I need to give my boss backup copies of some sensitive data. I want to encrypt the data, but she needs to be able to decrypt it on her own, without technical support (as long as she has the password, of course). Additionally, the decryption might need to be done on either a Mac or a Windows computer, and she may not have the ability to install new software on it. Running a self-contained program off a USB key would be okay.
Here's what I've already considered:
Use an encrypted ZIP file - Works fine on Windows, but on the Mac double-clicking an encrypted zip file gives an obscure, misleading error message. You have to open it using the terminal, which might not even have zip installed.
Use TrueCrypt - It would be fine if you could run TrueCrypt off of a USB key, but it looks like both the Mac and Windows versions require you to run an installer.
Put the plaintext file on a web server with .htpasswd authentication - This may be what I fall back on if no one has a better idea, even though it's a joke in terms of actual security.
Additionally, this post has a similar question. The two main suggestions are AxCrypt (doesn't work for me because it's Windows-only) and using a dedicated SCP account (which I don't have the ability to create).
Can anyone suggest a cross-platform, portable encryption method that would work for this scenario?