In Public-Key encryptions (PKE), can the final ciphertext be letters? because i think it can be only numbers.?

Maybe you haven't read this book, it's called "Codebreaker—the history of secret communication" by Stephen Pincock and Mark Frary, at the end, they give you 7 challenges for you to solve, i'm on the 5th one, and it is encrypted using PKE,"NCWLCBHOJHKOYMWTSUZLDUSANNUXRLVVKNRUIQWUWZGVAWZFMZL". I have tried to convert the letters into numbers, but i doubt that it's the case. so if you have any suggestions, please help. But whatever you say, don't say the answer, i want to figure it out myself!

Related posts: