NAME

     fido_dev_largeblob_get, fido_dev_largeblob_set, fido_dev_largeblob_remove, fido_dev_largeblob_get_array,
     fido_dev_largeblob_set_array — FIDO2 large blob API

SYNOPSIS

     #include <fido.h>

     int
     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);

     int
     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);

     int
     fido_dev_largeblob_remove(fido_dev_t *dev, const unsigned char *key_ptr, size_t key_len, const char *pin);

     int
     fido_dev_largeblob_get_array(fido_dev_t *dev, unsigned char **cbor_ptr, size_t *cbor_len);

     int
     fido_dev_largeblob_set_array(fido_dev_t *dev, const unsigned char *cbor_ptr, size_t cbor_len,
         const char *pin);

DESCRIPTION

     The “largeBlobs” API of libfido2 allows binary blobs residing on a CTAP 2.1 authenticator to be read,
     written, and inspected.  “largeBlobs” is a CTAP 2.1 extension.

     “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.

     Retrieval of a credential's encryption key is possible during enrollment with fido_cred_set_extensions(3)
     and fido_cred_largeblob_key_ptr(3), during assertion with fido_assert_set_extensions(3) and
     fido_assert_largeblob_key_ptr(3), or, in the case of a resident credential, via libfido2's credential
     management API.

     The “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 “largeBlobs” extension, please refer to the
     CTAP 2.1 spec.

     The fido_dev_largeblob_get() function retrieves the authenticator's “largeBlobs” CBOR array and, on
     success, returns the first blob (iterating from array index zero) that can be decrypted by key_ptr, where
     key_ptr points to key_len bytes.  On success, fido_dev_largeblob_get() sets blob_ptr to the body of the
     decrypted blob, and blob_len to the length of the decrypted blob in bytes.  It is the caller's
     responsibility to free blob_ptr.

     The fido_dev_largeblob_set() function uses key_ptr to encrypt blob_ptr and inserts the result in the
     authenticator's “largeBlobs” CBOR array.  Insertion happens at the end of the array if no existing element
     can be decrypted by key_ptr, or at the position of the first element (iterating from array index zero) that
     can be decrypted by key_ptr.  key_len holds the length of key_ptr in bytes, and blob_len the length of
     blob_ptr in bytes.  A pin or equivalent user-verification gesture is required.

     The fido_dev_largeblob_remove() function retrieves the authenticator's “largeBlobs” CBOR array and, on
     success, drops the first blob (iterating from array index zero) that can be decrypted by key_ptr, where
     key_ptr points to key_len bytes.  A pin or equivalent user-verification gesture is required.

     The fido_dev_largeblob_get_array() function retrieves the authenticator's “largeBlobs” CBOR array and, on
     success, sets cbor_ptr to the body of the CBOR array, and cbor_len to its corresponding length in bytes.
     It is the caller's responsibility to free cbor_ptr.

     Finally, the fido_dev_largeblob_set_array() function sets the authenticator's “largeBlobs” CBOR array to
     the data pointed to by cbor_ptr, where cbor_ptr points to cbor_len bytes.  A pin or equivalent user-
     verification gesture is required.

RETURN VALUES

     The functions fido_dev_largeblob_set(), fido_dev_largeblob_get(), fido_dev_largeblob_remove(),
     fido_dev_largeblob_get_array(), and fido_dev_largeblob_set_array() return FIDO_OK on success.  On error, an
     error code defined in <fido/err.h> is returned.

SEE ALSO

     fido_assert_largeblob_key_len(3), fido_assert_largeblob_key_ptr(3), fido_assert_set_extensions(3),
     fido_cred_largeblob_key_len(3), fido_cred_largeblob_key_ptr(3), fido_cred_set_extensions(3),
     fido_credman_get_dev_rk(3), fido_credman_get_dev_rp(3), fido_dev_get_assert(3), fido_dev_make_cred(3)

CAVEATS

     The “largeBlobs” extension is not meant to be used to store sensitive data.  When retrieved, a credential's
     “largeBlobs” encryption key is transmitted in the clear, and an authenticator's “largeBlobs” CBOR array can
     be read without user interaction or verification.