Settable minimum RSA key sizes on the client end for legacy devices.

[Philipp Marek]

Seems I wasn't clear enough on my main point -

     I'm not an SSH developer,
     and I _want_ to be protected by sane limits here!


>>> Minimal key size should have a "reasonable" default, and an explicit
>>> config parameter to override it and set to whatever value that
>>> *specific* installation needs.
>> 
>> No, that's too easy.
>> I've seen too many decisions made on such a basis - "just configure
>> security down until it works" - but these invariably lead to disaster.
> I don't think the right decision is to prevent people from doing
> things you don't like, and second guessing what they consider secure
> by over-riding defaults.  This is sort of the attitude I'm talking
> about.  It seems entirely reasonable to put the minimum key size as a
> runtime option rather than compile-time.  These are the people who own
> the computer in question.

Yeah, right. But owning something doesn't mean you know or can estimate
the second- or third-order changes you cause.

If the key-length runtime option goes down as far as you like, you
might be vulnerable to a timing attack - or a transmit-power
analysis for embedded CPUs with a Wifi, like a Raspberry Pi.


Security isn't a one-dimensional thing that can be measured in a single
bit length - it has so many facets, most of which one wouldn't even
think of.


> Maybe an admin want an even higher minimum key size than 1024 bits. 
> There's plenty of systems that might still be in service in 20 years,
> and perhaps the minimum key size would go up in that time.  Without
> the ability to set at runtime, you have to re-compile, which is always
> much harder for old systems.
Yeah, right. Current SSH allows already "-b 16384", that should be safe
enough for a few years.
So _longer_ keys than default isn't the issue here.


> Also, you can still make your system insecure if you want. telnetd is
> still supported on Debian system, and at least Centos 7, but not
> really recommended of course.
Yeah, of course security can be degraded in oh so many ways.


> The natural conclusion of the "I'm the parent trying to protect you
> from bad decisions" idea is that everyone else is a child.
Well, that's the point that I couldn't communicate.
I _long_ for
a) sane defaults that are good enough for a few years, and
b) sane limits that prevent me from doing something stupid.


> There's
> plenty of people that can understand exactly the risks they're taking
> with smaller key sizes, and are still willing to make the tradoff.
Sorry, no.
There are lots of people who hope to achieve something (faster
connection establishment, less CPU or battery use, backwards
compatibility, ...) by reducing security.

Some of these are true engineering trade-offs, and are okay.


I simply think that having some hard-coded limits
that are not easily removed via a google search and a config option
makes people ask other people for advice,
who then not only say "change #define and recompile" but also
mention some second-order consequences that are not obvious.



All that said, I'm well aware that for the problem that originated this
thread "recompile ssh-keygen and ssh to have smaller limits" is a valid
first-order technical solution.

OTOH, a complete threat-analysis would have to include the (monetary and
environmental) cost of a new device, the time spent analysing,
discussing, and then fixing the SSH key length, the potential risk of
this device being attacked and instructed to catch fire, and so on.