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+.\" $OpenBSD: CMS_decrypt.3,v 1.3 2019/08/10 23:41:22 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,
+.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+.\" OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd $Mdocdate: August 10 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
+was added to OpenSSL 0.9.8.
+.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 .