[Cryptography] What's a Plausible Attack On Random Number Generation?

[David Mercer]

On Sat, Nov 2, 2013 at 2:33 PM, Bill Stewart <bill.stewart at pobox.com> wrote:

> At 07:21 AM 11/1/2013, Jerry Leichter wrote:
>
>> On Nov 1, 2013, at 7:04 AM, Yaron Sheffer <yaronf.ietf at gmail.com> wrote:
>> > It sounds like a quick addition to DHCP - an extension that gets you
>> 256 bits from the server, would solve 99% of the problem we have with
>> embedded devices. It will not be sufficient for high-security environments,
>> because an attacker might be listening on the local LAN....
>> Ahem.  This is *exactly* the kind of reasoning I started this thread to
>> investigate.  (Though I certainly agree that a *single* DHCP packet
>> containing a random bit string is easily attacked.)
>>
>
> It's slightly backwards as far as timing goes - if you're trying to run a
> pure client, you normally have physical input from the user and access to a
> sound card before running anything that needs to generate encryption keys,
> so you don't really need it, and if you're running a server, you almost
> always want a fixed IP address rather than a random one from the DHCP pool,
> so you're probably not going to ask for DHCP.  Also, if you're starting a
> brand-new-out-of-the-box server, it doesn't matter if it takes a few
> minutes before there's enough entropy to generate keys, because it's new,
> while the case where you care most about startup time is restarting a
> previously running server that was shut down, so you would have saved a
> seed by then.  I guess that Cloud World may have occasion to care about how
> long it takes to provision a brand-new server from a canned image, and need
> to generate an ssh key so a user can log in to update the rest of their
> software, because they're paying by the millisecond, but are they likely to
> use DHCP as opposed to having Chef/Puppet give them an address?


Actually, in most of CloudWorld, as you put it, DHCP is used for servers
with fixed IP addresses.  The provisioning system knows the "hardware" MAC
address, and plops the private IP into the DHCP config.  Amazon AWS does
this, as do most other large cloud or VM hosts.  It is hand installed VM's
that will not use DHCP for static IP address allocation, and in that case
you have keyboard and mouse events.


-David Mercer
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