PCAG have started attaching a digital certificate to all our downloadable documents containing macros. This is to provide our users with the confidence that the macros in these files have originated from us and have not been tampered with or corrupted in their journey from PCAG to the website and onto the user. This should provide better protection than if we sent out the files by post on disc. This minimises the risk of macro viruses being added to our documents once they have left PCAG. At PCAG we take the usual precautions against infection at our end. To date we have not had any problems but its better to be safe…
Users will notice that when they open a spreadsheet with macros they will get a Security Warning indicating that the file has macros originating from us. We suggest you enable the macros each time. We are using BT Ignite as the trusted third party. The details are on the digital certificate that can be accessed by clicking Details on the dialogue that appears when the file opens. We would now recommend not using files containing macros that purport to come from us if they do not carry this certificate as this may indicate they have been interfered with. Please let us know if you come across them so that we can be aware of potential threats – as well as ensure we have not just forgotten to sign the relevant document!
In addition we are routinely signing all executable Winzip files (.EXE) such as the self-extracting files used to download query sets or templates. Users can now be confident that these files have not been tampered with (e.g. substituting a malicious executable with the same name), providing they carry a digital certificate from us. To check this, the file can be saved to disc rather than run from current location. Locate the file, right click and choose Properties. Check the certificate on the Digital Signatures tab. To extract the contents simply double click on the file after closing the Properties Dialogue and proceed as usual.
One way of thinking of a digital certificate is of a combination padlock with a very long code that has to be entered to lock and unlock it. The trusted third party who has checked our identity thoroughly supplies this to us. It has been arranged that only we know both halves of the combination. We place our document in a sealed container and lock it with the padlock. We then enter our private half of the combination (which only we know) and print the other half of the combination on the outside. Now anyone can open the container by simply entering the other half of the combination. If the container is locked when they get it they know it hasn’t been interfered with because no one else knows the whole combination. It will not be possible for someone to unlock it, alter the contents and then re-lock it because they do not know our private half of the combination. The recipient also knows that the document must have come from us because we are the only people who could have locked it in.
In reality this is all done with some nifty mathematics using very large prime numbers – but the computer handles this without you having to worry about it.