1 |
On Thursday 12 Jan 2012 14:06:26 Alan McKinnon wrote: |
2 |
> On Thu, 12 Jan 2012 08:58:02 -0500 |
3 |
> |
4 |
> Tanstaafl <tanstaafl@×××××××××××.org> wrote: |
5 |
|
6 |
> > But I still disagree. Would you also classify 'changing the locks on |
7 |
> > your house' as security through obscurity? Because changing the |
8 |
> > character set in PWM is just like changing the lock on a door... |
9 |
|
10 |
Changing locks (with the same number and quality of locks) is as good as not |
11 |
changing locks at all - unless some burglar happens to be half way through |
12 |
unpicking the current door lock mechanism. |
13 |
|
14 |
Changing locks with a higher quality lock (i.e. one with more levers in it) is |
15 |
like increasing the number of characters in your password. If the new levers |
16 |
are from a different 'character set' (different design class/pattern of lock |
17 |
levers) then it would be more difficult for the burglar to guess what these |
18 |
levers might look like (sort of adding more entropy - the levers would appear |
19 |
to be more random) and he'll have to try all combinations of levers. The |
20 |
addition of levers (or locks of the same design) is called protection through |
21 |
redundancy. If one falls in the hands of a skilled cracker the second should |
22 |
present the *same* level of protection. So we are essentially increasing the |
23 |
time it will take to crack the locks and thankfully the burglar's time is a |
24 |
limited resource. |
25 |
|
26 |
If on the other hand we add an entirely different *means* of protection - e.g. |
27 |
a guard dog, then we are increasing the level of protection not through |
28 |
redundancy, but through diversity. This means that systemic weaknesses of |
29 |
door lock lever design can be compensated for in our door protection system. |
30 |
Systemic weaknesses are important because they can be guessed (like which side |
31 |
of the qwerty keyboard the uber-geek typed his password) and so give the |
32 |
burglar a smaller set of solutions to try. There's no point in a burglar |
33 |
trying to guess how many or what type of levers a guard dog has. Indeed, his |
34 |
skills and resources at picking locks is now irrelevant - he's got to be a |
35 |
skilled dog whisperer too! |
36 |
|
37 |
We could think of the change of the port of sshd like adding redundancy |
38 |
protection, but we don't really. In reality we are adding (a very low degree |
39 |
of) diversity. This is because we're hiding the door of our hypothetical |
40 |
house. However, in doing so we're giving away the wrong signal to a non- |
41 |
opportunistic burglar. Since every other house door in the street is not |
42 |
hidden we are subliminally telling the burglars: "Hey! We have something |
43 |
worth hiding in this house." Then they'll set off picking the locks of our |
44 |
door, instead of the doors down the road. From a probability perspective |
45 |
though we are better off changing the sshd port, because all the opportunistic |
46 |
(botnet) burglars who just check port 22 will miss our door and never bother |
47 |
us. |
48 |
|
49 |
A strong security system will have both redundancy and diversity in its |
50 |
design. As an example an IPSec VPN set up which uses both SSL Certificates and |
51 |
XAuth with a long and random passphrase does just that. |
52 |
-- |
53 |
Regards, |
54 |
Mick |