Add support for the largeBlobKeys extension and the largeBlobs command

[robin-nitrokey]

This patch implements the largeBlobKeys extension and the largeBlobs command. I tried to split it up into atomic commits to make it easier to review.

Limitations:

• Currently, this implementation uses the core Trussed API, so the size of the entire large-blob array is limited to 1024 bytes as it has to fit into a Trussed message. The next step would be to add a chunked feature that replaces the SimpleStorage in src/ctap2/large_blobs.rs with a ChunkedStorage that uses the streaming extension. The implementation uses the Storage trait to try to ensure that the implementations can be easily switched.
• The size of the response sent when reading the large-blob array depends on the maximum message size that the authenticator declares in the getInfo command. ctap-types has a REALISTIC_MAX_MESSAGE_SIZE constant (1200) and assumes that this is the maximum message size. But in fact, the max message size can be configured by the runner and is set to usbd_ctaphid::constants::MESSAGE_SIZE (3072). This does not matter at the moment because our large-blob array has at most 1024 bytes, so even the buffer based on the REALISTIC_MAX_MESSAGE_SIZE is large enough. Once we implement chunked storage, we need to add a feature flag to ctap-types to use the message size from usbd-ctaphid instead.

Open issues:

• Chrome shows write error even if write was successful – maybe because we return an empty map instead of an empty response.
  • fixed by: Replace empty maps with an empty response trussed-dev/ctap-types#24

Fixes: #38

[robin-nitrokey]

For testing, use these PRs:

• fido-authenticator: Add large blobs support nitrokey-3-firmware#385
• fido2: Add list-large-blobs command pynitrokey#473

You can use the following tools for testing:

• nitropy fido2 list-large-blobs lists the number of entries in the large blob array, the number of credentials with large blob key, and the entries of the large blob array with information about the corresponding credential (if found).
• https://webauthn-large-blob.glitch.me can be used to create credentials with a large-blob key and to write entries to the large-blob table (does not work in Firefox).
• Chrome key management (chrome://settings/securityKeys → Sign-in data) performs garbage collection, i. e. it deletes all entries in the large-blob table without a matching credential.
• Performing a reset with Chrome or nitropy should also clear the large-blob array.
• Enabling logging in the firmware should provide some insight into the executed operations and their results.

Note that there is an open issue with Chrome reporting an error for writes even if the write is successful.

[sosthene-nitrokey]

Not yet tested on hardware.

[sosthene-nitrokey]

Do we really need a dedicated trait for that?

Can't it be just a group of free-standing functions where the implementation is selected based on the feature-flag?

No need to block merging on that since the storage will be revisited soon.

[robin-nitrokey]

The idea was to have two implementations, one using the Trussed core API and one using the streaming extension. The trait should make sure that both are exchangeable.

[robin-nitrokey]

Indeed Chrome no longer shows an error if we send an empty response instead of a response with an empty map when writing the large-blob array: trussed-dev/ctap-types#24

[robin-nitrokey]

This branch adds support for the streaming API: https://github.com/Nitrokey/fido-authenticator/tree/large-blobs-chunked

Requirements:

• https://github.com/Nitrokey/ctap-types/tree/usbd-ctaphid
• https://github.com/Nitrokey/trussed/tree/rename

[robin-nitrokey]

I’ve updated Nitrokey/nitrokey-3-firmware#385 with a maximum size of 4096 bytes for the large-blob array. It mostly works fine in the usbip simulation, but sometimes after writing a large file, there seems to be an issue with the filesystem and subsequent larger writes fail.