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FIDO_ASSERT_NEW(3) Library Functions Manual FIDO_ASSERT_NEW(3)


fido_assert_new, fido_assert_free, fido_assert_count, fido_assert_rp_id, fido_assert_user_display_name, fido_assert_user_icon, fido_assert_user_name, fido_assert_authdata_ptr, fido_assert_blob_ptr, fido_assert_clientdata_hash_ptr, fido_assert_hmac_secret_ptr, fido_assert_largeblob_key_ptr, fido_assert_user_id_ptr, fido_assert_sig_ptr, fido_assert_id_ptr, fido_assert_authdata_len, fido_assert_blob_len, fido_assert_clientdata_hash_len, fido_assert_hmac_secret_len, fido_assert_largeblob_key_len, fido_assert_user_id_len, fido_assert_sig_len, fido_assert_id_len, fido_assert_sigcount, fido_assert_flagsFIDO2 assertion API


#include <fido.h>

fido_assert_t *

fido_assert_free(fido_assert_t **assert_p);

fido_assert_count(const fido_assert_t *assert);

const char *
fido_assert_rp_id(const fido_assert_t *assert);

const char *
fido_assert_user_display_name(const fido_assert_t *assert, size_t idx);

const char *
fido_assert_user_icon(const fido_assert_t *assert, size_t idx);

const char *
fido_assert_user_name(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_authdata_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_clientdata_hash_ptr(const fido_assert_t *assert);

const unsigned char *
fido_assert_blob_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_hmac_secret_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_largeblob_key_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_user_id_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_sig_ptr(const fido_assert_t *assert, size_t idx);

const unsigned char *
fido_assert_id_ptr(const fido_assert_t *assert, size_t idx);

fido_assert_authdata_len(const fido_assert_t *assert, size_t idx);

fido_assert_clientdata_hash_len(const fido_assert_t *assert);

fido_assert_blob_len(const fido_assert_t *assert, size_t idx);

fido_assert_hmac_secret_len(const fido_assert_t *assert, size_t idx);

fido_assert_largeblob_key_len(const fido_assert_t *assert, size_t idx);

fido_assert_user_id_len(const fido_assert_t *assert, size_t idx);

fido_assert_sig_len(const fido_assert_t *assert, size_t idx);

fido_assert_id_len(const fido_assert_t *assert, size_t idx);

fido_assert_sigcount(const fido_assert_t *assert, size_t idx);

fido_assert_flags(const fido_assert_t *assert, size_t idx);


A FIDO2 assertion is a collection of statements, each statement a map between a challenge, a credential, a signature, and ancillary attributes. In , a FIDO2 assertion is abstracted by the fido_assert_t type. The functions described in this page allow a fido_assert_t type to be allocated, deallocated, and inspected. For other operations on fido_assert_t, please refer to fido_assert_set_authdata(3), fido_assert_allow_cred(3), fido_assert_verify(3), and fido_dev_get_assert(3).

The () function returns a pointer to a newly allocated, empty fido_assert_t type. If memory cannot be allocated, NULL is returned.

The () function releases the memory backing *assert_p, where *assert_p must have been previously allocated by fido_assert_new(). On return, *assert_p is set to NULL. Either assert_p or *assert_p may be NULL, in which case fido_assert_free() is a NOP.

The () function returns the number of statements in assert.

The () function returns a pointer to a NUL-terminated string holding the relying party ID of assert.

The (), (), and (), functions return pointers to the user display name, icon, and name attributes of statement idx in assert. If not NULL, the values returned by these functions point to NUL-terminated UTF-8 strings.

The (), (), (), (), (), (), and () functions return pointers to the CBOR-encoded authenticator data, client data hash, credential ID, user ID, signature, signature count, and authenticator data flags of statement idx in assert.

The () function returns a pointer to the hmac-secret attribute of statement idx in assert. The HMAC Secret Extension (hmac-secret) is a CTAP 2.0 extension. Note that the resulting hmac-secret varies according to whether user verification was performed by the authenticator.

The () and () functions return pointers to the “credBlob” and “largeBlobKey” attributes of statement idx in assert. Credential Blob (credBlob) and Large Blob Key (largeBlobKey) are CTAP 2.1 extensions.

The (), (), (), (), (), (), (), and () functions return the length of a given attribute.

Please note that the first statement in assert has an idx (index) value of 0.

The authenticator data and signature parts of an assertion statement are typically passed to a FIDO2 server for verification.


The authenticator data returned by fido_assert_authdata_ptr() is a CBOR-encoded byte string, as obtained from the authenticator.

The fido_assert_rp_id(), fido_assert_user_display_name(), fido_assert_user_icon(), fido_assert_user_name(), fido_assert_authdata_ptr(), fido_assert_clientdata_hash_ptr(), fido_assert_id_ptr(), fido_assert_user_id_ptr(), fido_assert_sig_ptr(), fido_assert_hmac_secret_ptr(), fido_assert_blob_ptr(), and fido_assert_largeblob_key_ptr() functions may return NULL if the respective field in assert is not set. If not NULL, returned pointers are guaranteed to exist until any API function that takes assert without the qualifier is invoked.


fido_assert_allow_cred(3), fido_assert_set_authdata(3), fido_assert_verify(3), fido_dev_get_assert(3), fido_dev_largeblob_get(3)

April 27, 2022 Debian