9.5 TLS Inner Application (TLS/IA) Functions

The following functions are used for TLS Inner Application (TLS/IA). Their prototypes lie in ‘gnutls/extra.h’. You need to link with ‘libgnutls-extra’ to be able to use these functions (see section GnuTLS-extra Functions).

The typical control flow in an TLS/IA client (that would not require an Application Phase for resumed sessions) would be similar to the following:

int client_avp (gnuls_session_t *session, void *ptr,
                const char *last, size_t lastlen,
		char **new, size_t *newlen)
{
...
}
...
int main ()
{
  gnutls_ia_client_credentials_t iacred;
...
  gnutls_init (&session, GNUTLS_CLIENT);
...
  /* Enable TLS/IA. */
  gnutls_ia_allocate_client_credentials(&iacred);
  gnutls_ia_set_client_avp_function(iacred, client_avp);
  gnutls_credentials_set (session, GNUTLS_CRD_IA, iacred);
...
  ret = gnutls_handshake (session);
  // Error handling...
...
  if (gnutls_ia_handshake_p (session))
    {
      ret = gnutls_ia_handshake (session);
      // Error handling...
...

See below for detailed descriptions of all the functions used above.

The function client_avp would have to be implemented by your application. The function is responsible for handling the AVP data. See gnutls_ia_set_client_avp_function below for more information on how that function should be implemented.

The control flow in a typical server is similar to the above, use gnutls_ia_server_credentials_t instead of gnutls_ia_client_credentials_t, and replace the call to the client functions with the corresponding server functions.

gnutls_ia_allocate_client_credentials

Function: int gnutls_ia_allocate_client_credentials (gnutls_ia_client_credentials_t * sc)

sc: is a pointer to a gnutls_ia_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Adding this credential to a session will enable TLS/IA, and will require an Application Phase after the TLS handshake (if the server support TLS/IA). Use gnutls_ia_enable() to toggle the TLS/IA mode.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_ia_allocate_server_credentials

Function: int gnutls_ia_allocate_server_credentials (gnutls_ia_server_credentials_t * sc)

sc: is a pointer to a gnutls_ia_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Adding this credential to a session will enable TLS/IA, and will require an Application Phase after the TLS handshake (if the client support TLS/IA). Use gnutls_ia_enable() to toggle the TLS/IA mode.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_ia_enable

Function: void gnutls_ia_enable (gnutls_session_t session, int allow_skip_on_resume)

session: is a gnutls_session_t structure.

allow_skip_on_resume: non-zero if local party allows to skip the TLS/IA application phases for a resumed session.

Specify whether we must advertise support for the TLS/IA extension during the handshake.

At the client side, we always advertise TLS/IA if gnutls_ia_enable was called before the handshake; at the server side, we also require that the client has advertised that it wants to run TLS/IA before including the advertisement, as required by the protocol.

Similarly, at the client side we always advertise that we allow TLS/IA to be skipped for resumed sessions if allow_skip_on_resume is non-zero; at the server side, we also require that the session is indeed resumable and that the client has also advertised that it allows TLS/IA to be skipped for resumed sessions.

After the TLS handshake, call gnutls_ia_handshake_p() to find out whether both parties agreed to do a TLS/IA handshake, before calling gnutls_ia_handshake() or one of the lower level gnutls_ia_* functions.

gnutls_ia_endphase_send

Function: int gnutls_ia_endphase_send (gnutls_session_t session, int final_p)

session: is a gnutls_session_t structure.

final_p: Set iff this should signal the final phase.

Send a TLS/IA end phase message.

In the client, this should only be used to acknowledge an end phase message sent by the server.

In the server, this can be called instead of gnutls_ia_send() if the server wishes to end an application phase.

Return value: Return 0 on success, or an error code.

gnutls_ia_extract_inner_secret

Function: void gnutls_ia_extract_inner_secret (gnutls_session_t session, char * buffer)

session: is a gnutls_session_t structure.

buffer: pre-allocated buffer to hold 48 bytes of inner secret.

