Can I use "XORing" in my thesis?
Some useful guidelines for writing are:


Who is my audience?
Will my audience understand what I am writing?
Will my usage of language be distracting or otherwise take attention away from the message I'm trying to make?


In this case, your audience is probably another computer scientist who has at least some knowledge of your area.  Therefore, yes, I would expect that another computer scientist will understand your meaning if you write "XORing" or "XORed".  Also, it's common enough that I don't think it will be distracting.  Whether it's too information is perhaps a bit subjective, but I don't view it as too informal.

I've seen it both ways: "xoring" and "XORing".  Personally, I find the all-caps jarring, so my eye prefers "xoring", but I think that is a matter of taste.

Note that "ciphertext" is normally spelled that as one word, and with an "i", not a "y".  Right: "ciphertext".  Wrong: "cyphertext", "cypher text", "cipher text".