=pod

=head1 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_CTX_set_block_padding_ex,
SSL_set_block_padding,
SSL_set_block_padding_ex - install callback to specify TLS 1.3 record padding

=head1 SYNOPSIS

 #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);
 int SSL_CTX_set_block_padding_ex(SSL_CTX *ctx, size_t app_block_size, size_t hs_block_size);
 int SSL_set_block_padding_ex(SSL *ssl, size_t app_block_size, size_t hs_block_size);

=head1 DESCRIPTION

SSL_CTX_set_record_padding_callback() or SSL_set_record_padding_callback()
can be used to assign a callback function I<cb> to specify the padding
for TLS 1.3 records. The value set in B<ctx> is copied to a new SSL by SSL_new().
Kernel TLS is not possible if the record padding callback is set, and the callback
function cannot be set if Kernel TLS is already configured for the current SSL object.

SSL_CTX_set_record_padding_callback_arg() and SSL_set_record_padding_callback_arg()
assign a value B<arg> that is passed to the callback when it is invoked. The value
set in B<ctx> is copied to a new SSL by SSL_new().

SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg()
retrieve the B<arg> value that is passed to the callback.

SSL_CTX_set_block_padding() and SSL_set_block_padding() pads the record to a multiple
of the B<block_size>. A B<block_size> of 0 or 1 disables block padding. The limit of
B<block_size> is SSL3_RT_MAX_PLAIN_LENGTH.

SSL_CTX_set_block_padding_ex() and SSL_set_block_padding_ex() do similarly but
allow the caller to separately specify the padding block size to be applied to
handshake and application data messages.

The callback is invoked for every record before encryption.
The B<type> parameter is the TLS record type that is being processed; may be
one of SSL3_RT_APPLICATION_DATA, SSL3_RT_HANDSHAKE, or SSL3_RT_ALERT.
The B<len> parameter is the current plaintext length of the record before encryption.
The B<arg> parameter is the value set via SSL_CTX_set_record_padding_callback_arg()
or SSL_set_record_padding_callback_arg().

These functions cannot be used with QUIC SSL objects.
SSL_set_record_padding_callback() and SSL_set_block_padding() fail if called on
a QUIC SSL object.

=head1 RETURN VALUES

The SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg()
functions return the B<arg> value assigned in the corresponding set functions.

The SSL_CTX_set_block_padding() and SSL_set_block_padding() functions return 1 on success
or 0 if B<block_size> is too large.

The B<cb> 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 (SSL3_RT_MAX_PLAIN_LENGTH) will pad out to the
maximum record size.

The SSL_CTX_get_record_padding_callback_arg() function returns 1 on success or 0 if
the callback function is not set because Kernel TLS is configured for the SSL object.

=head1 NOTES

The default behavior is to add no padding to the record.

A user-supplied padding callback function will override the behavior set by
SSL_set_block_padding() or SSL_CTX_set_block_padding(). Setting the user-supplied
callback to NULL will restore the configured block padding behavior.

These functions only apply to TLS 1.3 records being written.

Padding bytes are not added in constant-time.

=head1 SEE ALSO

L<ssl(7)>, L<SSL_new(3)>

=head1 HISTORY

The record padding API was added for TLS 1.3 support in OpenSSL 1.1.1.

The return type of SSL_CTX_set_record_padding_callback() function was
changed to int in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2017-2024 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
L<https://www.openssl.org/source/license.html>.

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