Computing MS-MPPE-Recv-Key and MS-MPPE-Send-Key in MS-CHAPv2 auth

[Jacob Lane]

Hi Hadi!  Feel free to drop me an email directly if you get really stuck (I've no experience with MS-CHAP, but I can just about make sense of RFCs), but in regards to mppe_add_reply - MS-CHAPv2 uses 128 bit send/receive session keys (16 bytes).  When calling mppe_add_reply, Alan is adding a keypair to the VP list that is 16 bytes long for each.  See this [https://datatracker.ietf.org/doc/html/draft-ietf-pppext-mschapv2-keys] IETF Draft from 1998 for session key reference, and RFC 2759 [https://datatracker.ietf.org/doc/html/rfc2759] for more info about the whole MS-CHAPv2 auth process.

Cheers!

Jacob
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On 06/01/2025 14:54:07, Hadi Rezaee <rezaee.hadi at gmail.com> wrote:
Hello Alan and thanks for your response.
You're absolutely right, I need to go back for more research and code
review :)

Regards,
Hadi

On Mon, Jan 6, 2025 at 5:34 PM Alan DeKok wrote:

> On Jan 5, 2025, at 2:37 PM, Hadi Rezaee wrote:
> >
> > I'm trying to figure out how MS-MPPE-Send-Key and MS-MPPE-Recv-Key are
> > getting generated by freeradius-server.
>
> The short answer is "read the RFCs and the code"
>
> > Here, it is the sample captured
> > traffic I've for a MS-CHAPv2 (Access-Accept) auth between a radius client
> > and server:
> > MS-MPPE-Recv-Key:
> > a660ce53f31ef08ed6cf209ece137a1dee40aeae5d8e5b9de0f1592324bc92569fc1
> > MS-MPPE-Send-Key:
> > a81579eb58f0bd25636599778c8689516129db8b25ec2d1e4c15797862efedabb3c
>
> Those are just random values. They don't mean anything.
>
> > Correct me if I'm wrong but here I read " mppe_sendkey" and "
> mppe_recvkey"
> > variables are initialized with 34 bytes (as i expected!), but later by
> > calling mppe_chap2_gen_keys128 only 16 bytes are copied. I was expecting
> > other items (such as 'Salt') to be taken into account too ..
>
> To be honest, I haven't looked at that code in a long time. If it's
> generating 34-byte keys, then it works. Which means that your reading of
> the code is wrong.
>
> If you want to know what the code is doing, use a debugger like gdb to
> step through it.
>
> Alan DeKok.
>
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