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.
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