Hand rolled keysharing

Question

Framing info

I'm traveling and don't have access to my paper documents, and don't have a working phone setup, or easy access to a printer or a fax. Yes, I could find an internet cafe and print things up and then fax them. But I hoped to find an easier, but still secure solution, that only relied on me having network connectivity, and the guy on the other end not being cryptographically sophisticated. Of course it'd be easier to fax or snail mail things in other circumstances.

And yes, I know the documents in question aren't very interesting. And yes, I know I could have been man-in-the-middled, and the emails from my mortgage guy may have in principled been spoofed. I wasn't trying to PUBLISH a new key-signing algorithm. Just to figure out what's the best, simple ad hoc solution I could craft in a MacGuyver-like spirit.

Question

I want to send documents securely to my mortgage guy to apply for a refinance. Normal people don't know public key encryption from public toilets. What to do?

What I did?

Make an account for him on a web server I control. In the https directories, though that may be overkill. Email him the url and instructions for how to generate the password (combine this word and your phone number kind of thing). He goes to that location and retrieves the file which is a second password. He emails me that he's obtained and copied the password. I log into the webserver, check the access logs that no one but me and him have accessed the password file. Then I delete that file, assign his account the password it contained, and restart Apache. I then scp the documents to a new url, secured by the new password which I know only he and I have seen. Unless I've been rootkitted and someone has changed my logs, but in that case I'm screwed lots of different ways.

Answer 1

I would have just stopped by his office and dropped off the documents...

Answer 2

I'm a little confused. I'm guessing the mortgage guy was too.

Why didn't you just call him and give him a password?

Or, since you understand PKI, why not get a certificate, send him a signed S/MIME email, and ask him to send you back an encrypted message with a password he chose? Most mail clients support S/MIME out of the box and make this pretty easy.

Answer 3

I didn't quite get what's your problem.

Normal people don't know public key encryption from public toilets. What to do?

You want to do a safe key exchange without public key cryptography or higher mathematical techniques like Diffie-Hellman?

Just tell him the passwort/meet him or use encrypted emails (e.g. enigmail)

Answer 4

Here are the most logical simple options:

1. Fax it.
2. Drop it off at his office.
3. Courier it by recorded delivery.

If software is really the option you wish to pursue, which doesn't seem really useful in this instance, and far more complicated than necessary, the simplest solution is probably the best:

1. Set up an HTTPS server which has authentication, email him the address and username from separate email accounts going to separate email acccounts, call him and give him the password and have him log into your HTTPS server and download the documents over SSL. This keeps the server address, username and password all separate and means that someone would have to go to fairly significant lengths to get all three pieces of information. So unless you're trying to foil the CIA or NSA... and I think it's pretty safe to say, they don't really care too much about your mortgage application, you'll be OK.

If I were the mortgage guy and you wanted me to go to all this effort just to get a document off you, I'd probably tell you that you're off your rocker and to fax it or bring it over, or I'd pick it up next time I was in your area.

What you could do is print it out, shred it and send every third strip in a ziploc bag with a different courier each numbered so that it would be simple to reconstruct at the other end. If he's missing any of the bits, he can email you to tell you which bits he's missing and you could shred another copy and send the missing bits with a separate courier.

Needless to say, I think you're over-engineering an unnecessary solution...