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.\" ========================================================================
.\"
.IX Title "CMS_DECRYPT 3"
.TH CMS_DECRYPT 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.nh
.SH "NAME"
CMS_decrypt \- decrypt content from a CMS envelopedData structure
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/cms.h>
\&
\& int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert,
\& BIO *dcont, BIO *out, unsigned int flags);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBCMS_decrypt()\fR extracts and decrypts the content from a \s-1CMS\s0 EnvelopedData
structure. \fBpkey\fR is the private key of the recipient, \fBcert\fR is the
recipient's certificate, \fBout\fR is a \s-1BIO\s0 to write the content to and
\&\fBflags\fR is an optional set of flags.
.PP
The \fBdcont\fR parameter is used in the rare case where the encrypted content
is detached. It will normally be set to \s-1NULL.\s0
.SH "NOTES"
.IX Header "NOTES"
Although the recipients certificate is not needed to decrypt the data it is
needed to locate the appropriate (of possible several) recipients in the \s-1CMS\s0
structure.
.PP
If \fBcert\fR is set to \s-1NULL\s0 all possible recipients are tried. This case however
is problematic. To thwart the \s-1MMA\s0 attack (Bleichenbacher's attack on
\&\s-1PKCS\s0 #1 v1.5 \s-1RSA\s0 padding) all recipients are tried whether they succeed or
not. If no recipient succeeds then a random symmetric key is used to decrypt
the content: this will typically output garbage and may (but is not guaranteed
to) ultimately return a padding error only. If \fBCMS_decrypt()\fR just returned an
error when all recipient encrypted keys failed to decrypt an attacker could
use this in a timing attack. If the special flag \fB\s-1CMS_DEBUG_DECRYPT\s0\fR is set
then the above behaviour is modified and an error \fBis\fR returned if no
recipient encrypted key can be decrypted \fBwithout\fR generating a random
content encryption key. Applications should use this flag with
\&\fBextreme caution\fR especially in automated gateways as it can leave them
open to attack.
.PP
It is possible to determine the correct recipient key by other means (for
example looking them up in a database) and setting them in the \s-1CMS\s0 structure
in advance using the \s-1CMS\s0 utility functions such as \fBCMS_set1_pkey()\fR. In this
case both \fBcert\fR and \fBpkey\fR should be set to \s-1NULL.\s0
.PP
To process KEKRecipientInfo types \fBCMS_set1_key()\fR or \fBCMS_RecipientInfo_set0_key()\fR
and \fBCMS_RecipientInfo_decrypt()\fR should be called before \fBCMS_decrypt()\fR and
\&\fBcert\fR and \fBpkey\fR set to \s-1NULL.\s0
.PP
The following flags can be passed in the \fBflags\fR parameter.
.PP
If the \fB\s-1CMS_TEXT\s0\fR flag is set \s-1MIME\s0 headers for type \fBtext/plain\fR are deleted
from the content. If the content is not of type \fBtext/plain\fR then an error is
returned.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBCMS_decrypt()\fR returns either 1 for success or 0 for failure.
The error can be obtained from \fBERR_get_error\fR\|(3)
.SH "BUGS"
.IX Header "BUGS"
The lack of single pass processing and the need to hold all data in memory as
mentioned in \fBCMS_verify()\fR also applies to \fBCMS_decrypt()\fR.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBERR_get_error\fR\|(3), \fBCMS_encrypt\fR\|(3)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2008\-2016 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
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