose of the truncation. Maybe we should be more explicit by stripping directly the padding and then it means we no longer need a window applied to the output list:

---- proposed.py
#!/usr/bin/env python3
import base64, secrets
str = base64.b32encode(secrets.token_bytes(16)).decode().rstrip('=').lower()
print('-'.join((str[i:i + 4] for i in range(0, len(str), 4))))
----

Results in 32 characters of output:
----
alex@hanzawa:~$ python3 proposed.py
46mx-u26s-4j2e-igby-akty-6zqu-zy
----

The document refers to 'urandom' which is no longer present, so should be amended to 'token_bytes'.

Cheers

Alex

[1] this may have been deliberate so to pad up to 32+ characters, but the aim here is to provide a clear and understandable implementation to show how easy it is to do the right thing