CMS_verify, CMS_get0_signers - verify a CMS SignedData structure
#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);
CMS_verify() is very similar to
PKCS7_verify(3). It verifies a
CMS SignedData structure contained in a structure of type
CMS_ContentInfo.
cms points to the
CMS_ContentInfo
structure to verify. The optional
certs parameter refers to a set of
certificates in which to search for signing certificates.
cms may
contain extra untrusted CA certificates that may be used for chain building as
well as CRLs that may be used for certificate validation.
store may be
NULL or point to the trusted certificate store to use for chain verification.
indata refers to the signed data if the content is detached from
cms. Otherwise
indata should be NULL and the signed data must be
in
cms. The content is written to the BIO
out unless it is NULL.
flags is an optional set of flags, which can be used to modify the
operation.
CMS_get0_signers() retrieves the signing certificate(s) from
cms,
it may only be called after a successful
CMS_verify() operation.
Normally the verify process proceeds as follows.
Initially some sanity checks are performed on
cms. The type of
cms
must be SignedData. There must be at least one signature on the data and if
the content is detached
indata cannot be NULL.
An attempt is made to locate all the signing certificate(s), first looking in
the
certs parameter (if it is not NULL) and then looking in any
certificates contained in the
cms structure unless
CMS_NOINTERN
is set. If any signing certificate cannot be located the operation fails.
Each signing certificate is chain verified using the
smimesign purpose
and using the trusted certificate store
store if supplied. Any internal
certificates in the message, which may have been added using
CMS_add1_cert(3), are used as untrusted CAs. If CRL checking is enabled
in
store and
CMS_NOCRL is not set, any internal CRLs, which may
have been added using
CMS_add1_crl(3), are used in addition to
attempting to look them up in
store. If
store is not NULL and
any chain verify fails an error code is returned.
Finally the signed content is read (and written to
out unless it is NULL)
and the signature is checked.
If all signatures verify correctly then the function is successful.
Any of the following flags (ored together) can be passed in the
flags
parameter to change the default verify behaviour.
If
CMS_NOINTERN is set the certificates in the message itself are not
searched when locating the signing certificate(s). This means that all the
signing certificates must be in the
certs parameter.
If
CMS_NOCRL is set and CRL checking is enabled in
store then any
CRLs in the message itself are ignored.
If the
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.
If
CMS_NO_SIGNER_CERT_VERIFY is set the signing certificates are not
chain verified, unless
CMS_CADES flag is also set.
If
CMS_NO_ATTR_VERIFY is set the signed attributes signature is not
verified, unless CMS_CADES flag is also set.
If
CMS_CADES is set, each signer certificate is checked against the ESS
signingCertificate or ESS signingCertificateV2 extension that is required in
the signed attributes of the signature.
If
CMS_NO_CONTENT_VERIFY is set then the content digest is not checked.
One application of
CMS_NOINTERN is to only accept messages signed by a
small number of certificates. The acceptable certificates would be passed in
the
certs parameter. In this case if the signer certificate is not one
of the certificates supplied in
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 signer certificates manually using the
signed data utility functions.
Care should be taken when modifying the default verify behaviour, for example
setting
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
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).
CMS_verify() returns 1 for a successful verification and 0 if an error
occurred.
CMS_get0_signers() returns all signers or NULL if an error occurred.
The error can be obtained from
ERR_get_error(3)
The trusted certificate store is not searched for the signing certificate. This
is primarily due to the inadequacies of the current
X509_STORE
functionality.
The lack of single pass processing means that the signed content must all be
held in memory if it is not detached.
PKCS7_verify(3),
CMS_add1_cert(3),
CMS_add1_crl(3),
OSSL_ESS_check_signing_certs(3),
ERR_get_error(3),
CMS_sign(3)
Copyright 2008-2022 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.