simple algorithm to encrypt/decrypt a text file
My advice would be to use a stream cipher.

First of all, it is easy to implement, because you won't have to think about dividing into blocks, padding, etc..

Secondly, the idea of stream ciphers is very easy: you generate a  pseudorandom sequence of bits out of the private key. Then you XOR this sequence with a plaintext. You will only have to code the generation, the rest is elementary. 

Finally, there are very easy stream ciphers, e.g.  RC4 http://en.wikipedia.org/wiki/RC4;  another example is Salsa20 http://en.wikipedia.org/wiki/Salsa20

And when I say "simple" I mean not only the amount of lines of code, but also the ease of debugging/ flaws search.