Add support for the largeBlobKeys extension and the largeBlobs command #41

robin-nitrokey · 2023-11-21T11:55:22Z

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

This patch updates the ctap-types dependency to pull in support for the largeBlobKey extension and the largeBlobs command.

This patch adds support for the largeBlobKey extension to the get_info command. It also adds a config entry to be able to enable or disable the extension.

This patch adds support for the largeBlobKey extension to make_credential. This means that we have to generate a 32-bit key and store it together with the credential if requested by the platform.

This patch adds support for the largeBlobKey extension to get_assertion. This means that we have to return the key stored together with the credential if it is present and requested by the platform.

This patch updates fido-authenticator to add support for the largeBlobKey extension and the largeBlobs command. See the fido-authenticator PR for more information: Nitrokey/fido-authenticator#41

robin-nitrokey · 2023-11-21T13:21:21Z

For testing, use these PRs:

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 · 2023-11-21T13:14:52Z

src/ctap2/large_blobs.rs

+trait Storage<C>: Sized {
+    fn read(client: &mut C, location: Location, offset: usize, length: usize) -> Result<Chunk>;
+
+    fn start_write(client: &mut C, offset: usize, expected_length: usize) -> Result<Self>;
+
+    fn extend_buffer(&mut self, client: &mut C, data: &[u8]) -> Result<usize>;
+
+    fn validate_checksum(&mut self, client: &mut C) -> bool;
+
+    fn commit(&mut self, client: &mut C, location: Location) -> Result<()>;
+}


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.

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.

src/ctap2.rs

robin-nitrokey · 2023-11-21T14:41:31Z

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 · 2023-11-22T23:37:21Z

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

Requirements:

robin-nitrokey · 2023-11-23T17:46:49Z

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.

This patch implements the largeBlobs command for reading and writing the large-blob array. Currently, the maximum size of the total array with metadata is 1024 bytes because it has to fit in a Trussed message. The storage location can be configured by the runner.

This patch updates the credential management implementation to include the largeBlobKey if present.

If a resident credential is passed in the allowlist, we don’t deserialize the full credential. This means that we previously did not have access to the largeBlobKey in that case. Therefore, this patch adds the largeBlobKey to the StrippedCredential so that we can always access it. The downside is that this inceases the size of the credential ID. So a better alternative would be to load the full credential from the filesystem instead.

This patch updates fido-authenticator to add support for the largeBlobKey extension and the largeBlobs command in the test configuration over USB. See the fido-authenticator PR for more information: Nitrokey/fido-authenticator#41

robin-nitrokey added 4 commits November 21, 2023 12:45

Update ctap-types

6800f49

This patch updates the ctap-types dependency to pull in support for the largeBlobKey extension and the largeBlobs command.

Add largeBlobKey support to get_info

71d14ff

This patch adds support for the largeBlobKey extension to the get_info command. It also adds a config entry to be able to enable or disable the extension.

Add largeBlobKey support to make_credential

c43da04

This patch adds support for the largeBlobKey extension to make_credential. This means that we have to generate a 32-bit key and store it together with the credential if requested by the platform.

Add largeBlobKey support to get_assertion

48d66c0

This patch adds support for the largeBlobKey extension to get_assertion. This means that we have to return the key stored together with the credential if it is present and requested by the platform.

robin-nitrokey requested review from daringer and sosthene-nitrokey November 21, 2023 11:58

robin-nitrokey mentioned this pull request Nov 21, 2023

fido-authenticator: Add large blobs support Nitrokey/nitrokey-3-firmware#385

Merged

sosthene-nitrokey reviewed Nov 21, 2023

View reviewed changes

sosthene-nitrokey approved these changes Nov 27, 2023

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robin-nitrokey added 3 commits November 28, 2023 10:46

Add largeBlobKey to credential management

aa9bb35

This patch updates the credential management implementation to include the largeBlobKey if present.

robin-nitrokey force-pushed the large-blobs branch from 681d730 to 019a5d1 Compare November 28, 2023 09:47

robin-nitrokey merged commit a4fff2f into main Nov 28, 2023

robin-nitrokey deleted the large-blobs branch November 28, 2023 09:47

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Add support for the largeBlobKeys extension and the largeBlobs command #41

Add support for the largeBlobKeys extension and the largeBlobs command #41

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Add support for the largeBlobKeys extension and the largeBlobs command #41

Add support for the largeBlobKeys extension and the largeBlobs command #41

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