=pod

=head1 NAME

SSL_CTX_set1_cert_comp_preference,
SSL_set1_cert_comp_preference,
SSL_CTX_compress_certs,
SSL_compress_certs,
SSL_CTX_get1_compressed_cert,
SSL_get1_compressed_cert,
SSL_CTX_set1_compressed_cert,
SSL_set1_compressed_cert - Certificate compression functions

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_CTX_set1_cert_comp_preference(SSL_CTX *ctx, int *algs, size_t len);
 int SSL_set1_cert_comp_preference(SSL *ssl, int *algs, size_t len);

 int SSL_CTX_compress_certs(SSL_CTX *ctx, int alg);
 int SSL_compress_certs(SSL *ssl, int alg);

 size_t SSL_CTX_get1_compressed_cert(SSL_CTX *ctx, int alg, unsigned char **data,
                                     size_t *orig_len);
 size_t SSL_get1_compressed_cert(SSL *ssl, int alg, unsigned char **data,
                                 size_t *orig_len);

 int SSL_CTX_set1_compressed_cert(SSL_CTX *ctx, int alg,
                                  unsigned char *comp_data,
                                  size_t comp_length, size_t orig_length);
 int SSL_set1_compressed_cert(SSL *ssl, int alg, unsigned char *comp_data,
                              size_t comp_length, size_t orig_length);


=head1 DESCRIPTION

These functions control the certificate compression feature. Certificate
compression is only available for TLSv1.3 as defined in RFC8879.

SSL_CTX_set1_cert_comp_preference() and SSL_set1_cert_comp_preference() are used
to specify the preferred compression algorithms. The B<algs> argument is an array
of algorithms, and B<length> is number of elements in the B<algs> array. Only
those algorithms enabled in the library will be accepted in B<algs>, unknown
algorithms in B<algs> are ignored. On an error, the preference order is left
unmodified.

The following compression algorithms (B<alg> arguments) may be used:

=over 4

=item * TLSEXT_comp_cert_brotli

=item * TLSEXT_comp_cert_zlib

=item * TLSEXT_comp_cert_zstd

=back

The above is also the default preference order. If a preference order is not
specified, then the default preference order is sent to the peer and the
received peer's preference order will be used when compressing a certificate.
Otherwise, the configured preference order is sent to the peer and is used
to filter the peer's preference order.

SSL_CTX_compress_certs() and SSL_compress_certs() are used to pre-compress all
the configured certificates on an SSL_CTX/SSL object with algorithm B<alg>. If
B<alg> is 0, then the certificates are compressed with the algorithms specified
in the preference list. Calling these functions on a client SSL_CTX/SSL object
will result in an error, as only server certificates may be pre-compressed.

SSL_CTX_get1_compressed_cert() and SSL_get1_compressed_cert() are used to get
the pre-compressed certificate most recently set that may be stored for later
use. Calling these functions on a client SSL_CTX/SSL object will result in an
error, as only server certificates may be pre-compressed. The B<data> and
B<orig_len> arguments are required.

The compressed certificate data may be passed to SSL_CTX_set1_compressed_cert()
or SSL_set1_compressed_cert() to provide a pre-compressed version of the
most recently set certificate. This pre-compressed certificate can only be used
by a server.

=head1 NOTES

Each side of the connection sends their compression algorithm preference list
to their peer indicating compressed certificate support. The received preference
list is filtered by the configured preference list (i.e. the intersection is
saved). As the default list includes all the enabled algorithms, not specifying
a preference will allow any enabled algorithm by the peer. The filtered peer's
preference order is used to determine what algorithm to use when sending a
compressed certificate.

Only server certificates may be pre-compressed. Calling any of these functions
(except SSL_CTX_set1_cert_comp_preference()/SSL_set1_cert_comp_preference())
on a client SSL_CTX/SSL object will return an error. Client certificates are
compressed on-demand as unique context data from the server is compressed along
with the certificate.

For SSL_CTX_set1_cert_comp_preference() and SSL_set1_cert_comp_preference()
the B<len> argument is the size of the B<algs> argument in bytes.

The compressed certificate returned by SSL_CTX_get1_compressed_cert() and
SSL_get1_compressed_cert() is the last certificate set on the SSL_CTX/SSL object.
The certificate is copied by the function and the caller must free B<*data> via
OPENSSL_free().

The compressed certificate data set by SSL_CTX_set1_compressed_cert() and
SSL_set1_compressed_cert() is copied into the SSL_CTX/SSL object.

SSL_CTX_compress_certs() and SSL_compress_certs() return an error under the
following conditions:

=over 4

=item * If no certificates have been configured.

=item * If the specified algorithm B<alg> is not enabled.

=item * If B<alg> is 0 and no compression algorithms are enabled.

=back

Sending compressed certificates may be disabled on a connection via the
SSL_OP_NO_TX_CERTIFICATE_COMPRESSION option. Receiving compressed certificates
may be disabled on a connection via the SSL_OP_NO_RX_CERTIFICATE_COMPRESSION
option.

=head1 RETURN VALUES

SSL_CTX_set1_cert_comp_preference(),
SSL_set1_cert_comp_preference(),
SSL_CTX_compress_certs(),
SSL_compress_certs(),
SSL_CTX_set1_compressed_cert(), and
SSL_set1_compressed_cert()
return 1 for success and 0 on error.

SSL_CTX_get1_compressed_cert() and
SSL_get1_compressed_cert()
return the length of the allocated memory on success and 0 on error.

=head1 SEE ALSO

L<SSL_CTX_set_options(3)>,
L<SSL_CTX_use_certificate(3)>

=head1 HISTORY

These functions were added in OpenSSL 3.2.

=head1 COPYRIGHT

Copyright 2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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>.

=cut
