Remember that I did not invent the PGP protocol. I wrote a tool that uses that protocol. So, I don't know if what you are suggesting is possible or not. But I can make an educated guess.
If what you are suggesting is possible, it would render the entire protocol (which has been around for something like 20 years) broken, invalid and insecure. It would undermine the integrity of vast untold quantities of data. Such a vulnerability would absolutely be newsworthy. And yet I've read no news about it. So of the possible explanations, what is most probable?
Such an obvious and easy to exploit vulnerability has existed for 20ish years, undiscovered/unexposed until one person on LW pointed it out?
The proposed security flaw sounds like maybe it might work, but doesnt.
I'd say #2 is more probable by several orders of magnitude
Such an obvious and easy to exploit vulnerability has existed for 20ish years, undiscovered/unexposed until one person on LW pointed it out?
It's not a vulnerability. I trust gnupg not to leak my private key, not the OpenPGP standard. I also trust gnupg not to delete all the files on my hard disk, etc. There's a difference between trusting software to securely implement a standard and trusting the standard itself.
For an even simpler "vulnerability" in OpenPGP look up section 13.1.1 in RFC4880; encoding a message before signing. Just replace the pseudo-random padding with bits from the private key. Decoding (section 13.1.2) does not make any requirements on the content of PS.
Information security is a pretty big passion of mine; I don't think someone needs to have "something to hide" in order to make use of digital signing, encryption, etc. Another passion of mine is making things easier for other people to do. I've written a couple of tools that I think can be useful for the LW crowd.
Online PGP Signature: This is an online javascript-based tool which allows you to sign messages using your PGP private key. I love the idea of PGP-signed messages (I remember someone under the pseudonym "Professor Quirrell" handing out PGP-verified Quirrell points a few years back). The problem is, I had yet to find an easy way to do this that didn't involve downloading command-line based software. So I wrote this tool that uses open-sourced, javascript-based PGP libraries to let you easily sign messages in your browser.
The whole thing is client-side so your private key is never seen by me, but be smart about security. If you don't trust me, that's fine, just don't use the tool. But also remember that you could have a virus, your computer could be monitored, someone could be watching over your shoulder, etc. so please be smart about your security. But hopefully this can be helpful.
Decoy: an iPhone App: I wrote this after "The Fappening", where I was basically appalled at the terrible security practices that pretty much everyone uses when sending pictures back and forth. Decoy uses a combination of steganography and AES encryption to let you send images back and forth without having to sign up for an account or use some outside service that can be hacked or otherwise compromised.
You take the original picture, then you come up with a passphrase, then you take a "decoy" picture. The original picture is converted to base64 image data, which is then AES-encrypted using your passphrase. The resulting cipher text is then encoded into the pixels of the "decoy" picture, which is what gets saved on your phone and sent out. The "decoy" pictures are indistinguishable from any other picture on your or your recipients' camera rolls, and unless you have the passphrase, then the original image is thoroughly inaccessible.
If your phone is lost, hacked, stolen, or (more benignly) someone happens to be looking through pictures on your phone, all anyone will see are the "decoy" pictures. Without the password, those pictures are worthless. Although the app is primarily branded for, *ahem*, "personal use", there are plenty of other ways to use it. For example, my wife and I use it for things like sending pictures of sensitive physical documents like credit cards, birth certificates, social security cards, etc.
(full disclosure: although Decoy is free, it is ad-supported so I do financially benefit from people using the app. But on the bright side I'm an avowed rationalist and if I make a quajillion dollars with this app I will spend the vast majority of it on LW-friendly causes!)