Comment 18 for bug 319423

Using Truecrypt for encryption is a possibility, yes, but being able to encrypt the diary within the program is better, because

a) Program may close diary file automatically after being idle for some time and protect the diary's secrecy. You may lose data if you try to accomplish this via Truecrypt (forcing unmount encrypted drive is not good)
b) this way you would not need extra tools
c) using Truecrypt may be complicated for many users
e) Truecrypt's reliability is in question these days (please check its website, strange things going on there)

Lifeograph has this ability, which is another open source project only for Linux users. Maybe its code would be useful for you if you take a look at it. It is already done there.

Developers know the best but I humbly agree others that this feature should be at top priority for a diary.

Thank you.