If your journal is typed in Microsoft Word of NeoOffice Writer, then you can save the document with a password. In Word click Options… in the save dialogue then enter a password there. In Writer, click Save As… then click the “Save with Password” checkbox. I do not know if that helps at all.

Also, if you are not using a word file, or even if you are but wanted further protection, then you could change the file permissions either in “Get info” or from the terminal, so that it the file could only be read when you are signed in under your own username, or with root privileges. To do this, you would open the “Get Info Dialogue,” scroll down to Permissions, then change everything but “Owner” to “No Access.” You may need to enter your password at some point. However, this would only help if all the users of your laptop use different usernames and passwords.

Finally, this will not password protect your file, but you could hide the file from prying eyes by opening the terminal, navigating to the directory in which your journal is stored, and renaming the file with a “.” before it. The command to rename is mv. For example:

mv MyJournal.txt .MyJournal.txt

would do the trick, obviously replacing “MyJournal.txt” with the name of your file. Then, just issue the reverse command to get it back when you would like to edit it:

mv .MyJournal.txt MyJournal.txt

You can also combine any or all of these three tricks for added security. Bear in mind that none of these methods actually encrypt your data, they just lock people out of it or hide it. For encryption, your best bet is the encrypted disk image mentioned above.

http://www.dashboardwidgets.com/showcase/details.php?wid=588

Thats not exactly on a file-by-file basis, but still – you could create a disk image at a certain size encrypt and password protect it and keep all your “secret” stuff there, and you wouldn’t have to look for 3rd party software to do the job ..