Copy the 48 bytes large inner secret into the specified buffer

This function is typically used after the TLS/IA handshake has concluded. The TLS/IA inner secret can be used as input to a PRF to derive session keys. Do not use the inner secret directly as a session key, because for a resumed session that does not include an application phase, the inner secret will be identical to the inner secret in the original session. It is important to include, for example, the client and server randomness when deriving a sesssion key from the inner secret.

gnutls_ia_free_client_credentials

Function: void gnutls_ia_free_client_credentials (gnutls_ia_client_credentials_t sc)

sc: is a gnutls_ia_client_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.

gnutls_ia_free_server_credentials

Function: void gnutls_ia_free_server_credentials (gnutls_ia_server_credentials_t sc)

sc: is a gnutls_ia_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.

gnutls_ia_generate_challenge

Function: int gnutls_ia_generate_challenge (gnutls_session_t session, size_t buffer_size, char * buffer)

session: is a gnutls_session_t structure.

buffer_size: size of output buffer.

buffer: pre-allocated buffer to contain buffer_size bytes of output.

Generate an application challenge that the client cannot control or predict, based on the TLS/IA inner secret.

Return value: Returns 0 on success, or an negative error code.

gnutls_ia_get_client_avp_ptr

Function: void * gnutls_ia_get_client_avp_ptr (gnutls_ia_client_credentials_t cred)

cred: is a gnutls_ia_client_credentials_t structure.

Returns the pointer that will be provided to the TLS/IA callback function as the first argument.

Returns: The client callback data pointer.

gnutls_ia_get_server_avp_ptr

Function: void * gnutls_ia_get_server_avp_ptr (gnutls_ia_server_credentials_t cred)

cred: is a gnutls_ia_client_credentials_t structure.

Returns the pointer that will be provided to the TLS/IA callback function as the first argument.

Returns: The server callback data pointer.

gnutls_ia_handshake_p

Function: int gnutls_ia_handshake_p (gnutls_session_t session)

session: is a gnutls_session_t structure.

Predicate to be used after gnutls_handshake() to decide whether to invoke gnutls_ia_handshake(). Usable by both clients and servers.

Return value: non-zero if TLS/IA handshake is expected, zero otherwise.

gnutls_ia_handshake

Function: int gnutls_ia_handshake (gnutls_session_t session)

session: is a gnutls_session_t structure.

Perform a TLS/IA handshake. This should be called after gnutls_handshake() iff gnutls_ia_handshake_p().

Returns: On success, GNUTLS_E_SUCCESS (zero) is returned, otherwise an error code is returned.

gnutls_ia_permute_inner_secret

Function: int gnutls_ia_permute_inner_secret (gnutls_session_t session, size_t session_keys_size, const char * session_keys)

session: is a gnutls_session_t structure.

session_keys_size: Size of generated session keys (0 if none).

session_keys: Generated session keys, used to permute inner secret (NULL if none).

Permute the inner secret using the generated session keys.

This can be called in the TLS/IA AVP callback to mix any generated session keys with the TLS/IA inner secret.

Return value: Return zero on success, or a negative error code.

gnutls_ia_recv

Function: ssize_t gnutls_ia_recv (gnutls_session_t session, char * data, size_t sizeofdata)

session: is a gnutls_session_t structure.

data: the buffer that the data will be read into, must hold >= 12 bytes.

sizeofdata: the number of requested bytes, must be >= 12.

Receive TLS/IA data. This function has the similar semantics with recv(). The only difference is that it accepts a GnuTLS session, and uses different error codes.

If the server attempt to finish an application phase, this function will return GNUTLS_E_WARNING_IA_IPHF_RECEIVED or GNUTLS_E_WARNING_IA_FPHF_RECEIVED. The caller should then invoke gnutls_ia_verify_endphase(), and if it runs the client side, also send an endphase message of its own using gnutls_ia_endphase_send.

If EINTR is returned by the internal push function (the default is code{recv()}) then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again, with the same parameters; alternatively you could provide a NULL pointer for data, and 0 for size.

Returns: The number of bytes received. A negative error code is returned in case of an error. The GNUTLS_E_WARNING_IA_IPHF_RECEIVED and GNUTLS_E_WARNING_IA_FPHF_RECEIVED errors are returned when an application phase finished message has been sent by the server.

gnutls_ia_send

Function: ssize_t gnutls_ia_send (gnutls_session_t session, const char * data, size_t sizeofdata)

session: is a gnutls_session_t structure.

data: contains the data to send

sizeofdata: is the length of the data

Send TLS/IA application payload data. This function has the similar semantics with send(). The only difference is that it accepts a GnuTLS session, and uses different error codes.

The TLS/IA protocol is synchronous, so you cannot send more than one packet at a time. The client always send the first packet.

To finish an application phase in the server, use gnutls_ia_endphase_send(). The client cannot end an application phase unilaterally; rather, a client is required to respond with an endphase of its own if gnutls_ia_recv indicates that the server has sent one.

