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

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);

DESCRIPTION

       SSL_CTX_set_record_padding_callback() or SSL_set_record_padding_callback() can be used to
       assign a callback function cb to specify the padding for TLS 1.3 records. The value set in
       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 arg that is passed to the callback when it is invoked. The value set in 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 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 block_size. A block_size of 0 or 1 disables block padding. The limit of block_size is
       SSL3_RT_MAX_PLAIN_LENGTH.

       The callback is invoked for every record before encryption.  The 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 len parameter is the current plaintext length of
       the record before encryption.  The arg parameter is the value set via
       SSL_CTX_set_record_padding_callback_arg() or SSL_set_record_padding_callback_arg().

RETURN VALUES

       The SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg()
       functions return the 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 block_size is too large.

       The 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.

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.

SEE ALSO

       ssl(7), SSL_new(3)

HISTORY

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

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 <https://www.openssl.org/source/license.html>.