---
name: "fido_assert_user_icon"
section: "3"
platform: "linux"
package: "libfido2"
version: "1.17.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/fido_assert_new"
---

# fido\_assert\_user\_icon(3)

FIDO2 assertion API

Part of [fido\_assert\_new(3)](/fido_assert_new/3.md).

## NAME

**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\_authdata\_raw\_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\_authdata\_raw\_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\_flags** – FIDO2 assertion API

## SYNOPSIS

**#include \<fido.h\>**

*fido\_assert\_t* *\**\

**fido\_assert\_new**(*void*);

*void*\

**fido\_assert\_free**(*fido\_assert\_t \*\*assert\_p*);

*size\_t*\

**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\_authdata\_raw\_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*);

*size\_t*\

**fido\_assert\_authdata\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_authdata\_raw\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_clientdata\_hash\_len**(*const fido\_assert\_t \*assert*);

*size\_t*\

**fido\_assert\_blob\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_hmac\_secret\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_largeblob\_key\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_user\_id\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_sig\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*size\_t*\

**fido\_assert\_id\_len**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*uint32\_t*\

**fido\_assert\_sigcount**(*const fido\_assert\_t \*assert*, *size\_t idx*);

*uint8\_t*\

**fido\_assert\_flags**(*const fido\_assert\_t \*assert*, *size\_t idx*);

## DESCRIPTION

A FIDO2 assertion is a collection of statements, each statement a map between a challenge, a credential, a signature, and ancillary attributes. In *libfido2*, 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_set_authdata/3.md), [fido\_assert\_allow\_cred(3)](/fido_assert_allow_cred/3.md), [fido\_assert\_verify(3)](/fido_assert_verify/3.md), and [fido\_dev\_get\_assert(3)](/fido_dev_get_assert/3.md).

The **fido\_assert\_new**() function returns a pointer to a newly allocated, empty *fido\_assert\_t* type. If memory cannot be allocated, NULL is returned.

The **fido\_assert\_free**() 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 **fido\_assert\_count**() function returns the number of statements in *assert*.

The **fido\_assert\_rp\_id**() function returns a pointer to a NUL-terminated string holding the relying party ID of *assert*.

The **fido\_assert\_user\_display\_name**(), **fido\_assert\_user\_icon**(), and **fido\_assert\_user\_name**(), 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 user display name, icon, and name attributes will typically only be returned by the authenticator if user verification was performed by the authenticator and multiple resident/discoverable credentials were involved in the assertion.

The **fido\_assert\_authdata\_ptr**(), **fido\_assert\_authdata\_raw\_ptr**(), **fido\_assert\_clientdata\_hash\_ptr**(), **fido\_assert\_id\_ptr**(), **fido\_assert\_user\_id\_ptr**(), **fido\_assert\_sig\_ptr**(), **fido\_assert\_sigcount**(), and **fido\_assert\_flags**() functions return pointers to the CBOR-encoded and raw authenticator data, client data hash, credential ID, user ID, signature, signature count, and authenticator data flags of statement *idx* in *assert*.

The **fido\_assert\_hmac\_secret\_ptr**() 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 **fido\_assert\_blob\_ptr**() and **fido\_assert\_largeblob\_key\_ptr**() 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 **fido\_assert\_authdata\_len**(), **fido\_assert\_authdata\_raw\_len**(), **fido\_assert\_clientdata\_hash\_len**(), **fido\_assert\_id\_len**(), **fido\_assert\_user\_id\_len**(), **fido\_assert\_sig\_len**(), **fido\_assert\_hmac\_secret\_len**(), **fido\_assert\_blob\_len**(), and **fido\_assert\_largeblob\_key\_len**() 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.

## RETURN VALUES

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 *const* qualifier is invoked.

## SEE ALSO

[fido\_assert\_allow\_cred(3)](/fido_assert_allow_cred/3.md), [fido\_assert\_set\_authdata(3)](/fido_assert_set_authdata/3.md), [fido\_assert\_verify(3)](/fido_assert_verify/3.md), [fido\_dev\_get\_assert(3)](/fido_dev_get_assert/3.md), [fido\_dev\_largeblob\_get(3)](/fido_dev_largeblob_get/3.md)