CryptMsgOpenToDecode

[DllImport("Crypt32.dll", SetLastError = true)]
public static extern IntPtr CryptMsgOpenToDecode(
   uint dwMsgEncodingType,
   uint dwFlags,
   uint dwMsgType,
   IntPtr hCryptProv,
   IntPtr pRecipientInfo,
   IntPtr pStreamInfo
);

Microsoft documentation: Link