- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- NOTES
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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.