FIDO_LARGEBLOB_GET(3) | Library Functions Manual | FIDO_LARGEBLOB_GET(3)

# 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

library “libfido2” libcbor libcrypto libz
`#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.

July 9, 2025 | Debian