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GPG works on OS X, Linux, and Windows with extensive selection of wrappers.
Pro Open-source and battle-tested
GPG is the oldest and most reliable encryption software available.
Pro Multiple types of encryption algorithms
GPG supports public key cryptography (RSA EIGamal, DSA), symmetrical key algorithms (Blowfish, AES, IDEA, etc), cryptographic hash functions (RIPEMD, SHA), and compression (ZIP, ZLIB, BZIP2).
Pro Volume and individual file encryption
With GPG you can encrypt you whole volume or files individually.
Pro Supports paired keys
Allows for encrypted communication.
Pro Supports expiring signatures
GPG keys by default expire after a set amount of time. The amount can be changed and this feature can be turned off.
Pro Both CLI and GUI versions available
GPG can be installed as a command line tool, or you can choose between several different GUI frontends available for it.
Con It may be hard to find a GUI frontend that suits your needs
If you decide not to use the CLI version of GPG, it may be hard to find a GPG GUI version that suits your needs simply because of the sheer number of different versions available.