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.IX Title "SSL_CTX_SET_RECORD_PADDING_CALLBACK 3ossl"
.TH SSL_CTX_SET_RECORD_PADDING_CALLBACK 3ossl "2024-09-03" "3.1.7+quic" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
SSL_CTX_set_record_padding_callback,
SSL_set_record_padding_callback,
SSL_CTX_set_record_padding_callback_arg,
SSL_set_record_padding_callback_arg,
SSL_CTX_get_record_padding_callback_arg,
SSL_get_record_padding_callback_arg,
SSL_CTX_set_block_padding,
SSL_set_block_padding \- install callback to specify TLS 1.3 record padding
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& void SSL_CTX_set_record_padding_callback(SSL_CTX *ctx, size_t (*cb)(SSL *s, int type, size_t len, void *arg));
\& int SSL_set_record_padding_callback(SSL *ssl, size_t (*cb)(SSL *s, int type, size_t len, void *arg));
\&
\& void SSL_CTX_set_record_padding_callback_arg(SSL_CTX *ctx, void *arg);
\& void *SSL_CTX_get_record_padding_callback_arg(const SSL_CTX *ctx);
\&
\& void SSL_set_record_padding_callback_arg(SSL *ssl, void *arg);
\& void *SSL_get_record_padding_callback_arg(const SSL *ssl);
\&
\& int SSL_CTX_set_block_padding(SSL_CTX *ctx, size_t block_size);
\& int SSL_set_block_padding(SSL *ssl, size_t block_size);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_CTX_set_record_padding_callback()\fR or \fBSSL_set_record_padding_callback()\fR
can be used to assign a callback function \fIcb\fR to specify the padding
for \s-1TLS 1.3\s0 records. The value set in \fBctx\fR is copied to a new \s-1SSL\s0 by \fBSSL_new()\fR.
Kernel \s-1TLS\s0 is not possible if the record padding callback is set, and the callback
function cannot be set if Kernel \s-1TLS\s0 is already configured for the current \s-1SSL\s0 object.
.PP
\&\fBSSL_CTX_set_record_padding_callback_arg()\fR and \fBSSL_set_record_padding_callback_arg()\fR
assign a value \fBarg\fR that is passed to the callback when it is invoked. The value
set in \fBctx\fR is copied to a new \s-1SSL\s0 by \fBSSL_new()\fR.
.PP
\&\fBSSL_CTX_get_record_padding_callback_arg()\fR and \fBSSL_get_record_padding_callback_arg()\fR
retrieve the \fBarg\fR value that is passed to the callback.
.PP
\&\fBSSL_CTX_set_block_padding()\fR and \fBSSL_set_block_padding()\fR pads the record to a multiple
of the \fBblock_size\fR. A \fBblock_size\fR of 0 or 1 disables block padding. The limit of
\&\fBblock_size\fR is \s-1SSL3_RT_MAX_PLAIN_LENGTH.\s0
.PP
The callback is invoked for every record before encryption.
The \fBtype\fR parameter is the \s-1TLS\s0 record type that is being processed; may be
one of \s-1SSL3_RT_APPLICATION_DATA, SSL3_RT_HANDSHAKE,\s0 or \s-1SSL3_RT_ALERT.\s0
The \fBlen\fR parameter is the current plaintext length of the record before encryption.
The \fBarg\fR parameter is the value set via \fBSSL_CTX_set_record_padding_callback_arg()\fR
or \fBSSL_set_record_padding_callback_arg()\fR.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The \fBSSL_CTX_get_record_padding_callback_arg()\fR and \fBSSL_get_record_padding_callback_arg()\fR
functions return the \fBarg\fR value assigned in the corresponding set functions.
.PP
The \fBSSL_CTX_set_block_padding()\fR and \fBSSL_set_block_padding()\fR functions return 1 on success
or 0 if \fBblock_size\fR is too large.
.PP
The \fBcb\fR returns the number of padding bytes to add to the record. A return of 0
indicates no padding will be added. A return value that causes the record to
exceed the maximum record size (\s-1SSL3_RT_MAX_PLAIN_LENGTH\s0) will pad out to the
maximum record size.
.PP
The \fBSSL_CTX_get_record_padding_callback_arg()\fR function returns 1 on success or 0 if
the callback function is not set because Kernel \s-1TLS\s0 is configured for the \s-1SSL\s0 object.
.SH "NOTES"
.IX Header "NOTES"
The default behavior is to add no padding to the record.
.PP
A user-supplied padding callback function will override the behavior set by
\&\fBSSL_set_block_padding()\fR or \fBSSL_CTX_set_block_padding()\fR. Setting the user-supplied
callback to \s-1NULL\s0 will restore the configured block padding behavior.
.PP
These functions only apply to \s-1TLS 1.3\s0 records being written.
.PP
Padding bytes are not added in constant-time.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(7), \fBSSL_new\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
The record padding \s-1API\s0 was added for \s-1TLS 1.3\s0 support in OpenSSL 1.1.1.
.PP
The return type of \fBSSL_CTX_set_record_padding_callback()\fR function was
changed to int in OpenSSL 3.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2017\-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>.