There are oodles of encryption algorithms and encrypting a file is quite simple.

# openssl enc -aes-256-cbc -in file -out file.enc
enter aes-256-cbc encryption password: password
Verifying - enter aes-256-cbc encryption password: password
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To decrypt, simply swap your -in and -out file parameters and add a -d to the command line:

# openssl enc -d -aes-256-cbc -in file.enc -out file
enter aes-256-cbc encryption password: password
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Source: http://www.unix.com/aix/28903-file-password-protection-encryption.html