SOURCE CODE FOR ARC/ZIP DECODING/ENCODING

Can anyone here point me in the right direction to find the above.  I
would prefer to get source in a flavour of BASIC for DOS, but I can
adapt just about anything.

I need this for a training course example for some beginnings in a
cryptographic/compression topic in a training course I'm putting
together at the Community Centre.

Any refs to websites or newsgroups where I might find this would be
appreciated.