ROT13

ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the alphabet. ROT13 is a special case of the Caesar cipher which was developed in ancient Rome.

https://www.youtube.com/watch?v=EzrngnWysv0

tl;dr

using VIM you have the following native shortcuts:

ggVGg?
ggg?G