Synonyms: Cipher, Encoding, Decoding, Codebreaking
Your character is an expert in encoding, decoding and codebreaking. If the cipher is known, he is able to write and read encoded messages at the same pace as normal text. He is capable to deciphering an unknown text within hours of hard work. Very few codes can resist him.
Cryptography have been used since antiquity for military communications, but medieval monastic orders such as the Templars have been known to use them to keep their most precious secrets.
Most ciphers are simple substitutions, but some more complex patterns already exist and your character probably invented some of them.
The character had access to all classical treaties on cryptography and read at length in Arabic the original work of Al-Khalil or the Risalah fi Istikhraj al-Mu'amma of Al-Kindi.
All characters with a specialised « Knowledge (something) » Feat are very literate. They are fluent in latin, greek and have at least some bases of a foreign language of their choice. They can read and write any languages they know.
On a 1,2,3,4 this Feat has no effect.