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.\"
.IX Title "SSL_CTX_USE_PSK_IDENTITY_HINT 3ossl"
.TH SSL_CTX_USE_PSK_IDENTITY_HINT 3ossl "2026-01-27" "3.5.5" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_psk_server_cb_func,
SSL_psk_find_session_cb_func,
SSL_CTX_use_psk_identity_hint,
SSL_use_psk_identity_hint,
SSL_CTX_set_psk_server_callback,
SSL_set_psk_server_callback,
SSL_CTX_set_psk_find_session_callback,
SSL_set_psk_find_session_callback
\&\- set PSK identity hint to use
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& typedef int (*SSL_psk_find_session_cb_func)(SSL *ssl,
\&                                             const unsigned char *identity,
\&                                             size_t identity_len,
\&                                             SSL_SESSION **sess);
\&
\&
\& void SSL_CTX_set_psk_find_session_callback(SSL_CTX *ctx,
\&                                            SSL_psk_find_session_cb_func cb);
\& void SSL_set_psk_find_session_callback(SSL *s, SSL_psk_find_session_cb_func cb);
\&
\& typedef unsigned int (*SSL_psk_server_cb_func)(SSL *ssl,
\&                                                const char *identity,
\&                                                unsigned char *psk,
\&                                                unsigned int max_psk_len);
\&
\& int SSL_CTX_use_psk_identity_hint(SSL_CTX *ctx, const char *hint);
\& int SSL_use_psk_identity_hint(SSL *ssl, const char *hint);
\&
\& void SSL_CTX_set_psk_server_callback(SSL_CTX *ctx, SSL_psk_server_cb_func cb);
\& void SSL_set_psk_server_callback(SSL *ssl, SSL_psk_server_cb_func cb);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
A server application wishing to use TLSv1.3 PSKs should set a callback
using either \fBSSL_CTX_set_psk_find_session_callback()\fR or
\&\fBSSL_set_psk_find_session_callback()\fR as appropriate.
.PP
The callback function is given a pointer to the \s-1SSL\s0 connection in \fBssl\fR and
an identity in \fBidentity\fR of length \fBidentity_len\fR. The callback function
should identify an \s-1SSL_SESSION\s0 object that provides the \s-1PSK\s0 details and store it
in \fB*sess\fR. The \s-1SSL_SESSION\s0 object should, as a minimum, set the master key,
the ciphersuite and the protocol version. See
\&\fBSSL_CTX_set_psk_use_session_callback\fR\|(3) for details.
.PP
It is also possible for the callback to succeed but not supply a \s-1PSK.\s0 In this
case no \s-1PSK\s0 will be used but the handshake will continue. To do this the
callback should return successfully and ensure that \fB*sess\fR is
\&\s-1NULL.\s0
.PP
Identity hints are not relevant for TLSv1.3. A server application wishing to use
\&\s-1PSK\s0 ciphersuites for TLSv1.2 and below may call \fBSSL_CTX_use_psk_identity_hint()\fR
to set the given \fB\s-1NUL\s0\fR\-terminated \s-1PSK\s0 identity hint \fBhint\fR for \s-1SSL\s0 context
object \fBctx\fR. \fBSSL_use_psk_identity_hint()\fR sets the given \fB\s-1NUL\s0\fR\-terminated \s-1PSK\s0
identity hint \fBhint\fR for the \s-1SSL\s0 connection object \fBssl\fR. If \fBhint\fR is
\&\fB\s-1NULL\s0\fR the current hint from \fBctx\fR or \fBssl\fR is deleted.
.PP
In the case where \s-1PSK\s0 identity hint is \fB\s-1NULL\s0\fR, the server does not send the
ServerKeyExchange message to the client.
.PP
A server application wishing to use PSKs for TLSv1.2 and below must provide a
callback function which is called when the server receives the
ClientKeyExchange message from the client. The purpose of the callback function
is to validate the received \s-1PSK\s0 identity and to fetch the pre-shared key used
during the connection setup phase. The callback is set using the functions
\&\fBSSL_CTX_set_psk_server_callback()\fR or \fBSSL_set_psk_server_callback()\fR. The callback
function is given the connection in parameter \fBssl\fR, \fB\s-1NUL\s0\fR\-terminated \s-1PSK\s0
identity sent by the client in parameter \fBidentity\fR, and a buffer \fBpsk\fR of
length \fBmax_psk_len\fR bytes where the pre-shared key is to be stored.
.PP
The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
recommended to use \fBSSL_CTX_set_psk_find_session_callback()\fR
or \fBSSL_set_psk_find_session_callback()\fR for this purpose instead. If TLSv1.3 has
been negotiated then OpenSSL will first check to see if a callback has been set
via \fBSSL_CTX_set_psk_find_session_callback()\fR or \fBSSL_set_psk_find_session_callback()\fR
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 \fBSSL_CTX_set_psk_server_callback()\fR or
\&\fBSSL_set_psk_server_callback()\fR and use that. In this case the handshake digest
will default to \s-1SHA\-256\s0 for any returned \s-1PSK.\s0 TLSv1.3 early data exchanges are
possible in \s-1PSK\s0 connections only with the \fBSSL_psk_find_session_cb_func\fR
callback, and are not possible with the \fBSSL_psk_server_cb_func\fR callback.
.PP
A connection established via a TLSv1.3 \s-1PSK\s0 will appear as if session resumption
has occurred so that \fBSSL_session_reused\fR\|(3) will return true.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fB\fBSSL_CTX_use_psk_identity_hint()\fB\fR and \fB\fBSSL_use_psk_identity_hint()\fB\fR return
1 on success, 0 otherwise.
.PP
Return values from the TLSv1.2 and below server callback are interpreted as
follows:
.IP "0" 4
\&\s-1PSK\s0 identity was not found. An \*(L"unknown_psk_identity\*(R" alert message
will be sent and the connection setup fails.
.IP ">0" 4
.IX Item ">0"
\&\s-1PSK\s0 identity was found and the server callback has provided the \s-1PSK\s0
successfully in parameter \fBpsk\fR. Return value is the length of
\&\fBpsk\fR in bytes. It is an error to return a value greater than
\&\fBmax_psk_len\fR.
.Sp
If the \s-1PSK\s0 identity was not found but the callback instructs the
protocol to continue anyway, the callback must provide some random
data to \fBpsk\fR and return the length of the random data, so the
connection will fail with decryption_error before it will be finished
completely.
.PP
The \fBSSL_psk_find_session_cb_func\fR callback should return 1 on success or 0 on
failure. In the event of failure the connection setup fails.
.SH "NOTES"
.IX Header "NOTES"
There are no known security issues with sharing the same \s-1PSK\s0 between TLSv1.2 (or
below) and TLSv1.3. However, the \s-1RFC\s0 has this note of caution:
.PP
\&\*(L"While there is no known way in which the same \s-1PSK\s0 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
\&\s-1TLS 1.3\s0 and \s-1TLS 1.2.\*(R"\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(7),
\&\fBSSL_CTX_set_psk_use_session_callback\fR\|(3),
\&\fBSSL_set_psk_use_session_callback\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fBSSL_CTX_set_psk_find_session_callback()\fR and \fBSSL_set_psk_find_session_callback()\fR
were added in OpenSSL 1.1.1.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2006\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
