Stu... I once had to write a "Secure SMTP" server in Java... The easiest, and quickest way to do this is to download and/or purchase PGP. They have an SDK that you can use to access in anything you want.
I'd have to go back and see if I had to write a COM wrapper, or if they already had one. (I wrote this SMTP server about 10 years ago). Anyways, don't get discouraged. About 5 years ago, I wrote an entire PGP based application (based on the openPGP RFC) in C++, but the catch was, I was NOT allowed to use any existing libraries. So I had to write all that stuff myself. And, I used GPG, OpenPGP, and PGP for testing, etc....
So, I could even provide help for you on how to decode this stuff in VBA. It's not impossible, (it may be slow as hell, but not impossible), and I'm NOT one to "shell out and run cmdline stuff to do work like this for you, as it will open you up to some SERIOUS security risks, as hurcane's suggestion (for example) will cause your passphrase to be displayed to tools like ProcExp). The first step is learning how PKE works, etc. Then, the steps you need to do to get what you want.
This is something I'd be interested in helping with since I'm always one to write code that everyone says can't be done. :) Plus, I own the source code of the app I wrote, because of of mergers, closures, etc...
It was originally written for the Oil and Gas industry, so I know it's secure. That's not to say I don't have ANY security flaws in the code, but I think it's stable. I know I have an issue with my Chinese Remainder Threory code.. For some reason when I use that short-cut, I can't decode the data correctly, but if I use the RSA "long way" it works...
Now, this application was never fully finished, so I don't support things like DSA Key-pairs, but I do support RSA key pairs, with SHA1, MD5, using IDEA, AES, (I THINK my 3DES code does not work correctly, but I may have fixed that since). I didn't implement compression yet, etc... But, I'd love a reason to go back and work on this code again.
I /COULD/ make you a COM object that you could call from VBA passing the original Base64 data in, along with the Base64 key data, (or a pointer to a key file on disk), and a passpsshrase to decode files....
Think about it... Let me know..
Over the years, I have collected vbScript code for doing things like MD5, SHA1, IDEA, and other crypto routines, but I didn't write them. Hell, you could probably just interface with Microsoft's CryptoAPI, and break each action down to it's core parts and still get it to work. (You will not find a Micosoft CryptoAPI call like "DecryptPGP()"... It'd all have to be done in chunks).
Lemme know if I can help.