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Key practice

 

 

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david.russell.scotland at gmail

Nov 14, 2009, 11:58 AM

Post #1 of 5 (765 views)
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Key practice

I've just bought a netbook with Ubuntu preinstalled, and since
integrating with GnuPG is much easier than it is on Windows I thought it
would be a good idea to start using it properly. However I don't know
what the 'best practice' is with regards to keypairs and so on. I've
read in a couple of places that it's a good idea to have a non-expiring
'master key', which is only used to sign (time-expiring) subkeys that
one then actually uses for signing and encrypting purposes. The problem
is that I'm not particularly hot on cryptography, so it has all combined
to pass far over my head!

Could some kind soul please explain, in layman's terms, what I should
generate and how I should use it? I'm on Ubuntu 8.04 so it's GnuPG 1.4.6
(not the newer version which defaults to RSA - I've read enough FAQs to
establish that DSA is a Bad Thing) if that matters.

Thanks

David Russell

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rjh at sixdemonbag

Nov 14, 2009, 3:43 PM

Post #2 of 5 (705 views)
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Re: Key practice [In reply to]

David Alexander Russell wrote:
> However I don't know what the 'best practice' is with regards to
> keypairs and so on.

GnuPG best practices, in a single sentence: "Unless you know what
you're doing and why, stick with the defaults."

This one sentence is useful for about 95% of new users' questions.
GnuPG is meant to be secure by default: you don't need to know a ton of
niggling little details just to use it safely.

> The problem is that I'm not particularly hot on cryptography, so it
> has all combined to pass far over my head!

You're in good company. :) People who write these sorts of articles
mean well, but it's very hard to figure out which authors actually know
what they're talking about and which are just talking a good game. On
top of that, even if you find an article written by someone who knows
the subject, the author's recommendations might not make sense in your
particular environment.

> Could some kind soul please explain, in layman's terms, what I should
> generate and how I should use it? I'm on Ubuntu 8.04 so it's GnuPG
> 1.4.6 (not the newer version which defaults to RSA - I've read enough
> FAQs to establish that DSA is a Bad Thing) if that matters.

DSA is not a Bad Thing. Whoever it was who told you this did you a
disservice. If you'd like to tell us what you've heard about DSA, we
would be happy to correct the misinformation you were given.

My suggestion is to "gpg --gen-key". At each step of the way, if you
ever don't know what to do, just hit RETURN and go on. GnuPG will
produce a high-quality keypair for you.


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david.russell.scotland at gmail

Nov 14, 2009, 3:51 PM

Post #3 of 5 (710 views)
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Re: Key practice [In reply to]

Robert J. Hansen wrote:
> DSA is not a Bad Thing. Whoever it was who told you this did you a
> disservice. If you'd like to tell us what you've heard about DSA, we
> would be happy to correct the misinformation you were given.
>
> My suggestion is to "gpg --gen-key". At each step of the way, if you
> ever don't know what to do, just hit RETURN and go on. GnuPG will
> produce a high-quality keypair for you.
>
>
Robert,

Essentially what I read was that the default 1024-bit DSA key isn't
strong enough, due to some flaw in SHA-1 which is the hash used for that
size of DSA (that's as much detail as I absorbed I'm afraid) - the main
link I have is the Debian website
http://www.debian-administration.org/users/dkg/weblog/48 . It was my
understanding that in the latest version of GnuPG, 1.4.10, the default
had been changed to 2048-bit RSA for precisely this reason.

Thanks

David R

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rjh at sixdemonbag

Nov 14, 2009, 5:06 PM

Post #4 of 5 (707 views)
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Re: Key practice [In reply to]

David Alexander Russell wrote:
> Essentially what I read was that the default 1024-bit DSA key isn't
> strong enough, due to some flaw in SHA-1 which is the hash used for that
> size of DSA (that's as much detail as I absorbed I'm afraid)

Don't believe the hype.

I don't like DSA-1024, for a lot of reasons similar to the ones in the
website you linked. However, there's a big difference between saying "I
don't like DSA-1024," and "DSA-1024 is insecure and shouldn't be used."

At present, it appears that breaking DSA-1024 is within the realm of
plausibility for ridiculously well-equipped adversaries who are willing
to spend astronomically absurd sums on breaking your key. Some people
think this means "DSA-1024 is broken, don't use it." This seems to be
pretty ignorant of history.

During the Cold War, the NSA spent absurd amounts of money designing
beautiful, elegant ciphers, and training very skilled cipher clerks.
The KGB spent small amounts of money on beautiful, elegant women and
sending them to these lonely, far-from-home cipher clerks. You can
figure out who was in the habit of winning those games of Spy-Vs.-Spy.

The moral of the story: no one with two brain cells to rub together is
going to attack DSA-1024 cryptanalytically. Not now, and not for the
reasonable future. It's going to be much, much faster and cheaper to
use other kinds of attacks, attacks which are just as useful against
RSA-4096 as DSA-1024.

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rjh at sixdemonbag

Nov 14, 2009, 5:13 PM

Post #5 of 5 (705 views)
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Re: Key practice [In reply to]

Also --

Keep in mind that I am not criticizing that weblog entry. I am only
saying, "don't believe the hype." Much of what it says is accurate: it
is a good idea to migrate towards better digest algorithms. Just don't
believe anyone who tells you that DSA-1024 is insecure: it isn't.

That said, you can migrate to a different digest algorithm quite easily.
Add these two lines to your gpg.conf file:



enable-dsa2
personal-digest-preferences SHA256 RIPEMD160 SHA1



Ta-da! Simple. :)


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