openpgp smartcard: ssh auth speed vs. RSA key size

[Thomas Jarosch]

Hello together,

here's an interesting observation on ssh auth speed
when using different key sizes on the openpgp smartcard:

RSA 2048 bit key: 0.7s
RSA 4096 bit key: 3.1s

Card used is an openpgp smartcard V3.3
with gnupg 2.2.4. The ssh key is accessed via gpg-agent.

We found this while creating our keys with 4096 bit and now reverted to 2048 
bit. It's secure enough and the speed hit is almost not noticeable.

The time was measured with:

	$ time ssh SERVERNAME /bin/true

Cheers,
Thomas