Cryptography and SSL/TLS Toolkit



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


 #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);


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().


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.


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.


ssl(7), SSL_new(3)


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

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