Verifying a Countersignature

To verify a countersignature

1. Call CryptMsgOpenToDecode to get a handle to the signed message.
2. Get a pointer to the countersigner's certificate ( CERT_INFO).
3. Call CryptMsgGetParam to retrieve the signer information from the message.
4. Call CryptMsgGetParam to retrieve the countersignature from the message.
5. Call CryptMsgVerifyCountersignatureEncoded to verify the countersignature.

If the CryptMsgVerifyCountersignatureEncoded function call succeeds, the countersignature is verified.