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DESCRIPTION

CMS_get0_signers() retrieves the signing certificate(s) from cms. It must be called after a successful CMS_verify() operation.

Normally the verify process proceeds as follows.

Initially some sanity checks are performed on cms. There must be at least one signature on the data. 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 itself. If any signing certificate cannot be located, the operation fails.

Each signing certificate is chain verified using the 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 store, any internal CRLs are used in addition to attempting to look them up in store. If any chain verify fails, an error code is returned.

Finally the signed content is read (and written to out if it is not NULL) and the signature is checked.

If all signatures verify correctly, then the function is successful.

Any of the following flags (OR'ed together) can be passed to change the default verify behaviour:

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 certs parameter.

CMS_NOCRL

If CRL checking is enabled in store, then any CRLs in the message itself are ignored.

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.

CMS_NO_SIGNER_CERT_VERIFY

Do not verify signing certificates.

CMS_NO_ATTR_VERIFY

Do not check the signed attributes signature.

CMS_NO_CONTENT_VERIFY

Do not check the content digest.

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 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 signers 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).