Subject: RE: SNMPv3 Diffie-Helman key mib

From: Yost William <[email-masked]>
Date: Mon, 2 Aug 1999 09:26:56 -0500

My concern was for the software developer.  When I developed the SNMP V3
software for our cable modem I had to check that the a MIB browser could
talk to the cable modem using the docsisProv user after the modem did it's
diffie-hellman process.  The mib browser I used required that I type in a
readable password.

We have automated that process now by writing a proprietary program that
creates the secondary users and passwords so that isn't a concern here now.

By the way, what SNMP V3 manager does @Home use?  @Home has previously
stated that all of their vendors were developing SNMP V3 cable modems and
the technical note TN-010 requires it.  I'd like to be sure that we work
with the manager you are currently using.

----------- William H. Yost, Thomson Consumer Electronics ------------ 
.     Home of RCA, GE, and Proscan (317) 587-4816 [email-masked]   .

-----Original Message-----
From: Mike StJohns [mailto:[email-masked]]
Sent: Friday, July 30, 1999 12:58 PM
To: [email-masked]
Subject: Re: SNMPv3 Diffie-Helman key mib 

> The description of usmDHKickstartMgrPublic describes how to convert the
> shared secret into the underlying keys.  I believe it is better to first
> convert the shared secret into a readable 16 character password and then
use
> the password to key algorighm in RFC2274.  This is because a lot of MIB
> browsers and maybe SNMP managers do not allow manual entry of keys, only
> passwords you can type in by the keyboard.  The internally they use the
> password to key algorithm to make a key.
> 

I think you're missing the point here.  The idea is to be able to kick start
this automatically, untouched by human hands.  And were talking lots and
lots of values here - one per managed device.  Any manager that implements
this is going to have to be able to install a key for a device based on data
it can retrieve from the agent without falling back to 'printable'
passwords.

BTW - I consider the ability to 'pick' a key one of the biggest weakenesses
in USM.  Humans are NOT a good source of random numbers and hence are not a
good source of key.

In any event, converting the shared secret into a readable password would
probably involve the loss of entropy with a parallel weakening of security.
I guess you could convert the shared secret into a hex representation  -
but then why go through all of this?

---

From: Yost William <[email-masked]>
Date: Tue, 17 Aug 1999 16:16:26 -0500

Here is an examle of a readable password generated by our modem using your
original Diffie-Hellman key transfer scheme (The one that defined the
docsDevKickStart group) with my addition of forcing it to be a readable
password.  You can see you don't get easily remembered passwords and I don't
think you are losing any randomness because there are still 16 characters.
Each character is limited to 96 values instead of the full 256 8-bit
combinations.  This means the possible combinations are (96**16).  This
password can be entered into an SNMP MIB browser available today (MG-Soft)
and can be used for testing.

0P_L\L$k$(~<5}^L

This scheme is still startable untouched by human hands.  I am only
proposing that after the generation of the 16 byte key, each byte is forced
into the printable ascii range of 33 to 126 decimal.  Both the cable modem
and the outside manager do this conversion and then use the password to key
generation algorithm in RFC2274.  But like I said earlier, I am seeing the
problem from a software developer's viewpoint.  In developing the original
code I had to test it by using an SNMP V3 browser which required me to enter
a password.

I understand if your security concerns are greater than this provides.  I
just wanted to be sure you understood what I was proposing.

----------- William H. Yost, Thomson Consumer Electronics ------------ 
..     Home of RCA, GE, and Proscan (317) 587-4816 [email-masked]   .

-----Original Message-----
From: Mike StJohns [mailto:[email-masked]]
Sent: Friday, July 30, 1999 12:58 PM
To: [email-masked]
Subject: Re: SNMPv3 Diffie-Helman key mib 

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