FIDO_ASSERT_SET_AUTHDATA(3)

NAME
fido_assert_set_authdata, fido_assert_set_authdata_raw, fido_assert_set_clientdata, fido_assert_set_clientdata_hash, fido_assert_set_count, fido_assert_set_extensions, fido_assert_set_hmac_salt, fido_assert_set_hmac_secret, fido_assert_set_up, fido_assert_set_uv, fido_assert_set_rp, fido_assert_set_sig - set parameters of a FIDO 2 assertion

SYNOPSIS
#include <fido.h>

    typedef enum {
    	FIDO_OPT_OMIT = 0, /* use authenticator's default */
    	FIDO_OPT_FALSE,    /* explicitly set option to false */
            FIDO_OPT_TRUE,     /* explicitly set option to true */
    } fido_opt_t;

int
fido_assert_set_authdata(fido_assert_t *assert, size_t idx, const unsigned char *ptr, size_t len);
int
fido_assert_set_authdata_raw(fido_assert_t *assert, size_t idx, const unsigned char *ptr, size_t len);
int
fido_assert_set_clientdata(fido_assert_t *assert, const unsigned char *ptr, size_t len);
int
fido_assert_set_clientdata_hash(fido_assert_t *assert, const unsigned char *ptr, size_t len);
int
fido_assert_set_count(fido_assert_t *assert, size_t n);
int
fido_assert_set_extensions(fido_assert_t *assert, int flags);
int
fido_assert_set_hmac_salt(fido_assert_t *assert, const unsigned char *ptr, size_t len);
int
fido_assert_set_hmac_secret(fido_assert_t *assert, const unsigned char *ptr, size_t len);
int
fido_assert_set_up(fido_assert_t *assert, fido_opt_t up);
int
fido_assert_set_uv(fido_assert_t *assert, fido_opt_t uv);
int
fido_assert_set_rp(fido_assert_t *assert, const char *id);
int
fido_assert_set_sig(fido_assert_t *assert, size_t idx, const unsigned char *ptr, size_t len);

DESCRIPTION
The fido_assert_set_authdata set of functions define the various parameters of a FIDO 2 assertion, allowing a fido_assert_t type to be prepared for a subsequent call to fido_dev_get_assert(3) or fido_assert_verify(3). For the complete specification of a FIDO 2 assertion and the format of its constituent parts, please refer to the Web Authentication (webauthn) standard.

The fido_assert_set_count() function sets the number of assertion statements in assert to n.

The fido_assert_set_authdata() and fido_assert_set_sig() functions set the authenticator data and signature parts of the statement with index idx of assert to ptr, where ptr points to len bytes. A copy of ptr is made, and no references to the passed pointer are kept. Please note that the first assertion statement of assert has an idx of 0. The authenticator data passed to fido_assert_set_authdata() must be a CBOR-encoded byte string, as obtained from fido_assert_authdata_ptr(). Alternatively, a raw binary blob may be passed to fido_assert_set_authdata_raw().

The fido_assert_set_clientdata_hash(), fido_assert_set_hmac_salt(), and fido_assert_set_hmac_secret() functions set the client data hash and hmac-salt parts of assert to ptr, where ptr points to len bytes. A copy of ptr is made, and no references to the passed pointer are kept.

The fido_assert_set_clientdata() function allows an application to set the client data hash of assert by specifying the assertion's unhashed client data. This is required by Windows Hello, which calculates the client data hash internally. For compatibility with Windows Hello, applications should use fido_assert_set_clientdata() instead of fido_assert_set_clientdata_hash().

The fido_assert_set_rp() function sets the relying party id of assert, where id is a NUL-terminated UTF-8 string. The content of id is copied, and no references to the passed pointer are kept.

The fido_assert_set_extensions() function sets the extensions of assert to the bitmask flags. At the moment, only the FIDO_EXT_CRED_BLOB, FIDO_EXT_HMAC_SECRET, and FIDO_EXT_LARGEBLOB_KEY extensions are supported. If flags is zero, the extensions of assert are cleared.

The fido_assert_set_up() and fido_assert_set_uv() functions set the up (user presence) and uv (user verification) attributes of assert. Both are FIDO_OPT_OMIT by default, allowing the authenticator to use its default settings.

Use of the fido_assert_set_authdata set of functions may happen in two distinct situations: when asking a FIDO device to produce a series of assertion statements, prior to fido_dev_get_assert(3) (i.e, in the context of a FIDO client), or when verifying assertion statements using fido_assert_verify(3) (i.e, in the context of a FIDO server).

For a complete description of the generation of a FIDO 2 assertion and its verification, please refer to the FIDO 2 specification. An example of how to use the fido_assert_set_authdata set of functions can be found in the examples/assert.c file shipped with libfido2.

fido_assert_set_hmac_secret() is not normally useful in a FIDO client or server — it is provided to enable testing other functionality that relies on retrieving the HMAC secret from an assertion obtained from an authenticator.

RETURN VALUES
The fido_assert_set_authdata functions return FIDO_OK on success. The error codes returned by the fido_assert_set_authdata set of functions are defined in <fido/err.h>.

SEE ALSO
fido_assert_allow_cred(3), fido_assert_verify(3), fido_dev_get_assert(3)