To create encrypted archives that can be opened both in Mac OSX and in Windows, we recommend an open-source application called TrueCrypt. This software allows for a similar user experience in OSX and in Windows - while assuring that encrypted archives created in OSX can be opened in Windows (and vice versa).
Head to truecrypt.org, where you can download the latest version of TrueCrypt for your operating system. The software is open-source, so it's free.
In OSX or in Windows, it's now easy to create a cross-platform encrypted archive. Although the instructions are nearly identical for OSX and Windows, users who wish to see directions with Windows screenshots should follow TrueCrypt's beginner's tutorial.
- Select the TrueCrypt application from your applications folder or Windows start menu. The main TrueCrypt window should appear. Click create volume, outlined in red in the image below.
- The creation wizard should appear. Select Create an encrypted file container and click next.
- In the next menu, select Standard TrueCrypt volume and click next.
- Next, you need to specify where you want the TrueCrypt volume (file container) to be created. A TrueCrypt container acts as any other file in that it can be moved, deleted, and so on. Click select file.
- A standard window will appear. Here you should name your file container and choose its save location. For these instructions I have named the file container 'Top Confidentiality'. Click save when finished. The file selector window will disappear and our focus will return to the volume creation wizard.
- In the volume creation wizard, click next.
- Here you can choose an encryption algorithm and a hash algorithm for the volume. We recommend sticking with the defaults and clicking next.
- Next you'll have a chance to specify volume size. For the purposes of this demonstration I've chosen 500 MB. Make sure to create a volume large enough to accommodate all of the files and folders you want to encrypt. Once you've specified the volume size, click next.
- It's time to choose a password! Make sure to read the information in the wizard about password selection. It should be strong, tough to crack, but memorable for you. After entering your chosen password in both input fields, click next.
- In the next step you may choose what type of filesystem your volume will be. Choosing the FAT filesystem will ensure that you can read and write to the file container whether on a Mac or Windows. When you're finished selecting, click next.
- This is the most entertaining part of the creation wizard. Move your mouse randomly in every which way for as long as you can. It is recommended you do so for at least 30 seconds, though the longer you do it, the better. This will strengthen the encryption keys. When you've had enough wild movements, click Format.
- Success! A window will appear with this message.
- After clicking OK, the creation wizard will ask if you'd like to create another volume or exit. Unless you need to create a new volume, click Exit.
Now it's time to learn how to use your file container.
We will now return to the main TrueCrypt window. It should still be open, but if it is not, relaunch TrueCrypt from your applications folder.
- Select a drive. On OSX these will be labeled as numbers; on Windows these will be labeled as letters. In the image below I have selected drive 2. Then click Select file. The usual file selector window will appear. Navigate to your file volume and click open.
- In the main TrueCrypt window, click Mount.
- The password prompt dialog should appear. Enter your password and click OK. TrueCrypt will attempt to mount the volume.
- Double-click the volume mounted in order to use it. After, a window will appear, much like opening an empty folder. This is your file container. You can copy files (or folders) to and from the TrueCrypt volume just as you would any normal disk.
- When you are done and ready to stop using your file container, remember to dismount. Go back to the TrueCrypt main window, select the mounted volume, and then click dismount. Of course, if you would like to access the files again, you will have to remount the volume, repeating steps 1-4.
Note: You can attach an encrypted archive to an email just as you would attach any other file. Remember, though, that you'll also need to relay the archive password to the recipient. Whatever you do, don't include the password in the text of a message, along with the attached encrypted archive!