Arms Companion has an actual Cryptography skill with DP costs as Poetic Improvisation, putting it in the Linguistic category.
CRYPTOGRAPHY (In/Re) (Static Action) – The skill at creating and breaking written codes. Character must study any of these before he may decipher them. Treat this as a maneuver roll with appropriate difficulty (i.e., for detailed languages, such as Egyptian hieroglyphics, Phenomenal; for Latin, Very Hard, etc.) When creating a code, the character makes a skill roll. The results determine the complexity. (Note, even the shortest and least descriptive codes can be insanely complex by virtue of their simplicity). The language or code can only be broken by a successful Cryptography roll of appropriate difficulty, see below.
Total Skill Roll ............................ Difficulty to break code/language
Under 1 ...................................................................... Mundane
1-25 .......................................................................... ......Trivial
26-50 .......................................................................... ..... Easy
51-75 .......................................................................... ..... Light
76-100 ......................................................................... Medium
101-125 .......................................................................... . Hard
126-150 .................................................................... Very Hard
151-175 ............................................................ Extremely Hard
176-200 .................................................................. Sheer Folly
201-225 ........................................................................ Absurd
226-250 ........................................................................ Insane
251-275 ................................................................. Phenomenal
276+ ........................................................... Virtually Impossible