.\" Copyright (c) 2020 Yubico AB. All rights reserved.
.\" Use of this source code is governed by a BSD-style
.\" license that can be found in the LICENSE file.
.\"
.Dd $Mdocdate: August 31 2022 $
.Dt FIDO_LARGEBLOB_GET 3
.Os
.Sh NAME
.Nm fido_dev_largeblob_get ,
.Nm fido_dev_largeblob_set ,
.Nm fido_dev_largeblob_remove ,
.Nm fido_dev_largeblob_get_array ,
.Nm fido_dev_largeblob_set_array
.Nd FIDO2 large blob API
.Sh SYNOPSIS
.In fido.h
.Ft int
.Fn fido_dev_largeblob_get "fido_dev_t *dev" "const unsigned char *key_ptr" "size_t key_len" "unsigned char **blob_ptr" "size_t *blob_len"
.Ft int
.Fn fido_dev_largeblob_set "fido_dev_t *dev" "const unsigned char *key_ptr" "size_t key_len" "const unsigned char *blob_ptr" "size_t blob_len" "const char *pin"
.Ft int
.Fn fido_dev_largeblob_remove "fido_dev_t *dev" "const unsigned char *key_ptr" "size_t key_len" "const char *pin"
.Ft int
.Fn fido_dev_largeblob_get_array "fido_dev_t *dev" "unsigned char **cbor_ptr" "size_t *cbor_len"
.Ft int
.Fn fido_dev_largeblob_set_array "fido_dev_t *dev" "const unsigned char *cbor_ptr" "size_t cbor_len" "const char *pin"
.Sh DESCRIPTION
The
.Dq largeBlobs
API of
.Em libfido2
allows binary blobs residing on a CTAP 2.1 authenticator to be
read, written, and inspected.
.Dq largeBlobs
is a CTAP 2.1 extension.
.Pp
.Dq largeBlobs
are stored as elements of a CBOR array.
Confidentiality is ensured by encrypting each element with a
distinct, credential-bound 256-bit AES-GCM key.
The array is otherwise shared between different credentials and
FIDO2 relying parties.
.Pp
Retrieval of a credential's encryption key is possible during
enrollment with
.Xr fido_cred_set_extensions 3
and
.Xr fido_cred_largeblob_key_ptr 3 ,
during assertion with
.Xr fido_assert_set_extensions 3
and
.Xr fido_assert_largeblob_key_ptr 3 ,
or, in the case of a resident credential, via
.Em libfido2's
credential management API.
.Pp
The
.Dq largeBlobs
CBOR array is opaque to the authenticator.
Management of the array is left at the discretion of FIDO2 clients.
For further details on CTAP 2.1's
.Dq largeBlobs
extension, please refer to the CTAP 2.1 spec.
.Pp
The
.Fn fido_dev_largeblob_get
function retrieves the authenticator's
.Dq largeBlobs
CBOR array and, on success, returns the first blob
.Pq iterating from array index zero
that can be
decrypted by 
.Fa key_ptr ,
where
.Fa key_ptr
points to
.Fa key_len
bytes.
On success,
.Fn fido_dev_largeblob_get
sets
.Fa blob_ptr
to the body of the decrypted blob, and
.Fa blob_len
to the length of the decrypted blob in bytes.
It is the caller's responsibility to free
.Fa blob_ptr .
.Pp
The
.Fn fido_dev_largeblob_set
function uses
.Fa key_ptr
to encrypt
.Fa blob_ptr
and inserts the result in the authenticator's
.Dq largeBlobs
CBOR array.
Insertion happens at the end of the array if no existing element
can be decrypted by
.Fa key_ptr ,
or at the position of the first element
.Pq iterating from array index zero
that can be decrypted by
.Fa key_ptr .
.Fa key_len
holds the length of
.Fa key_ptr
in bytes, and
.Fa blob_len
the length of
.Fa blob_ptr
in bytes.
A
.Fa pin
or equivalent user-verification gesture is required.
.Pp
The
.Fn fido_dev_largeblob_remove
function retrieves the authenticator's
.Dq largeBlobs
CBOR array and, on success, drops the first blob
.Pq iterating from array index zero
that can be decrypted by
.Fa key_ptr ,
where
.Fa key_ptr
points to
.Fa key_len
bytes.
A
.Fa pin
or equivalent user-verification gesture is required.
.Pp
The
.Fn fido_dev_largeblob_get_array
function retrieves the authenticator's
.Dq largeBlobs
CBOR array and, on success,
sets
.Fa cbor_ptr
to the body of the CBOR array, and
.Fa cbor_len
to its corresponding length in bytes.
It is the caller's responsibility to free
.Fa cbor_ptr .
.Pp
Finally, the
.Fn fido_dev_largeblob_set_array
function sets the authenticator's
.Dq largeBlobs
CBOR array to the data pointed to by
.Fa cbor_ptr ,
where
.Fa cbor_ptr
points to
.Fa cbor_len
bytes.
A
.Fa pin
or equivalent user-verification gesture is required.
.Sh RETURN VALUES
The functions
.Fn fido_dev_largeblob_set ,
.Fn fido_dev_largeblob_get ,
.Fn fido_dev_largeblob_remove ,
.Fn fido_dev_largeblob_get_array ,
and
.Fn fido_dev_largeblob_set_array
return
.Dv FIDO_OK
on success.
On error, an error code defined in
.In fido/err.h
is returned.
.Sh SEE ALSO
.Xr fido_assert_largeblob_key_len 3 ,
.Xr fido_assert_largeblob_key_ptr 3 ,
.Xr fido_assert_set_extensions 3 ,
.Xr fido_cred_largeblob_key_len 3 ,
.Xr fido_cred_largeblob_key_ptr 3 ,
.Xr fido_cred_set_extensions 3 ,
.Xr fido_credman_get_dev_rk 3 ,
.Xr fido_credman_get_dev_rp 3 ,
.Xr fido_dev_get_assert 3 ,
.Xr fido_dev_make_cred 3
.Sh CAVEATS
The
.Dq largeBlobs
extension is not meant to be used to store sensitive data.
When retrieved, a credential's
.Dq largeBlobs
encryption key is transmitted in the clear, and an authenticator's
.Dq largeBlobs
CBOR array can be read without user interaction or verification.