[kwlug-disc] Using 4096-bit RSA for keysigning party (NOT defaults)

Denver Gingerich denver at ossguy.com
Sat Sep 11 17:36:17 EDT 2010

On Sat, Sep 11, 2010 at 5:14 PM, Chris Frey <cdfrey at foursquare.net> wrote:
> On Sat, Sep 11, 2010 at 01:34:21PM -0400, Denver Gingerich wrote:
>> Unfortunately, they might not be. ?On most distros released before
>> about May 2009 (and probably more), the default GnuPG settings will
>> give you a 1024-bit DSA key, which is quite vulnerable to attacks due
>> to its reliance on SHA-1:
> As another person in private has mentioned, I think considering
> SHA-1 to be "quite vulnerable" to be overstating it.  When I first
> heard of the SHA-1 issue, I was concerned too, but as far as I know
> there have been no actual collisions found in the wild.  It has just
> been proven that collisions will be easier to find than first thought.

Right.  This was mentioned to me in a separate conversation as well.
So we probably don't need to go avoid it like a plague, but it's good
to know about.

>> http://www.debian-administration.org/users/dkg/weblog/48
>> As recommended in the above article, users should select RSA and I
>> would personally recommend using the maximum key size of 4096 bits.
>> So please do NOT use the defaults and instead choose 4096-bit RSA.
>> This will give us a much stronger web of trust.
> I would encourage anyone to use a larger key size if they choose.
> The first key in the pair is used for signing, while the second one is
> used for encryption.  The web of trust relies on the signing key.
> Unfortunately, some versions of GPG don't make it easy to create a
> signing key larger than 1024 bits.
> For example, if you are using gpg 1.4.9, the defaults give you a 1024 bit
> signing key and a 2048 bit encryption key.  I don't seen an option to
> create an RSA/RSA pair, either.   On 1.4.10, the bit sizes seem to be
> the same for both, defaulting to RSA for both keys, with 2048 bits.
> And practically speaking, 2048 bits is pretty danged good. :-)

Yes, I do recall that it was challenging to create an RSA/RSA version
in prior versions of GnuPG, but 1.4.10 (in Ubuntu 10.04) seems to work
pretty well.

>> > ? ? ?gpg --fingerprint dc6371d5
>> > ? ? ?pub 1024D/DC6371D5 2006-12-02 [expires: 2011-12-01]
>> > ? ? ?Key fingerprint = 7D71 47F2 3F61 B0E1 5F3C 68A4 819A 39D8 DC63
>> > 71D5
>> > ? ? ?uid Chris Frey (cube)
>> > ? ? ?sub 4096g/C2855553 2006-12-02 [expires: 2011-12-01]
>> I hate to break it to Chris, but his key is one of the potentially
>> vulnerable. ?"pub 1024D" means 1024-bit DSA. ?I would especially
>> recommend that Chris generate a new key before the meeting, being the
>> keymaster and all.
> I've had this key for a while now, so it might be worth changing anyhow.
> I don't think it is worth a panic yet.  It is good you brought this up,
> though, because my next key, whenever I do create it, should have
> a larger key. :-)
> Thanks for the heads-up!

You're welcome.  Thanks very much for putting together the keysigning event.


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