How to programitically create password protected file

Clarifications

If they haven't got PGP, then you could try GnuPG which is Linux/Windos/Mac supported (http://www.gnupg.org/), free, but whoever you're sending to will need it as well.

If this isn't an option, you could try 7zip (http://www.7-zip.org/download.html), again, there's versions for windows/linux etc to create a zip file which is password protected. This again is free and has a command line version which means you can do it programatically.

1. 7zip, you might need to install the p7zip package, and you can make a password protected zip file.

EXAMPLE:
To encrypt
[pre]7za a -tzip protected.zip somefile.txt -ppassword[/pre]

To extract
[pre]7za x protected.zip[/pre]
(then type the password "password" when promoted, or use the -p option to skip the prompt)

2. Openssl, its installed on most linux distros and macs. There are several encryption options depending on the cyphyer you want, I used des3 for the example since its easy to type :)

To encrypt a file:
[pre]openssl des3 -a -salt -k password -in normalfile.txt -out encryptedfile.txt[/pre]

To decrypt the file:
[pre]openssl des3 -d -a -k password -in encryptedfile.txt -out normalfile.txt[/pre]

You can remove the -k and will be prompted to type the password on the fly.