FIDO_ASSERT_SET_AUTHDATA(3) | Library Functions Manual | 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`, `fido_assert_set_winhello_appid` — set parameters of a FIDO2 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, size_t idx, 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);

int
`fido_assert_set_winhello_appid`(fido_assert_t *assert, const char *id);

# DESCRIPTION

The `fido_assert_set_authdata` set of functions define the various parameters of a FIDO2 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 FIDO2 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`() function sets the client data hash 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_hmac_salt`() and `fido_assert_set_hmac_secret`() functions set the hmac-salt and hmac-secret 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 HMAC Secret (hmac-secret) Extension is a CTAP 2.0 extension. Note that the resulting hmac-secret varies according to whether user verification was performed by the authenticator. The `fido_assert_set_hmac_secret`() function is normally only useful when writing tests.

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.

The `fido_assert_set_winhello_appid`() function sets the U2F application id (“U2F AppID”) 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_winhello_appid`() function is a no-op unless assert is passed to fido_dev_get_assert(3) with a device dev on which fido_dev_is_winhello(3) holds true. In this case, libfido2 will instruct Windows Hello to try the assertion twice, first with the id passed to `fido_assert_set_rp`(), and a second time with the id passed to `fido_assert_set_winhello_appid`(). If the second assertion succeeds, fido_assert_rp_id(3) will point to the U2F AppID once fido_dev_get_assert(3) completes. This mechanism exists in Windows Hello to ensure U2F backwards compatibility without the application inadvertently prompting the user twice. Note that `fido_assert_set_winhello_appid`() is not needed on platforms offering CTAP primitives, since the authenticator can be silently probed for the existence of U2F credentials.

Use of the `fido_assert_set_authdata` set of functions may happen in two distinct situations: when asking a FIDO2 device to produce a series of assertion statements, prior to fido_dev_get_assert(3) (i.e, in the context of a FIDO2 client), or when verifying assertion statements using fido_assert_verify(3) (i.e, in the context of a FIDO2 server).

For a complete description of the generation of a FIDO2 assertion and its verification, please refer to the FIDO2 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.

# 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), fido_dev_is_winhello(3)

April 8, 2023 | Yubico AB