F_ROT13

Entrypoint rot13 compatible with UTF-8

 Input       CSTRING(254)     String to encrypt
 Output      CSTRING(254)     String enrypted with ROT13

 From Wikipedia:
 ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple substitution cipher used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the 
 casual glance. ROT13 has been described as the "Usenet equivalent of a magazine printing the answer to a quiz upside down". ROT13 is a variation of the Caesar cipher, developed in ancient Rome.
 Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary.
 ROT13 is its own inverse; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding. The algorithm provides no real cryptographic security and is not 
 normally used for such. It is often cited as a canonical example of weak encryption.
 TestSQL
 SELECT F_ROT13(F_ROT13('This is not a real encryption')), F_ROT13('This is not a real encryption') FROM RDB$DATABASE;
 SELECT NULL AS ISCORRECT, F_ROT13(NULL) FROM RDB$DATABASE;