=pod
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=head1 NAME
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CMS_compress - create a CMS CompressedData structure
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=head1 SYNOPSIS
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#include <openssl/cms.h>
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CMS_ContentInfo *CMS_compress(BIO *in, int comp_nid, unsigned int flags);
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=head1 DESCRIPTION
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CMS_compress() creates and returns a CMS CompressedData structure. B<comp_nid>
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is the compression algorithm to use or B<NID_undef> to use the default
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algorithm (zlib compression). B<in> is the content to be compressed.
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B<flags> is an optional set of flags.
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=head1 NOTES
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The only currently supported compression algorithm is zlib using the NID
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NID_zlib_compression.
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If zlib support is not compiled into OpenSSL then CMS_compress() will return
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an error.
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If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are
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prepended to the data.
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Normally the supplied content is translated into MIME canonical format (as
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required by the S/MIME specifications) if B<CMS_BINARY> is set no translation
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occurs. This option should be used if the supplied data is in binary format
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otherwise the translation will corrupt it. If B<CMS_BINARY> is set then
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B<CMS_TEXT> is ignored.
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If the B<CMS_STREAM> flag is set a partial B<CMS_ContentInfo> structure is
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returned suitable for streaming I/O: no data is read from the BIO B<in>.
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The compressed data is included in the CMS_ContentInfo structure, unless
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B<CMS_DETACHED> is set in which case it is omitted. This is rarely used in
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practice and is not supported by SMIME_write_CMS().
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=head1 NOTES
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If the flag B<CMS_STREAM> is set the returned B<CMS_ContentInfo> structure is
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B<not> complete and outputting its contents via a function that does not
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properly finalize the B<CMS_ContentInfo> structure will give unpredictable
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results.
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Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
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PEM_write_bio_CMS_stream() finalize the structure. Alternatively finalization
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can be performed by obtaining the streaming ASN1 B<BIO> directly using
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BIO_new_CMS().
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Additional compression parameters such as the zlib compression level cannot
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currently be set.
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=head1 RETURN VALUES
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CMS_compress() returns either a CMS_ContentInfo structure or NULL if an error
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occurred. The error can be obtained from ERR_get_error(3).
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=head1 SEE ALSO
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L<ERR_get_error(3)>, L<CMS_uncompress(3)>
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=head1 HISTORY
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The B<CMS_STREAM> flag was added in OpenSSL 1.0.0.
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=head1 COPYRIGHT
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Copyright 2008-2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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