FIDO_CRED_SET_AUTHDATA(3) | Library Functions Manual | FIDO_CRED_SET_AUTHDATA(3)

# NAME

`fido_cred_set_authdata`, `fido_cred_set_authdata_raw`, `fido_cred_set_attstmt`, `fido_cred_set_x509`, `fido_cred_set_sig`, `fido_cred_set_id`, `fido_cred_set_clientdata`, `fido_cred_set_clientdata_hash`, `fido_cred_set_rp`, `fido_cred_set_user`, `fido_cred_set_extensions`, `fido_cred_set_blob`, `fido_cred_set_pin_minlen`, `fido_cred_set_prot`, `fido_cred_set_rk`, `fido_cred_set_uv`, `fido_cred_set_fmt`, `fido_cred_set_type` — set parameters of a FIDO2 credential

# SYNOPSIS

library “libfido2” libcbor libcrypto libz
`#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_cred_set_authdata`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_authdata_raw`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_attstmt`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_x509`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_sig`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_id`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_clientdata`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_clientdata_hash`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_rp`(fido_cred_t *cred, const char *id, const char *name);

int
`fido_cred_set_user`(fido_cred_t *cred, const unsigned char *user_id, size_t user_id_len, const char *name, const char *display_name, const char *icon);

int
`fido_cred_set_extensions`(fido_cred_t *cred, int flags);

int
`fido_cred_set_blob`(fido_cred_t *cred, const unsigned char *ptr, size_t len);

int
`fido_cred_set_pin_minlen`(fido_cred_t *cred, size_t len);

int
`fido_cred_set_prot`(fido_cred_t *cred, int prot);

int
`fido_cred_set_rk`(fido_cred_t *cred, fido_opt_t rk);

int
`fido_cred_set_uv`(fido_cred_t *cred, fido_opt_t uv);

int
`fido_cred_set_fmt`(fido_cred_t *cred, const char *ptr);

int
`fido_cred_set_type`(fido_cred_t *cred, int cose_alg);

# DESCRIPTION

The `fido_cred_set_authdata` set of functions define the various parameters of a FIDO2 credential, allowing a fido_cred_t type to be prepared for a subsequent call to fido_dev_make_cred(3) or fido_cred_verify(3). For the complete specification of a FIDO2 credential and the format of its constituent parts, please refer to the Web Authentication (webauthn) standard.

The `fido_cred_set_authdata`(), `fido_cred_set_attstmt`(), `fido_cred_set_x509`(), `fido_cred_set_sig`(), `fido_cred_set_id`(), and `fido_cred_set_clientdata_hash`() functions set the authenticator data, attestation statement, attestation certificate, attestation signature, id, and client data hash parts of cred to ptr, where ptr points to len bytes. A copy of ptr is made, and no references to the passed pointer are kept.

The authenticator data passed to `fido_cred_set_authdata`() must be a CBOR-encoded byte string, as obtained from `fido_cred_authdata_ptr`(). Alternatively, a raw binary blob may be passed to `fido_cred_set_authdata_raw`(). An application calling `fido_cred_set_authdata`() does not need to call `fido_cred_set_id`(). The latter is meant to be used in contexts where the credential's authenticator data is not available.

The attestation statement passed to `fido_cred_set_attstmt`() must be a CBOR-encoded map, as obtained from `fido_cred_attstmt_ptr`(). An application calling `fido_cred_set_attstmt`() does not need to call `fido_cred_set_x509`() or `fido_cred_set_sig`(). The latter two are meant to be used in contexts where the credential's complete attestation statement is not available or required.

The `fido_cred_set_clientdata`() function allows an application to set the client data hash of cred by specifying the credential'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_cred_set_clientdata`() instead of `fido_cred_set_clientdata_hash`().

The `fido_cred_set_rp`() function sets the relying party id and name parameters of cred, where id and name are NUL-terminated UTF-8 strings. The contents of id and name are copied, and no references to the passed pointers are kept.

The `fido_cred_set_user`() function sets the user attributes of cred, where user_id points to user_id_len bytes and name, display_name, and icon are NUL-terminated UTF-8 strings. The contents of user_id, name, display_name, and icon are copied, and no references to the passed pointers are kept. Previously set user attributes are flushed. The user_id, name, display_name, and icon parameters may be NULL.

The `fido_cred_set_extensions`() function sets the extensions of cred to the bitmask flags. At the moment, only the `FIDO_EXT_CRED_BLOB`, `FIDO_EXT_CRED_PROTECT`, `FIDO_EXT_HMAC_SECRET`, `FIDO_EXT_MINPINLEN`, and `FIDO_EXT_LARGEBLOB_KEY` extensions are supported. If flags is zero, the extensions of cred are cleared.

The `fido_cred_set_blob`() function sets the “credBlob” to be stored with cred to the data pointed to by ptr, which must be len bytes long.

The `fido_cred_set_pin_minlen`() function enables the CTAP 2.1 `FIDO_EXT_MINPINLEN` extension on cred and sets the expected minimum PIN length of cred to len, where len is greater than zero. If len is zero, the `FIDO_EXT_MINPINLEN` extension is disabled on cred.

The `fido_cred_set_prot`() function enables the CTAP 2.1 `FIDO_EXT_CRED_PROTECT` extension on cred and sets the protection of cred to the scalar prot. At the moment, only the `FIDO_CRED_PROT_UV_OPTIONAL`, `FIDO_CRED_PROT_UV_OPTIONAL_WITH_ID`, and `FIDO_CRED_PROT_UV_REQUIRED` protections are supported. If prot is zero, the protection of cred is cleared.

The `fido_cred_set_rk`() and `fido_cred_set_uv`() functions set the rk (resident/discoverable key) and uv (user verification) attributes of cred. Both are `FIDO_OPT_OMIT` by default, allowing the authenticator to use its default settings.

The `fido_cred_set_fmt`() function sets the attestation format of cred to fmt, where fmt must be packed (the format used in FIDO2), fido-u2f (the format used by U2F), or none. A copy of fmt is made, and no references to the passed pointer are kept. Note that not all authenticators support FIDO2 and therefore may not be able to generate packed.

The `fido_cred_set_type`() function sets the type of cred to cose_alg, where cose_alg is `COSE_ES256`, `COSE_RS256`, or `COSE_EDDSA`. The type of a credential may only be set once. Note that not all authenticators support COSE_RS256 or COSE_EDDSA.

Use of the `fido_cred_set_authdata` set of functions may happen in two distinct situations: when generating a new credential on a FIDO2 device, prior to fido_dev_make_cred(3) (i.e, in the context of a FIDO2 client), or when validating a generated credential using fido_cred_verify(3) (i.e, in the context of a FIDO2 server).

For a complete description of the generation of a FIDO2 credential and its verification, please refer to the FIDO2 specification. A concrete utilisation example of the `fido_cred_set_authdata` set of functions can be found in the cred.c example shipped with libfido2.

# RETURN VALUES

The error codes returned by the `fido_cred_set_authdata` set of functions are defined in `<fido/err.h>`. On success, `FIDO_OK` is returned.

# SEE ALSO

fido_cred_exclude(3), fido_cred_verify(3), fido_dev_make_cred(3)

July 9, 2025 | Debian