Re: Concepts: Security and Obscurity

[Daniel Miessler <daniel () dmiessler com>]

On Apr 12, 2007, at 2:34 AM, Craig Wright wrote:

> Port knocking has the issue that it is not completely silent as is
> presumed. Most routers are not set to stop sending ICMP, TCP etc
> responses to other routers. In fact to do so is a violation of the
> internet standards. As such, information on ports is often available
> from the network infrastructure. Drop on firewalls does not stop an
> attacker finding what ports are running - it just means that they have
> to be a little more creative.

More creative? Ok, let's try it this way:

1. You send me a SYN to a given port
2.  I send you an RST/ACK for that port (or I don't answer at all)

Either way, what now? So what if you can ping the firewall? We're  
talking about ACLs here. An ACL saying traffic gets dropped or  
rejected to port X. So unless you have some revolutionary way to  
simply bypass firewall ACLs, you're basking in the darkness of  
futility here, my friend.

> Systems that ONLY drop packets stand out. They are not "stealthy" but
> rather the hole they make makes them extrememly visible.

Huh ?!?

Kind of like the hole caused by systems that aren't online? LOL. So I  
guess all those systems that don't exist are being put on a master  
hacker list somewhere to be investigated later? Dude, you're  
frightening me.

> In port knocking the control is not highly effective, to take a quote:
>
> In 'Critique of Port Knocking', Arvind Narayana states:
> "Suppose you decide on a list of 32 valid ports (the current
> implementation
> allows up to 256). How long does the port knock sequence
> need to be? You might think that since each port is a 16-bit integer,
> you need 8 knocks, so that you get 8*16 bits or 128 bits of security
> (virtually unbreakable). But since each port has only 32 possible
> values (5 bits), what you actually get is only 8*5=40 bits of security
> (trivially breakable)!"

Portknocking isn't the point; I mentioned SPA as another alternative,  
as the technology doesn't mater much. The point is that adding  
obscurity ON TOP of solid security doesn't *diminish* said security.  
That's all. Simple point. Nothing fancy. Basic stuff.

(or at least I thought so)

> Applied Cryptography by Bruce Schneier:
> "If I take a letter, lock it in a safe, hide the safe somewhere in New
> York, then tell you to read the letter, that's not security. Thats
> obscurity. On the other hand, if I take a letter and lock it in a
> safe, and then give you the safe along with the design specifications
> of the safe and a hundred identical safes with their combinations
> so that you and the worlds best safecrackers can study the locking
> mechanism - and you still can't open the safe and read the letter -
> thats security."

Interesting, well what if you let people crack on our "safe" all day  
long (e.g. your SSH or VPN software) through OTHER PEOPLE'S SYSTEMS,  
but you tuck YOURS behind a firewall that only your users can get  
through?

So in other words, you get the benefit of scrutiny by using well- 
tested systems, but you don't have the downside of wide-open  
exposure. Surely with all your education and credentials you can see  
that this is a positive thing.

If I'm wrong here please show me how...

--
Daniel Miessler
E: daniel () dmiessler com
W: http://dmiessler.com
G: 0xDA6D50EAC

Attachment: PGP.sig
Description: This is a digitally signed message part