SSL_psk_client_cb_func, SSL_psk_use_session_cb_func,
SSL_CTX_set_psk_client_callback, SSL_set_psk_client_callback,
SSL_CTX_set_psk_use_session_callback, SSL_set_psk_use_session_callback - set
PSK client callback
#include <openssl/ssl.h>
typedef int (*SSL_psk_use_session_cb_func)(SSL *ssl, const EVP_MD *md,
const unsigned char **id,
size_t *idlen,
SSL_SESSION **sess);
void SSL_CTX_set_psk_use_session_callback(SSL_CTX *ctx,
SSL_psk_use_session_cb_func cb);
void SSL_set_psk_use_session_callback(SSL *s, SSL_psk_use_session_cb_func cb);
typedef unsigned int (*SSL_psk_client_cb_func)(SSL *ssl,
const char *hint,
char *identity,
unsigned int max_identity_len,
unsigned char *psk,
unsigned int max_psk_len);
void SSL_CTX_set_psk_client_callback(SSL_CTX *ctx, SSL_psk_client_cb_func cb);
void SSL_set_psk_client_callback(SSL *ssl, SSL_psk_client_cb_func cb);
A client application wishing to use TLSv1.3 PSKs should use either
SSL_CTX_set_psk_use_session_callback() or
SSL_set_psk_use_session_callback() as appropriate. These functions
cannot be used for TLSv1.2 and below PSKs.
The callback function is given a pointer to the SSL connection in
ssl.
The first time the callback is called for a connection the
md parameter
is NULL. In some circumstances the callback will be called a second time. In
that case the server will have specified a ciphersuite to use already and the
PSK must be compatible with the digest for that ciphersuite. The digest will
be given in
md. The PSK returned by the callback is allowed to be
different between the first and second time it is called.
On successful completion the callback must store a pointer to an identifier for
the PSK in
*id. The identifier length in bytes should be stored in
*idlen. The memory pointed to by
*id remains owned by the
application and should be freed by it as required at any point after the
handshake is complete.
Additionally the callback should store a pointer to an SSL_SESSION object in
*sess. This is used as the basis for the PSK, and should, at a minimum,
have the following fields set:
- The master key
- This can be set via a call to
SSL_SESSION_set1_master_key(3).
- A ciphersuite
- Only the handshake digest associated with the ciphersuite
is relevant for the PSK (the server may go on to negotiate any ciphersuite
which is compatible with the digest). The application can use any TLSv1.3
ciphersuite. If md is not NULL the handshake digest for the
ciphersuite should be the same. The ciphersuite can be set via a call to
< SSL_SESSION_set_cipher(3)>. The handshake digest of an
SSL_CIPHER object can be checked using <
SSL_CIPHER_get_handshake_digest(3)>.
- The protocol version
- This can be set via a call to
SSL_SESSION_set_protocol_version(3) and should be
TLS1_3_VERSION.
Additionally the maximum early data value should be set via a call to
SSL_SESSION_set_max_early_data(3) if the PSK will be used for sending
early data.
Alternatively an SSL_SESSION created from a previous non-PSK handshake may also
be used as the basis for a PSK.
Ownership of the SSL_SESSION object is passed to the OpenSSL library and so it
should not be freed by the application.
It is also possible for the callback to succeed but not supply a PSK. In this
case no PSK will be sent to the server but the handshake will continue. To do
this the callback should return successfully and ensure that
*sess is
NULL. The contents of
*id and
*idlen will be ignored.
A client application wishing to use PSK ciphersuites for TLSv1.2 and below must
provide a different callback function. This function will be called when the
client is sending the ClientKeyExchange message to the server.
The purpose of the callback function is to select the PSK identity and the
pre-shared key to use during the connection setup phase.
The callback is set using functions
SSL_CTX_set_psk_client_callback() or
SSL_set_psk_client_callback(). The callback function is given the
connection in parameter
ssl, a
NUL-terminated PSK identity hint
sent by the server in parameter
hint, a buffer
identity of
length
max_identity_len bytes (including the
NUL-terminator)
where the resulting
NUL-terminated identity is to be stored, and a
buffer
psk of length
max_psk_len bytes where the resulting
pre-shared key is to be stored.
The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
recommended to use
SSL_CTX_set_psk_use_session_callback() or
SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3
has been negotiated then OpenSSL will first check to see if a callback has
been set via
SSL_CTX_set_psk_use_session_callback() or
SSL_set_psk_use_session_callback() and it will use that in preference.
If no such callback is present then it will check to see if a callback has
been set via
SSL_CTX_set_psk_client_callback() or
SSL_set_psk_client_callback() and use that. In this case the
hint value will always be NULL and the handshake digest will default to
SHA-256 for any returned PSK. TLSv1.3 early data exchanges are possible in PSK
connections only with the
SSL_psk_use_session_cb_func callback, and are
not possible with the
SSL_psk_client_cb_func callback.
Note that parameter
hint given to the callback may be
NULL.
A connection established via a TLSv1.3 PSK will appear as if session resumption
has occurred so that
SSL_session_reused(3) will return true.
There are no known security issues with sharing the same PSK between TLSv1.2 (or
below) and TLSv1.3. However, the RFC has this note of caution:
"While there is no known way in which the same PSK might produce related
output in both versions, only limited analysis has been done. Implementations
can ensure safety from cross-protocol related output by not reusing PSKs
between TLS 1.3 and TLS 1.2."
Return values from the
SSL_psk_client_cb_func callback are interpreted as
follows:
On success (callback found a PSK identity and a pre-shared key to use) the
length (> 0) of
psk in bytes is returned.
Otherwise or on errors the callback should return 0. In this case the connection
setup fails.
The SSL_psk_use_session_cb_func callback should return 1 on success or 0 on
failure. In the event of failure the connection setup fails.
ssl(7),
SSL_CTX_set_psk_find_session_callback(3),
SSL_set_psk_find_session_callback(3)
SSL_CTX_set_psk_use_session_callback() and
SSL_set_psk_use_session_callback() were added in OpenSSL 1.1.1.
Copyright 2006-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.