Hecho a la fuerza bruta:
CREATE FUNCTION rot13(stringIn VARCHAR(500)) RETURNS VARCHAR(500)
BEGIN
DECLARE v1 INT DEFAULT 1;
DECLARE stringOut VARCHAR(200) DEFAULT '';
DECLARE str VARCHAR(1) DEFAULT '';
WHILE v1 <= LENGTH(stringIn) DO
SET str = SUBSTR(stringIn,v1,1);
CASE BINARY str
WHEN 'A' THEN SET stringOut = CONCAT(stringOut , 'N');
WHEN 'a' THEN SET stringOut = CONCAT(stringOut , 'n');
WHEN 'B' THEN SET stringOut = CONCAT(stringOut , 'O');
WHEN 'b' THEN SET stringOut = CONCAT(stringOut , 'o');
WHEN 'C' THEN SET stringOut = CONCAT(stringOut , 'P');
WHEN 'c' THEN SET stringOut = CONCAT(stringOut , 'p');
WHEN 'D' THEN SET stringOut = CONCAT(stringOut , 'Q');
WHEN 'd' THEN SET stringOut = CONCAT(stringOut , 'q');
WHEN 'E' THEN SET stringOut = CONCAT(stringOut , 'R');
WHEN 'e' THEN SET stringOut = CONCAT(stringOut , 'r');
WHEN 'F' THEN SET stringOut = CONCAT(stringOut , 'S');
WHEN 'f' THEN SET stringOut = CONCAT(stringOut , 's');
WHEN 'G' THEN SET stringOut = CONCAT(stringOut , 'T');
WHEN 'g' THEN SET stringOut = CONCAT(stringOut , 't');
WHEN 'H' THEN SET stringOut = CONCAT(stringOut , 'U');
WHEN 'h' THEN SET stringOut = CONCAT(stringOut , 'u');
WHEN 'I' THEN SET stringOut = CONCAT(stringOut , 'V');
WHEN 'i' THEN SET stringOut = CONCAT(stringOut , 'v');
WHEN 'J' THEN SET stringOut = CONCAT(stringOut , 'W');
WHEN 'j' THEN SET stringOut = CONCAT(stringOut , 'w');
WHEN 'K' THEN SET stringOut = CONCAT(stringOut , 'X');
WHEN 'k' THEN SET stringOut = CONCAT(stringOut , 'x');
WHEN 'L' THEN SET stringOut = CONCAT(stringOut , 'Y');
WHEN 'l' THEN SET stringOut = CONCAT(stringOut , 'y');
WHEN 'M' THEN SET stringOut = CONCAT(stringOut , 'Z');
WHEN 'm' THEN SET stringOut = CONCAT(stringOut , 'z');
WHEN 'N' THEN SET stringOut = CONCAT(stringOut , 'A');
WHEN 'n' THEN SET stringOut = CONCAT(stringOut , 'a');
WHEN 'O' THEN SET stringOut = CONCAT(stringOut , 'B');
WHEN 'o' THEN SET stringOut = CONCAT(stringOut , 'b');
WHEN 'P' THEN SET stringOut = CONCAT(stringOut , 'C');
WHEN 'p' THEN SET stringOut = CONCAT(stringOut , 'c');
WHEN 'Q' THEN SET stringOut = CONCAT(stringOut , 'D');
WHEN 'q' THEN SET stringOut = CONCAT(stringOut , 'd');
WHEN 'R' THEN SET stringOut = CONCAT(stringOut , 'E');
WHEN 'r' THEN SET stringOut = CONCAT(stringOut , 'e');
WHEN 'S' THEN SET stringOut = CONCAT(stringOut , 'F');
WHEN 's' THEN SET stringOut = CONCAT(stringOut , 'f');
WHEN 'T' THEN SET stringOut = CONCAT(stringOut , 'G');
WHEN 't' THEN SET stringOut = CONCAT(stringOut , 'g');
WHEN 'U' THEN SET stringOut = CONCAT(stringOut , 'H');
WHEN 'u' THEN SET stringOut = CONCAT(stringOut , 'h');
WHEN 'V' THEN SET stringOut = CONCAT(stringOut , 'I');
WHEN 'v' THEN SET stringOut = CONCAT(stringOut , 'i');
WHEN 'W' THEN SET stringOut = CONCAT(stringOut , 'J');
WHEN 'w' THEN SET stringOut = CONCAT(stringOut , 'j');
WHEN 'X' THEN SET stringOut = CONCAT(stringOut , 'K');
WHEN 'x' THEN SET stringOut = CONCAT(stringOut , 'k');
WHEN 'Y' THEN SET stringOut = CONCAT(stringOut , 'L');
WHEN 'y' THEN SET stringOut = CONCAT(stringOut , 'l');
WHEN 'Z' THEN SET stringOut = CONCAT(stringOut , 'M');
WHEN 'z' THEN SET stringOut = CONCAT(stringOut , 'm');
END CASE;
SET v1 = v1 + 1;
END WHILE;
RETURN stringOut;
END;
