Re: SNMPv2 security

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Frank Kastenholz says:
> If you manage to intercept all transactions, you will have only 2
> transactions which contain any given key-value. In effect, you have
> two numbers, each of which is the exclusive or of a constant (the
> key) and a different, unknown random number.
> Just to show the point, assume that keys are 8-bits long. Let us say
> that you have intercepted the first ten key changes from a manager to
> a specific agent (e.g. the (K1 xor K0) ... (K10 xor K9) exchanges,
> above). The data that you have received are: 0xbf, 0x99, 0x25, 0xc1,
> 0x2e, 0xc7, 0x9d, 0xc4, 0x51, and 0x45. What are the keys?

Just to get back to cryptographic view of this problem...

What is the reason people change their [secret] keys?
Why not leave the same one for indefinite time?

Maybe it's because there's a desire to make sure,  that
if at any moment an adversary _somehow_ (i.e. black bag
job, cryptanalysis, bribery, torture) acquires our key,
it's "usability" will be limited,  and our traffic will
be compromised only during a lifetime of the key  [that
we lost].

Now in this XOR scheme, it doesn't matter at what point
in time I get your key -  but your traffic is open to
me from now on, both in past (pre-recorded messages)
and whatever you'll send in the future. Also, if
there's is a cryptanalytic attack on the key,
then these XORs make sure I'll be able to
exploit _all_ the traffic, rather than
just those messages, encrypted or
protected by that particular
key I'm trying to crack.

Thus - what do we save by "changing" the key in
such a funny way?  What kind of "attack" are we
twarting? Inquiring minds want to know (:-).
--
Regards,
Uri         [e-mail]      scifi!angmar!uri 	N2RIU
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