Basically, you make an simple encryption system, encrypt a basic message that has some fairly obvious words, and see if anyone can decrypt it.
Here's mine:
J vjkpm lw'v e kfnwqd yglk ila bpib wxwn yp jzf iuxx penpx hvwg fjxrzan.
Uijt okijv vhhp pmoi ymj iujk pz kwuxtmf; uz ernyvgl, yj'i aljk s labym un xzmovdi lqjypqwpekj.
Ifsf't c klqw almgl nx aykrkyy: jzf'ww tmhq fbyirq hvs rdst je xvk lzak fxlltzx!
Ju uvctvu dv e xnruqj xuz-1 jpwoly. Ng soqv heprt, afujwskw max mbczn vhjpio kja. Cv hdfk gsqqe ymj tshkx pz w.
Gps ugokeqnqpu lqfuhdvh xli xmnky zu aoyll, myvyxc dstqe uf sbhe. Max zoff nojk eo cfsb.
Tipvme vjg urwdwlrq viegl ejwt, rcux jqnjm yxo -- iq'dq onpx oh gdi-1.
...Y'c efk vgafy oxkr qyff vo atlhwejejc qefp, bn K?
If you're feeling generous, you can give hints. I won't give out any hints for mine :3