Though not technically a "secret code" i am including Ogham in my line up of keys and ciphers because it is relevant in many of todays Gaelic (Irish) Pantheon practices:
"Dating ogham is difficult and often problematic: although the alphabet itself was created rather earlier, the evidence suggests that the surviving inscriptions of ogham in Ireland belong predominantly to the fifth and sixth centuries. The original form of ogham represented approximately 80 sounds from Gaelic, with 20 symbols arranged in four groups of five. Each group, or aicme, was made up of single strokes, easily carved in wood or stone, with each letter represented by one, two, three, four or five strokes and grouped in sequences of one to five located to the left, right, diagonally across or in the middle of a central stem-line (one stroke to the right is a ‘b’, two strokes is ‘l’, three strokes ‘v/f’, and so on).
Ogham was developed during the Roman Empire and demonstrates the spread of its influence far beyond the imperial frontiers; the fact that ogham has five vowel symbols (although Gaelic has ten such sounds) is one of the reasons scholars believe that the Latin alphabet, which also uses five vowels, was an influence on the invention of the system. Ogham was not a single, fixed system and the surviving stones show modifications, as new symbols were invented and older ones were lost. A few stones also show Christian influence, bearing carved crosses and the Primitive Irish word KOI, which is thought to be a translation of the Christian Latin burial formula hic iacit or ‘here lies’.
The ancient rune-like writing system is carved into stones across Ireland.
As a result of Ogham deriving from an archaic writing system based off the Gaelic phonetics, it is tedious in translating most modern words and words of other languages into Ogham text.
How i utilize the writing system, and with most other systems from historical record, is to either use the archaic version or root version of the word and if there is none to get the phonetics close enough to the system without changing the context drastically.
Ogham is interesting in particular because when written out horizontally it actually mimics patterns of sound waves as seen on modern digital screens. I havent been able to confirm this hypothesis because of my lack of sound equipment and synthisizer.