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=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_set_block_padding - 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));
 void 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);

=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().

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.

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().

=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.

=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.

=head1 COPYRIGHT

Copyright 2017-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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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