A cipher is a code, something that stands in the place of another thing to keep it concealed. It also means a nonentity or a nothing. Since my online identity is what I use so I don't have to give out real life details about myself, and that my online identity is a nonentity in the sense that it's not my legal name, it seemed appropriate.
I added the 'e' because the word 'cipher' looks a bit incomplete and so it's not as widely used as cipher or cypher. If you Google 'Ciepher', nearly all the results refer to me.
I also use the name 'Rip Van Winkle', and variants of Rip, after a relatively obscure character in a relatively obscure manga called Hellsing... luckily, both the character and manga are becoming more popular. I identify with her an awful lot, and have made a little fansite.