Um, you say RC4 or SHA in your subject line, but those are as different
as chalk and cheese.
RC4 is a symmetric stream cipher. You can encrypt any amount of data
using RC4 and a key of your choice, then decrypt it using RC4 (again)
and the same key.
SHA is a hash function. When you process any amount of data with SHA &
its variants, you get a fixed-length result ('x' bits - I can't
remember 'x'). It is computationally impossible to go back to the
original input, from the fixed-length result. So, SHA & variants are
not what you want, at all.
There are hundreds of implementations of RC4 in basic/VB/VBA on the web
- but they're normally called ARCFOUR, or similar, for trademark
reasons. Some quick googling will find them for you. Make sure you also
find some "test vectors" to check the code. A test vector is a known
combination of (input, key, output) for that cipher.
Unfortunately, encryption is full of traps for the unwary. Believe me,
you'll be very lucky to get it right, without some experience.
HTH,
TC
