Hi what methods of encryption could I use to encrypt a text document. It must use C and not C++.

Cheers Sam

Is this for an assignment, or for real cryptographic use?

It's very easy to do a simple encryption scheme yourself for an assignment, but it's quite another thing to devise a scheme to make it secure from attack. For real work, it's best to use a proven library - no doubt about it. For a one time scheme, it's hard to beat the One Time Pad method (go to Wiki for more info). It's cumbersome, but for one time, quite doable.

This is a PDF on "lightweight" stream ciphers - quite enlightening. Wikipedia has a whole portal on Cryptology btw, which covers several popular ciphers.

http://biblion.epfl.ch/EPFL/theses/2008/4040/EPFL_TH4040.pdf
(multi-lingual but plenty of English)

The biggest mistakes people make in this regard, is thinking that some cipher is "unbreakable", or that a clever untested cipher is secure. Cryptologists have come up with amazingly sophisticated ways to decode messages.

Of course, it can't go unmentioned that no cryptographic system is secure, if the humans running it are in any way, careless in how they handle the details thereof.