The Gaius Julius Code

Lxwpajcdujcrxwb!
Hxd’en bxuenm cqn orabc ydiiun.
Cqn orabc xw vrwh.
Rw cqn wngc yjajpajgq,
njlq wdvkna anyanbnwcb j unccna’b
yujln rw cqn juyqjknc.

18, 15, 6.
12, 5, 20, 20, 5.
5, 5, 18, 8, 20.
5, 22, 12, 15, 19.
5, 8, 20.
4, 18, 15, 23.
8, 3, 18, 1, 5, 19.
14, 5, 18, 20.
23, 15, 12, 12, 15, 6.
5, 8, 20.
14, 5, 16, 7, 9, 16.
19, 14, 15, 9, 20, 3, 20, 18, 20, 19, 14, 9.
23, 15, 12, 5, 2.

Best of luck.