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.\" $OpenBSD: CMS_decrypt.3,v 1.4 2019/08/11 12:46:38 schwarze Exp $
.\" full merge up to: OpenSSL e9b77246 Jan 20 19:58:49 2017 +0100
.\"
.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
.\" Copyright (c) 2008, 2014 The OpenSSL Project. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: August 11 2019 $
.Dt CMS_DECRYPT 3
.Os
.Sh NAME
.Nm CMS_decrypt
.Nd decrypt content from a CMS envelopedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft int
.Fo CMS_decrypt
.Fa "CMS_ContentInfo *cms"
.Fa "EVP_PKEY *pkey"
.Fa "X509 *cert"
.Fa "BIO *dcont"
.Fa "BIO *out"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
.Fn CMS_decrypt
extracts and decrypts the content from a CMS EnvelopedData structure.
.Fa pkey
is the private key of the recipient,
.Fa cert
is the recipient's certificate,
.Fa out
is a
.Vt BIO
to write the content to and
.Fa flags
is an optional set of flags.
.Pp
The
.Fa dcont
parameter is used in the rare case where the encrypted content is
detached.
It will normally be set to
.Dv NULL .
.Pp
Although the recipients certificate is not needed to decrypt the data it
is needed to locate the appropriate (of possible several) recipients in
the CMS structure.
.Pp
If
.Fa cert
is set to
.Dv NULL ,
all possible recipients are tried.
This case however is problematic.
To thwart the MMA attack (Bleichenbacher's attack on PKCS #1 v1.5 RSA
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
.Fn CMS_decrypt
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
.Dv CMS_DEBUG_DECRYPT
is set, then the above behaviour is modified and an error
.Em is
returned if no recipient encrypted key can be decrypted
.Em without
generating a random content encryption key.
Applications should use this flag with extreme caution
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 CMS
structure in advance using the CMS utility functions such as
.Xr CMS_set1_pkey 3 .
In this case both
.Fa cert
and
.Fa pkey
should be set to
.Dv NULL .
.Pp
To process KEKRecipientInfo types,
.Xr CMS_set1_key 3
or
.Xr CMS_RecipientInfo_set0_key 3
and
.Xr CMS_RecipientInfo_decrypt 3
should be called before
.Fn CMS_decrypt
and
.Fa cert
and
.Fa pkey
set to
.Dv NULL .
.Pp
The following flags can be passed in the
.Fa flags
parameter:
.Pp
If the
.Dv CMS_TEXT
flag is set, MIME headers for type text/plain
are deleted from the content.
If the content is not of type text/plain,
then an error is returned.
.Sh RETURN VALUES
.Fn CMS_decrypt
returns either 1 for success or 0 for failure.
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr CMS_encrypt 3
.Sh HISTORY
.Fn CMS_decrypt
first appeared in OpenSSL 0.9.8h
and has been available since
.Ox 6.6 .
.Sh BUGS
The lack of single pass processing and the need to hold all data in
memory as mentioned in
.Xr CMS_verify 3
also applies to
.Fn CMS_decrypt .
|
