Callback used with CRYPT_VERIFY_MESSAGE_PARA to locate a certficate by issuer and serial nbr. This function will receive 4 args: 1. Arbitrary context object given as GetArg in CRYPT_VERIFY_MESSAGE_PARA 2. CertEncodingType (int) - specifies the type of encoding used 3. SignerId - Dict containing issuer and serial nbr that uniquely identifies a certificate 4. PyCERTSTORE containing certificates extracted from the message Function must return a PyCERT_CONTEXT. If no certificate could be found, it should raise pywintypes.error(winerror.CRYPT_E_NO_MATCH) If this function is not specified, the default action is to locate a certificate encoded in the message.