Well you've done plenty of encryption and decryption with the key.
What if you don't have the key?

Here is a little puzzle - figure out the encrypted text below,
using your brain and paper and pencil, but no computer or internet.
Email the plain text of the first line to nick@cs.stanford.edu with
the subject "puzzle". The first three students with the solution will
receive a small prize and infinite bragging rights.

Ywob, C'h ajiio rofw hy bjpqw rj rbw jgt rjvi pjot
C'h ajiio pctw 'rcg C doi'r ij hjpw
C'h ajiio rofw hy bjpqw rj rbw jgt rjvi pjot
C'h ajiio pctw 'rcg C doi'r ij hjpw