Subject: [IPsec] eap-md5 based authentication

From: "Prashant Batra (prbatra)" <[email-masked]>
Date: Mon, 24 Oct 2011 21:41:06 +0530

Hello,

I am facing some problem in calculating md5-challenge response.

What I am doing is simply MD5(Identifier | <secret> | <Challenge value
received in the challenge request>).

The challenge response is somehow wrong.

Is it correct to say that Challenge value used as input to md5 is the
same value what is in the EAP payload (type md5-challenge request)?

Regards,

Prashant

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From: Yoav Nir <[email-masked]>
Date: Tue, 25 Oct 2011 10:35:04 +0200

Hi Prashant.

I think in the challenge request, the first byte is the challenge length (usually 16) followed by the challenge itself, and then followed by some server name. I guess the reasoning is that this allows the client to choose the correct password based on the server name.

Yoav

________________________________
From: [email-masked] [mailto:[email-masked]] On Behalf Of Prashant Batra (prbatra)
Sent: 24 October 2011 18:11
To: [email-masked]
Subject: [IPsec] eap-md5 based authentication

Hello,

I am facing some problem in calculating md5-challenge response.
What I am doing is simply MD5(Identifier | <secret> | <Challenge value received in the challenge request>).
The challenge response is somehow wrong.

Is it correct to say that Challenge value used as input to md5 is the same value what is in the EAP payload (type md5-challenge request)?

Regards,
Prashant

---

From: Glen Zorn <[email-masked]>
Date: Tue, 25 Oct 2011 17:15:52 +0700

On 10/25/2011 3:35 PM, Yoav Nir wrote:

> Hi Prashant.
>  
> I think in the challenge request, the first byte is the challenge length
> (usually 16) followed by the challenge itself, and then followed by some
> server name. I guess the reasoning is that this allows the client to
> choose the correct password based on the server name.

The format is defined in Section 4.1 of RFC 1994

...

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From: "Prashant Batra (prbatra)" <[email-masked]>
Date: Tue, 25 Oct 2011 17:40:25 +0530

Thanks Yaov and Glen,

I could successfully calculate the challenge response.
Now, after the challenge response is successful, the server will send
EAP-SUCCESS, then the client has to send a AUTH payload.
As eap-md5 doesn't result in any key like eap-aka/sim, the client will
use the same password(used for calculating challenge response) to
calculate AUTH payload.
If so, why there is an explicit auth required here. EAP-SUCCESS, can
itself indicate that the client is authenticated.

Maybe, it is required for some extra authentication?

Regards,
Prashant

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Glen Zorn
Sent: Tuesday, October 25, 2011 3:46 PM
To: Yoav Nir
Cc: [email-masked]; Prashant Batra (prbatra)
Subject: Re: [IPsec] eap-md5 based authentication

...
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From: Yoav Nir <[email-masked]>
Date: Tue, 25 Oct 2011 14:28:58 +0200

No, you don't use the same password for calculating the AUTH payload.
>From section 2.15:

   There are two types of EAP authentication (described in
   Section 2.16), and each type uses different values in the AUTH
   computations shown above.  If the EAP method is key-generating,
   substitute master session key (MSK) for the shared secret in the
   computation.  For non-key-generating methods, substitute SK_pi and
   SK_pr, respectively, for the shared secret in the two AUTH
   Computations.

So the client in your case is going to use SK_pi.

If you don't mind the question, how did this come up?  If you're writing
your own client, why not use something better than EAP-MD5 such as EAP-EKE
or EAP-pwd?  If you're using a third-party client (like Microsoft's Win7
client) I think they're using EAP-MSChapv2.  Where did you find a client
with EAP-MD5?

Yoav

On 10/25/11 2:10 PM, "Prashant Batra (prbatra)" <[email-masked]> wrote:

>Scanned by Check Point Total Security Gateway.

---

From: "Prashant Batra (prbatra)" <[email-masked]>
Date: Tue, 25 Oct 2011 20:30:55 +0530

Thanks Yoav,

I think I missed it somehow from the RFC.

Regarding the usage of EAP-MD5, it is just that the client should
support all types of authentication,
So even the weakest ones also and rfc-3748 says that md5-challenge is a
must.
Moreover, it is the server that has to decide what is the best suited
auth mechanisms for a particular client.

Regards,
Prashant

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Yoav Nir
Sent: Tuesday, October 25, 2011 5:59 PM
To: Prashant Batra (prbatra); Glen Zorn
Cc: [email-masked]
Subject: Re: [IPsec] eap-md5 based authentication

If you don't mind the question, how did this come up?  If you're writing
your own client, why not use something better than EAP-MD5 such as
EAP-EKE
or EAP-pwd?  If you're using a third-party client (like Microsoft's Win7
client) I think they're using EAP-MSChapv2.  Where did you find a client
with EAP-MD5?

Yoav