4: Noisy Channel
This message was transmitted through a noisy channel. Recovering the original message is trivial (this is left as an exercise for the reader), but it is not the key concept here.
C klp ilkktfx tn ngdlqitfx qicq dtnblcpn gk ptnqkcmqn ekgd c klblscfq gk tdhgkqcfq tnnrl. Tq dcw ol ltqilk c bgxtmcb ecbbcmw gk c btqlkckw plstml qicq blcpn klcplkn gk crptlfmln qguckpn c ecbnl mgfmbrntgf.