If the EINTR is returned by the internal push function (the default is send()} then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again, with the same parameters; alternatively you could provide a NULL pointer for data, and 0 for size.

Returns: The number of bytes sent, or a negative error code.

gnutls_ia_set_client_avp_function

Function: void gnutls_ia_set_client_avp_function (gnutls_ia_client_credentials_t cred, gnutls_ia_avp_func avp_func)

cred: is a gnutls_ia_client_credentials_t structure.

avp_func: is the callback function

Set the TLS/IA AVP callback handler used for the session.

The AVP callback is called to process AVPs received from the server, and to get a new AVP to send to the server.

The callback’s function form is: int (*avp_func) (gnutls_session_t session, void *ptr, const char *last, size_t lastlen, char **next, size_t *nextlen);

The session parameter is the gnutls_session_t structure corresponding to the current session. The ptr parameter is the application hook pointer, set through gnutls_ia_set_client_avp_ptr(). The AVP received from the server is present in last of lastlen size, which will be NULL on the first invocation. The newly allocated output AVP to send to the server should be placed in *next of *nextlen size.

The callback may invoke gnutls_ia_permute_inner_secret() to mix any generated session keys with the TLS/IA inner secret.

Return 0 (GNUTLS_IA_APPLICATION_PAYLOAD) on success, or a negative error code to abort the TLS/IA handshake.

Note that the callback must use allocate the next parameter using gnutls_malloc(), because it is released via gnutls_free() by the TLS/IA handshake function.

gnutls_ia_set_client_avp_ptr

Function: void gnutls_ia_set_client_avp_ptr (gnutls_ia_client_credentials_t cred, void * ptr)

cred: is a gnutls_ia_client_credentials_t structure.

ptr: is the pointer

Sets the pointer that will be provided to the TLS/IA callback function as the first argument.

gnutls_ia_set_server_avp_function

Function: void gnutls_ia_set_server_avp_function (gnutls_ia_server_credentials_t cred, gnutls_ia_avp_func avp_func)

cred: is a gnutls_ia_server_credentials_t structure.

Set the TLS/IA AVP callback handler used for the session.

The callback’s function form is: int (*avp_func) (gnutls_session_t session, void *ptr, const char *last, size_t lastlen, char **next, size_t *nextlen);

The session parameter is the gnutls_session_t structure corresponding to the current session. The ptr parameter is the application hook pointer, set through gnutls_ia_set_server_avp_ptr(). The AVP received from the client is present in last of lastlen size. The newly allocated output AVP to send to the client should be placed in *next of *nextlen size.

The AVP callback is called to process incoming AVPs from the client, and to get a new AVP to send to the client. It can also be used to instruct the TLS/IA handshake to do go into the Intermediate or Final phases. It return a negative error code, or a gnutls_ia_apptype_t message type.

The callback may invoke gnutls_ia_permute_inner_secret() to mix any generated session keys with the TLS/IA inner secret.

Specifically, return GNUTLS_IA_APPLICATION_PAYLOAD (0) to send another AVP to the client, return GNUTLS_IA_INTERMEDIATE_PHASE_FINISHED (1) to indicate that an IntermediatePhaseFinished message should be sent, and return GNUTLS_IA_FINAL_PHASE_FINISHED (2) to indicate that an FinalPhaseFinished message should be sent. In the last two cases, the contents of the next and nextlen parameter is not used.

Note that the callback must use allocate the next parameter using gnutls_malloc(), because it is released via gnutls_free() by the TLS/IA handshake function.

gnutls_ia_set_server_avp_ptr

Function: void gnutls_ia_set_server_avp_ptr (gnutls_ia_server_credentials_t cred, void * ptr)

cred: is a gnutls_ia_client_credentials_t structure.

ptr: is the pointer

Sets the pointer that will be provided to the TLS/IA callback function as the first argument.

gnutls_ia_verify_endphase

Function: int gnutls_ia_verify_endphase (gnutls_session_t session, const char * checksum)

session: is a gnutls_session_t structure.

checksum: 12-byte checksum data, received from gnutls_ia_recv().

Verify TLS/IA end phase checksum data. If verification fails, the GNUTLS_A_INNER_APPLICATION_VERIFICATION alert is sent to the other sie.

This function is called when gnutls_ia_recv() return GNUTLS_E_WARNING_IA_IPHF_RECEIVED or GNUTLS_E_WARNING_IA_FPHF_RECEIVED.

Return value: Return 0 on successful verification, or an error code. If the checksum verification of the end phase message fails, GNUTLS_E_IA_VERIFY_FAILED is returned.