---
name: "gnutls_prf"
section: "3"
platform: "linux"
package: "gnutls"
version: "3.8.13-3"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/gnutls_prf"
---

# gnutls\_prf(3)

API function

## NAME

gnutls\_prf - API function

## SYNOPSIS

**#include \<gnutls/gnutls.h\>**

**int gnutls\_prf(gnutls\_session\_t** *session***, size\_t** *label\_size***, const char \*** *label***, int** *server\_random\_first***, size\_t** *extra\_size***, const char \*** *extra***, size\_t** *outsize***, char \*** *out***);**

## ARGUMENTS

- gnutls\_session\_t session

  is a **gnutls\_session\_t** type.

- size\_t label\_size

  length of the  *label* variable.

- const char \* label

  label used in PRF computation, typically a short string.

- int server\_random\_first

  non-zero if server random field should be first in seed

- size\_t extra\_size

  length of the  *extra* variable.

- const char \* extra

  optional extra data to seed the PRF with.

- size\_t outsize

  size of pre-allocated output buffer to hold the output.

- char \* out

  pre-allocated buffer to hold the generated data.

## DESCRIPTION

Applies the TLS Pseudo-Random-Function (PRF) on the master secret and the provided data, seeded with the client and server random fields. For the key expansion specified in RFC5705 see **gnutls\_prf\_rfc5705()**.

The  *label* variable usually contains a string denoting the purpose for the generated data.  The  *server\_random\_first* indicates whether the client random field or the server random field should be first in the seed.  Non-zero indicates that the server random field is first, 0 that the client random field is first.

The  *extra* variable can be used to add more data to the seed, after the random variables.  It can be used to make sure the generated output is strongly connected to some additional data (e.g., a string used in user authentication).

The output is placed in  *out* , which must be pre-allocated.

## NOTE

This function produces identical output with **gnutls\_prf\_rfc5705()** when  *server\_random\_first* is set to 0 and  *extra* is **NULL**. Under TLS1.3 this function will only operate when these conditions are true, or otherwise return **GNUTLS\_E\_INVALID\_REQUEST**.

## RETURNS

**GNUTLS\_E\_SUCCESS** on success, or an error code.

## REPORTING BUGS

Report bugs to \<bugs@gnutls.org\>.\
 Home page: https://www.gnutls.org

## COPYRIGHT

Copyright © 2001-2023 Free Software Foundation, Inc., and others.\
 Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.

## SEE ALSO

The full documentation for **gnutls** is maintained as a Texinfo manual. If the /usr/share/doc/gnutls/ directory does not contain the HTML form visit

- https://www.gnutls.org/manual/