1=pod
2
3=head1 NAME
4
5CMS_verify, CMS_get0_signers - verify a CMS SignedData structure
6
7=head1 SYNOPSIS
8
9 #include <openssl/cms.h>
10
11 int CMS_verify(CMS_ContentInfo *cms, STACK_OF(X509) *certs, X509_STORE *store,
12                BIO *indata, BIO *out, unsigned int flags);
13
14 STACK_OF(X509) *CMS_get0_signers(CMS_ContentInfo *cms);
15
16=head1 DESCRIPTION
17
18CMS_verify() verifies a CMS SignedData structure. B<cms> is the CMS_ContentInfo
19structure to verify. B<certs> is a set of certificates in which to search for
20the signing certificate(s). B<store> is a trusted certificate store used for
21chain verification. B<indata> is the detached content if the content is not
22present in B<cms>. The content is written to B<out> if it is not NULL.
23
24B<flags> is an optional set of flags, which can be used to modify the verify
25operation.
26
27CMS_get0_signers() retrieves the signing certificate(s) from B<cms>, it may only
28be called after a successful CMS_verify() operation.
29
30=head1 VERIFY PROCESS
31
32Normally the verify process proceeds as follows.
33
34Initially some sanity checks are performed on B<cms>. The type of B<cms> must
35be SignedData. There must be at least one signature on the data and if
36the content is detached B<indata> cannot be B<NULL>.
37
38An attempt is made to locate all the signing certificate(s), first looking in
39the B<certs> parameter (if it is not NULL) and then looking in any
40certificates contained in the B<cms> structure itself. If any signing
41certificate cannot be located the operation fails.
42
43Each signing certificate is chain verified using the B<smimesign> purpose and
44the supplied trusted certificate store. Any internal certificates in the message
45are used as untrusted CAs. If CRL checking is enabled in B<store> any internal
46CRLs are used in addition to attempting to look them up in B<store>. If any
47chain verify fails an error code is returned.
48
49Finally the signed content is read (and written to B<out> if it is not NULL)
50and the signature's checked.
51
52If all signature's verify correctly then the function is successful.
53
54Any of the following flags (ored together) can be passed in the B<flags>
55parameter to change the default verify behaviour.
56
57If B<CMS_NOINTERN> is set the certificates in the message itself are not
58searched when locating the signing certificate(s). This means that all the
59signing certificates must be in the B<certs> parameter.
60
61If B<CMS_NOCRL> is set and CRL checking is enabled in B<store> then any
62CRLs in the message itself are ignored.
63
64If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
65from the content. If the content is not of type B<text/plain> then an error is
66returned.
67
68If B<CMS_NO_SIGNER_CERT_VERIFY> is set the signing certificates are not
69verified, unless CMS_CADES flag is also set.
70
71If B<CMS_NO_ATTR_VERIFY> is set the signed attributes signature is not
72verified, unless CMS_CADES flag is also set.
73
74If B<CMS_CADES> is set, each signer certificate is checked against the
75ESS signingCertificate or ESS signingCertificateV2 extension
76that is required in the signed attributes of the signature.
77
78If B<CMS_NO_CONTENT_VERIFY> is set then the content digest is not checked.
79
80=head1 NOTES
81
82One application of B<CMS_NOINTERN> is to only accept messages signed by
83a small number of certificates. The acceptable certificates would be passed
84in the B<certs> parameter. In this case if the signer is not one of the
85certificates supplied in B<certs> then the verify will fail because the
86signer cannot be found.
87
88In some cases the standard techniques for looking up and validating
89certificates are not appropriate: for example an application may wish to
90lookup certificates in a database or perform customised verification. This
91can be achieved by setting and verifying the signers certificates manually
92using the signed data utility functions.
93
94Care should be taken when modifying the default verify behaviour, for example
95setting B<CMS_NO_CONTENT_VERIFY> will totally disable all content verification
96and any modified content will be considered valid. This combination is however
97useful if one merely wishes to write the content to B<out> and its validity
98is not considered important.
99
100Chain verification should arguably be performed using the signing time rather
101than the current time. However, since the signing time is supplied by the
102signer it cannot be trusted without additional evidence (such as a trusted
103timestamp).
104
105=head1 RETURN VALUES
106
107CMS_verify() returns 1 for a successful verification and zero if an error
108occurred.
109
110CMS_get0_signers() returns all signers or NULL if an error occurred.
111
112The error can be obtained from L<ERR_get_error(3)>
113
114=head1 BUGS
115
116The trusted certificate store is not searched for the signing certificate,
117this is primarily due to the inadequacies of the current B<X509_STORE>
118functionality.
119
120The lack of single pass processing means that the signed content must all
121be held in memory if it is not detached.
122
123=head1 SEE ALSO
124
125L<OSSL_ESS_check_signing_certs(3)>,
126L<ERR_get_error(3)>, L<CMS_sign(3)>
127
128=head1 COPYRIGHT
129
130Copyright 2008-2021 The OpenSSL Project Authors. All Rights Reserved.
131
132Licensed under the Apache License 2.0 (the "License").  You may not use
133this file except in compliance with the License.  You can obtain a copy
134in the file LICENSE in the source distribution or at
135L<https://www.openssl.org/source/license.html>.
136
137=cut
138