Set reasonable maximum numbers for persistent data

[daringer]

In particular + my suggestions:

• maximum number of FIDO2 RKs (20)
• maximum number of secrets-app entries (100)

These are very conservative entries, which shall be higher in the future once we can promise better performance for e.g., secrets-app entries.

Best would be that these are passed as options into the app(s) instead of hard-coding them inside the apps - so later we can configure them through the admin-app

[robin-nitrokey]

For FIDO2, there are two approaches for implementing this:

1. Checking the number of existing resident keys when creating a new one. Downside: This is potentially a slow operation if there are many keys.
2. Keeping track of the number of resident keys in the state. Downside: Requires initial setup for old states, more error-prone because it is easier to miss an execution path, additional writes on the filesystem, can be problematic if e. g. deleting a key is successful but writing the state fails.

So to keep things simple, I’d prefer 1.

[szszszsz]

Why is there a limit on FIDO RKs? Is it due to listing performance?

[robin-nitrokey]

Yes. Currently, there is no limit, but we wanted to add one to make sure that the key does not become unusable due to the listing issue.

[robin-nitrokey]

Implemented for fido-authenticator in Nitrokey/fido-authenticator#12 and #239.