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+.\" $OpenBSD: CMS_verify.3,v 1.3 2019/08/10 23:41:22 schwarze Exp $
+.\" full merge up to: OpenSSL 35fd9953 May 28 14:49:38 2019 +0200
+.\"
+.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
+.\" Copyright (c) 2008 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_VERIFY 3
+.Os
+.Sh NAME
+.Nm CMS_verify ,
+.Nm CMS_get0_signers
+.Nd verify a CMS SignedData structure
+.Sh SYNOPSIS
+.In openssl/cms.h
+.Ft int
+.Fo CMS_verify
+.Fa "CMS_ContentInfo *cms"
+.Fa "STACK_OF(X509) *certs"
+.Fa "X509_STORE *store"
+.Fa "BIO *indata"
+.Fa "BIO *out"
+.Fa "unsigned int flags"
+.Fc
+.Ft STACK_OF(X509) *
+.Fo CMS_get0_signers
+.Fa "CMS_ContentInfo *cms"
+.Fc
+.Sh DESCRIPTION
+.Fn CMS_verify
+verifies a CMS SignedData structure.
+.Fa cms
+is the
+.Vt CMS_ContentInfo
+structure to verify.
+.Fa certs
+is a set of certificates in which to search for the signing
+certificate(s).
+.Fa store
+is a trusted certificate store used for chain verification.
+.Fa indata
+is the detached content if the content is not present in
+.Fa cms .
+The content is written to
+.Fa out
+if it is not
+.Dv NULL .
+.Pp
+.Fa flags
+is an optional set of flags, which can be used to modify the verify
+operation.
+.Pp
+.Fn CMS_get0_signers
+retrieves the signing certificate(s) from
+.Fa cms .
+It must be called after a successful
+.Fn CMS_verify
+operation.
+.Pp
+Normally the verify process proceeds as follows.
+.Pp
+Initially some sanity checks are performed on
+.Fa cms .
+The type of
+.Fa cms
+must be SignedData.
+There must be at least one signature on the data and if the content is
+detached;
+.Fa indata
+cannot be
+.Dv NULL .
+.Pp
+An attempt is made to locate all the signing certificate(s), first
+looking in the
+.Fa certs
+parameter (if it is not
+.Dv NULL )
+and then looking in any certificates contained in the
+.Fa cms
+structure itself.
+If any signing certificate cannot be located, the operation fails.
+.Pp
+Each signing certificate is chain verified using the
+.Sy smimesign
+purpose and the supplied trusted certificate store.
+Any internal certificates in the message are used as untrusted CAs.
+If CRL checking is enabled in
+.Fa store ,
+any internal CRLs are used in addition to attempting to look them up in
+.Fa store .
+If any chain verify fails, an error code is returned.
+.Pp
+Finally the signed content is read (and written to
+.Fa out
+if it is not
+.Dv NULL )
+and the signature is checked.
+.Pp
+If all signatures verify correctly, then the function is successful.
+.Pp
+Any of the following flags (OR'ed together) can be passed in the
+.Fa flags
+parameter to change the default verify behaviour:
+.Bl -tag -width Ds
+.It Dv CMS_NOINTERN
+Do not use the certificates in the message itself when
+locating the signing certificate(s).
+This means that all the signing certificates must be in the
+.Fa certs
+parameter.
+.It Dv CMS_NOCRL
+If CRL checking is enabled in
+.Fa store ,
+then any CRLs in the message itself are ignored.
+It Dv CMS_TEXT
+MIME headers for type text/plain are deleted from the content.
+If the content is not of type text/plain, an error is returned.
+.It Dv CMS_NO_SIGNER_CERT_VERIFY
+Do not verify signing certificates.
+.It Dv CMS_NO_ATTR_VERIFY
+Do not check the signed attributes signature.
+.It Dv CMS_NO_CONTENT_VERIFY
+Do not check the content digest.
+.El
+.Pp
+One application of
+.Dv CMS_NOINTERN
+is to only accept messages signed by a small number of certificates.
+The acceptable certificates would be passed in the
+.Fa certs
+parameter.
+In this case, if the signer is not one of the certificates supplied in
+.Fa certs ,
+then the verify will fail because the signer cannot be found.
+.Pp
+In some cases the standard techniques for looking up and validating
+certificates are not appropriate: for example an application may wish to
+lookup certificates in a database or perform customised verification.
+This can be achieved by setting and verifying the signers certificates
+manually using the signed data utility functions.
+.Pp
+Care should be taken when modifying the default verify behaviour, for
+example setting
+.Dv CMS_NO_CONTENT_VERIFY
+will totally disable all content verification and any modified content
+will be considered valid.
+This combination is however useful if one merely wishes to write the
+content to
+.Fa out
+and its validity is not considered important.
+.Pp
+Chain verification should arguably be performed using the signing time
+rather than the current time.
+However since the signing time is supplied by the signer it cannot be
+trusted without additional evidence (such as a trusted timestamp).
+.Sh RETURN VALUES
+.Fn CMS_verify
+returns 1 for a successful verification or zero if an error occurred.
+.Pp
+.Fn CMS_get0_signers
+returns all signers or
+.Dv NULL
+if an error occurred.
+.Pp
+The error can be obtained from
+.Xr ERR_get_error 3 .
+.Sh SEE ALSO
+.Xr CMS_sign 3
+.Sh HISTORY
+.Fn CMS_verify
+was added to OpenSSL 0.9.8.
+.Sh BUGS
+The trusted certificate store is not searched for the signing certificate.
+This is primarily due to the inadequacies of the current
+.Vt X509_STORE
+functionality.
+.Pp
+The lack of single pass processing means that the signed content must
+all be held in memory if it is not detached.