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FreeOTFE4PDA is an encryption system which allows "virtual storage cards" to be created on a Windows Mobile device. Files stored on one of these virtual storage cards are automatically encrypted and decrypted on-the-fly.
Many different options are available, including many cyphers such as AES, Twofish, Serpent, MARS and hash algorithms (e.g. SHA512, RIPEMD, Tiger, Whirlpool, etc)
A PC version (FreeOTFE) is also available, and fully compatiable with the PDA version.
Source(s): Encryption system