Did the Romans ever develop any form of cryptography, where either words were replaced with other words or letters were replaced with other letters? Do we have any remaining examples, and if so have we translated them?
Yes. We know that Caesar was famous for using a cipher, which is still named for him:
Some letters of his to the senate are also preserved, and he seems to have been the first to reduce such documents to pages and the form of a note-book, whereas previously consuls and generals sent their reports written right across the sheet. There are also letters of his to Cicero, as well as to his intimates on private affairs, and in the latter, if he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others.
Suet. Div. Jul. 56.6