UPDATE: TrueCrypt abruptly shut down on May 28, claiming its software "may contain unfixed security issues." If you still wish to use TrueCrypt, we recommend you use version 7.1, as it appears that 7.2 is the version to which the creators were referring. An independent security audit is currently investigating TrueCrypt's code, but for now TrueCrypt 7.1 is probably still safe.
The documents leaked by former NSA contractor Edward Snowden have revealed that the U.S. government has the ability to spy on the electronic communications of almost anyone in the world — whether or not it actually does.
To protect your files from anyone who wants to read them, use encryption software. One of the best free encryption programs is called TrueCrypt.
By default, most files are stored in "plaintext" (i.e., a text that is readable to humans). Encrypted files, however, are stored in an encoded form, so without the proper decryption password they look like random characters to human eyes.
To encrypt a file you'll need two things: an encryption algorithm and a password. These are roughly analogous to a lock and a key; the algorithm is the means by which the to-be-encrypted data will be scrambled, and the password is the key that, when put through the algorithm, will decrypt the data and make it readable again.
Theoretically, there's no such thing as "uncrackable" encryption. But the more complex the password is, the longer it'll take any code-breaking software to crack it. A long password that contains no recognizable words or patterns such as birth dates could take years to break.
That's why security expert Jennifer Granick, director of civil liberties at Stanford Law School's Center for Internet and Society,told Tom's Guide that "encryption and decryption is probably the best answer we have so far" for keeping data safe.