1=pod
2
3=head1 NAME
4
5SSL_CTX_set_record_padding_callback,
6SSL_set_record_padding_callback,
7SSL_CTX_set_record_padding_callback_arg,
8SSL_set_record_padding_callback_arg,
9SSL_CTX_get_record_padding_callback_arg,
10SSL_get_record_padding_callback_arg,
11SSL_CTX_set_block_padding,
12SSL_set_block_padding - install callback to specify TLS 1.3 record padding
13
14=head1 SYNOPSIS
15
16 #include <openssl/ssl.h>
17
18 void SSL_CTX_set_record_padding_callback(SSL_CTX *ctx, size_t (*cb)(SSL *s, int type, size_t len, void *arg));
19 int SSL_set_record_padding_callback(SSL *ssl, size_t (*cb)(SSL *s, int type, size_t len, void *arg));
20
21 void SSL_CTX_set_record_padding_callback_arg(SSL_CTX *ctx, void *arg);
22 void *SSL_CTX_get_record_padding_callback_arg(const SSL_CTX *ctx);
23
24 void SSL_set_record_padding_callback_arg(SSL *ssl, void *arg);
25 void *SSL_get_record_padding_callback_arg(const SSL *ssl);
26
27 int SSL_CTX_set_block_padding(SSL_CTX *ctx, size_t block_size);
28 int SSL_set_block_padding(SSL *ssl, size_t block_size);
29
30=head1 DESCRIPTION
31
32SSL_CTX_set_record_padding_callback() or SSL_set_record_padding_callback()
33can be used to assign a callback function I<cb> to specify the padding
34for TLS 1.3 records. The value set in B<ctx> is copied to a new SSL by SSL_new().
35Kernel TLS is not possible if the record padding callback is set, and the callback
36function cannot be set if Kernel TLS is already configured for the current SSL object.
37
38SSL_CTX_set_record_padding_callback_arg() and SSL_set_record_padding_callback_arg()
39assign a value B<arg> that is passed to the callback when it is invoked. The value
40set in B<ctx> is copied to a new SSL by SSL_new().
41
42SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg()
43retrieve the B<arg> value that is passed to the callback.
44
45SSL_CTX_set_block_padding() and SSL_set_block_padding() pads the record to a multiple
46of the B<block_size>. A B<block_size> of 0 or 1 disables block padding. The limit of
47B<block_size> is SSL3_RT_MAX_PLAIN_LENGTH.
48
49The callback is invoked for every record before encryption.
50The B<type> parameter is the TLS record type that is being processed; may be
51one of SSL3_RT_APPLICATION_DATA, SSL3_RT_HANDSHAKE, or SSL3_RT_ALERT.
52The B<len> parameter is the current plaintext length of the record before encryption.
53The B<arg> parameter is the value set via SSL_CTX_set_record_padding_callback_arg()
54or SSL_set_record_padding_callback_arg().
55
56=head1 RETURN VALUES
57
58The SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg()
59functions return the B<arg> value assigned in the corresponding set functions.
60
61The SSL_CTX_set_block_padding() and SSL_set_block_padding() functions return 1 on success
62or 0 if B<block_size> is too large.
63
64The B<cb> returns the number of padding bytes to add to the record. A return of 0
65indicates no padding will be added. A return value that causes the record to
66exceed the maximum record size (SSL3_RT_MAX_PLAIN_LENGTH) will pad out to the
67maximum record size.
68
69The SSL_CTX_get_record_padding_callback_arg() function returns 1 on success or 0 if
70the callback function is not set because Kernel TLS is configured for the SSL object.
71
72=head1 NOTES
73
74The default behavior is to add no padding to the record.
75
76A user-supplied padding callback function will override the behavior set by
77SSL_set_block_padding() or SSL_CTX_set_block_padding(). Setting the user-supplied
78callback to NULL will restore the configured block padding behavior.
79
80These functions only apply to TLS 1.3 records being written.
81
82Padding bytes are not added in constant-time.
83
84=head1 SEE ALSO
85
86L<ssl(7)>, L<SSL_new(3)>
87
88=head1 HISTORY
89
90The record padding API was added for TLS 1.3 support in OpenSSL 1.1.1.
91
92The return type of SSL_CTX_set_record_padding_callback() function was
93changed to int in OpenSSL 3.0.
94
95=head1 COPYRIGHT
96
97Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.
98
99Licensed under the Apache License 2.0 (the "License").  You may not use
100this file except in compliance with the License.  You can obtain a copy
101in the file LICENSE in the source distribution or at
102L<https://www.openssl.org/source/license.html>.
103
104=cut
105