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=pod
=head1 NAME
CMS_verify - verify a CMS signedData structure
=head1 SYNOPSIS
#include <openssl/cms.h>
int CMS_verify(CMS_ContentInfo *cms, STACK_OF(X509) *certs, X509_STORE *store, BIO *indata, BIO *out, unsigned int flags);
STACK_OF(X509) *CMS_get0_signers(CMS_ContentInfo *cms);
=head1 DESCRIPTION
CMS_verify() verifies a CMS signedData structure. B<cms> is the CMS_ContentInfo
structure to verify. B<certs> is a set of certificates in which to search for
the signer's certificate. B<store> is a trusted certficate store (used for
chain verification). B<indata> is the signed data if the content is not
present in B<cms> (that is it is detached). The content is written to B<out>
if it is not NULL.
B<flags> is an optional set of flags, which can be used to modify the verify
operation.
CMS_get0_signers() retrieves the signer's certificate(s) from B<cms>, it must
be called after a succeful CMS_verify() operation.
=head1 VERIFY PROCESS
Normally the verify process proceeds as follows.
Initially some sanity checks are performed on B<cms>. The type of B<cms> must
be signedData. There must be at least one signature on the data and if
the content is detached B<indata> cannot be B<NULL>.
An attempt is made to locate all the signer's certificates, first looking in
the B<certs> parameter (if it is not B<NULL>) and then looking in any
certificates contained in the B<cms> structure itself. If any signer's
certificates cannot be located the operation fails.
Each signer's certificate is chain verified using the B<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 B<store> any internal
CRLs are used in addition to attempting to look the up in B<store>. If any
chain verify fails an error code is returned.
Finally the signed content is read (and written to B<out> is it is not NULL)
and the signature's checked.
If all signature's verify correctly then the function is successful.
Any of the following flags (ored together) can be passed in the B<flags>
parameter to change the default verify behaviour.
If B<CMS_NOINTERN> is set the certificates in the message itself are not
searched when locating the signer's certificate. This means that all the signers
certificates must be in the B<certs> parameter.
If B<CMS_NOCRL> is set and CRL checking is enabled in B<store> then any
CRLs in the message itself are ignored.
If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
from the content. If the content is not of type B<text/plain> then an error is
returned.
If B<CMS_NO_SIGNER_CERT_VERIFY> is set the signer's certificates are not
verified.
If B<CMS_NO_ATTR_VERIFY> is set the signed attributes signature is not
verified.
If B<CMS_NO_CONTENT_VERIFY> is set then the content digest is not checked.
=head1 NOTES
One application of B<CMS_NOINTERN> is to only accept messages signed by
a small number of certificates. The acceptable certificates would be passed
in the B<certs> parameter. In this case if the signer is not one of the
certificates supplied in B<certs> then the verify will fail because the
signer cannot be found.
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.
Care should be taken when modifying the default verify behaviour, for example
setting B<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 B<out> and its validity
is not considered important.
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).
=head1 RETURN VALUES
CMS_verify() returns 1 for a successful verification and zero if an error
occured.
CMS_get0_signers() returns all signers or B<NULL> if an error occurred.
The error can be obtained from L<ERR_get_error(3)|ERR_get_error(3)>
=head1 BUGS
The trusted certificate store is not searched for the signers certificate,
this is primarily due to the inadequacies of the current B<X509_STORE>
functionality.
The lack of single pass processing and need to hold all data in memory as
mentioned in CMS_sign() also applies to CMS_verify().
=head1 SEE ALSO
L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_sign(3)|CMS_sign(3)>
=head1 HISTORY
CMS_verify() was added to OpenSSL 0.9.8
=cut
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