Do not deselect applications if the application was not found

[sosthene-nitrokey]

From NIST SP 800-73-4:

If the currently selected application is the PIV Card Application when the SELECT command is given
and the AID in the data field of the SELECT command is an invalid AID not supported by the ICC, then
the PIV Card Application shall remain the currently selected application and all PIV Card Application
security status indicators shall remain unchanged.

[dengert]

Thanks.

The section of 800-74-4 lists 4 cases. Since these cards can also have an openpgp applet,
if openpgp AID was received, 800-73-4 would require the PIV to lose login state, but that makes it harder
if user wants to have one application using PIV and another with OpenPGP without having to re enter PIN or use a cached pin.
If user only wants to use one of the applet, Is there a way to disable one or more of the applets so only one is active, to avoid this situation? (Yubikey can do that.)

I am not familiar with rust, but does the change return "not found" for an unknown AID? 6A 82

[sosthene-nitrokey]

if openpgp AID was received, 800-73-4 would require the PIV to lose login state,

From what I understand the previous behaviour was already correct for the case of selecting an existing AID.

If user only wants to use one of the applet, Is there a way to disable one or more of the applets so only one is active, to avoid this situation

Yes: nitropy nk3 set-config piv.disabled true and nitropy nk3 set-config opcard.disabled true for disabling PIV and OpenPGP respectively. To re-enable them, it's the same command with false instead of `true. We're working on exposing this config in the nitrokey app 2. It's also an issue with some software

I am not familiar with rust, but does the change return "not found" for an unknown AID? 6A 82

Yes, this does not change

[dengert]

Looks good. I will be looking for the next firmware update to try it.

[robin-nitrokey]

nit: Can we use Option::replace instead? IMHO easier to read.

Suggested change

	let old_aid = mem::replace(&mut self.current_aid, Some(aid));
	let old_aid = self.current_aid.replace(aid);

[sosthene-nitrokey]

Good catch. Probably should be a clippy lint.