FAQ: Re: key length

[NIIBE Yutaka]

On 2014-08-06 at 21:40 +0200, Bernd Eckenfels wrote:
> does the Card interface (and therefore the private key) in any way
> influence what signing/encryption keys are accepted or checked?

In the OpenPGP card protocol specification, there are "key
attributes".  When we execute the command line: 'gpg --card-status',
we see the information which is something like:

	Key attributes ...: 2048R 2048R 2048R

(It's the case for Gnuk Token.)  It means that it supports RSA
2048-bit for signing, decryption, and authentication.

Card implementation can support multiple key lengths.  There is a bit
defined in the extended card capabilities, and when it's set, host can
write the key attributes to change its key length (if such a key
length is supported by card).

> If I have a large key I dont care if others can only produce smaller
> keys, as long as they can communicate with me, right?

How/Where to put private keys is up to users, and majority of users
don't use OpenPGP cards (yet), but there are no problem for OpenPGP
communication itself.  Besides, OpenPGP card only handles private
keys; it helps use of OpenPGP, but it doesn't cover all of OpenPGP
operations.

It just means: You can't put your private keys to (some) OpenPGP card
implementation if its key size or algorithm is not supported by the
card/token.

When we consider a use-case of OpenPGP encryption to send a message to
multiple users, easiest target (to be attacked) could be weakest link.
So, it matters, smallest key size, weakest algorithm, or where private
keys are.
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