COMP_CTX_NEW(3SSL) OpenSSL COMP_CTX_NEW(3SSL)
NAME
COMP_CTX_new, COMP_CTX_get_method, COMP_CTX_get_type, COMP_get_type,
COMP_get_name, COMP_CTX_free, COMP_compress_block, COMP_expand_block,
COMP_zlib, COMP_zlib_oneshot, COMP_brotli, COMP_brotli_oneshot,
COMP_zstd, COMP_zstd_oneshot, BIO_f_zlib, BIO_f_brotli, BIO_f_zstd -
Compression support
SYNOPSIS
#include <openssl/comp.h>
COMP_CTX *COMP_CTX_new(COMP_METHOD *meth);
void COMP_CTX_free(COMP_CTX *ctx);
const COMP_METHOD *COMP_CTX_get_method(const COMP_CTX *ctx);
int COMP_CTX_get_type(const COMP_CTX* comp);
int COMP_get_type(const COMP_METHOD *meth);
const char *COMP_get_name(const COMP_METHOD *meth);
int COMP_compress_block(COMP_CTX *ctx, unsigned char *out, int olen,
unsigned char *in, int ilen);
int COMP_expand_block(COMP_CTX *ctx, unsigned char *out, int olen,
unsigned char *in, int ilen);
COMP_METHOD *COMP_zlib(void);
COMP_METHOD *COMP_zlib_oneshot(void);
COMP_METHOD *COMP_brotli(void);
COMP_METHOD *COMP_brotli_oneshot(void);
COMP_METHOD *COMP_zstd(void);
COMP_METHOD *COMP_zstd_oneshot(void);
const BIO_METHOD *BIO_f_zlib(void);
const BIO_METHOD *BIO_f_brotli(void);
const BIO_METHOD *BIO_f_zstd(void);
DESCRIPTION
These functions provide compression support for OpenSSL. Compression is
used within the OpenSSL library to support TLS record and certificate
compression.
COMP_CTX_new() is used to create a new COMP_CTX structure used to
compress data.
COMP_CTX_free() is used to free the returned COMP_CTX. If the argument
is NULL, nothing is done.
COMP_CTX_get_method() returns the COMP_METHOD of the given ctx.
COMP_CTX_get_type() and COMP_get_type() return the NID for the COMP_CTX
and COMP_METHOD, respectively. COMP_get_name() returns the name of the
algorithm of the given COMP_METHOD.
COMP_compress_block() compresses b<ilen> bytes from the buffer in into
the buffer b<out> of size olen using the algorithm specified by ctx.
COMP_expand_block() expands ilen bytes from the buffer in into the
buffer out of size olen using the algorithm specified by ctx.
Methods (COMP_METHOD) may be specified by one of these functions. These
functions will be available even if their corresponding compression
algorithm is not configured into the OpenSSL library. In such a case,
NULL will be returned.
• COMP_zlib() returns a COMP_METHOD for stream-based ZLIB compression.
• COMP_zlib_oneshot() returns a COMP_METHOD for one-shot ZLIB
compression.
• COMP_brotli() returns a COMP_METHOD for stream-based Brotli
compression.
• COMP_brotli_oneshot() returns a COMP_METHOD for one-shot Brotli
compression.
• COMP_zstd() returns a COMP_METHOD for stream-based Zstandard
compression.
• COMP_zstd_oneshot() returns a COMP_METHOD for one-shot Zstandard
compression.
BIO_f_zlib(), BIO_f_brotli() BIO_f_zstd() each return a BIO_METHOD that
may be used to create a BIO via BIO_new(3) to read and write compressed
files or streams. The functions are only available if the corresponding
algorithm is compiled into the OpenSSL library. NULL may be returned if
the algorithm fails to load dynamically.
NOTES
While compressing non-compressible data, the output may be larger than
the input. Care should be taken to size output buffers appropriate for
both compression and expansion.
Compression support and compression algorithms must be enabled and built
into the library before use. Refer to the INSTALL.md file when
configuring OpenSSL.
ZLIB may be found at <https://zlib.net>
Brotli may be found at <https://github.com/google/brotli>.
Zstandard may be found at <https://github.com/facebook/zstd>.
Compression of SSL/TLS records is not recommended, as it has been shown
to lead to the CRIME attack <https://en.wikipedia.org/wiki/CRIME>. It
is disabled by default, and may be enabled by clearing the
SSL_OP_NO_COMPRESSION option and setting the security level as
appropriate. See the documentation for the SSL_CTX_set_options(3) and
SSL_set_options(3) functions.
Compression is also used to support certificate compression as described
in RFC8879 <https://datatracker.ietf.org/doc/html/rfc8879>. It may be
disabled via the SSL_OP_NO_TX_CERTIFICATE_COMPRESSION and
SSL_OP_NO_RX_CERTIFICATE_COMPRESSION options of the
SSL_CTX_set_options(3) or SSL_set_options(3) functions.
COMP_zlib(), COMP_brotli() and COMP_zstd() are stream-based compression
methods. Internal state (including compression dictionary) is
maintained between calls. If an error is returned, the stream is
corrupted, and should be closed.
COMP_zlib_oneshot(), COMP_brotli_oneshot() and COMP_zstd_oneshot() are
not stream-based. These methods do not maintain state between calls. An
error in one call does not affect future calls.
RETURN VALUES
COMP_CTX_new() returns a COMP_CTX on success, or NULL on failure.
COMP_CTX_get_method(), COMP_zlib(), COMP_zlib_oneshot(), COMP_brotli(),
COMP_brotli_oneshot(), COMP_zstd(), and COMP_zstd_oneshot() return a
COMP_METHOD on success, or NULL on failure.
COMP_CTX_get_type() and COMP_get_type() return a NID value. On failure,
NID_undef is returned.
COMP_compress_block() and COMP_expand_block() return the number of bytes
stored in the output buffer out. This may be 0. On failure, -1 is
returned.
COMP_get_name() returns a const char * that must not be freed on
success, or NULL on failure.
BIO_f_zlib(), BIO_f_brotli() and BIO_f_zstd() return NULL on error, and
a BIO_METHOD on success.
SEE ALSO
BIO_new(3), SSL_CTX_set_options(3), SSL_set_options(3)
HISTORY
Brotli and Zstandard functions were added in OpenSSL 3.2.
COPYRIGHT
Copyright 2022-2024 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
<https://www.openssl.org/source/license.html>.
3.5.7 2026-06-09 COMP_CTX_NEW(3SSL)
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