The invention of writing brought with it the need to prevent the wrong people from reading documents. The most common methods involved either concealing the text itself (invisible ink) or disguising the message using codes or ciphers. Equipment for sending secret messages is LC4.
Codes use symbols or groups of letters to represent words or phrases (a complex message requires a codebook), while ciphers substitute one letter for another using a predefined scheme. Any written language could serve as a secret code if only a select group of people understand how to read it. An example of a cipher was used by Julius Caesar (a cipher now called the “Caesar shift”): He replaced every letter in the alphabet with one three places down in position so that a = D, b = E, c = F, etc. To extract the original message, the recipient simply reversed the process.
It’s almost impossible to read a short, coded message if you don’t know the code or cipher. The longer the message, the easier it is to decode. See Cryptography (p. B186) for rules.
This seems to have been used first in classical Greece. It consists of a strip of leather or paper wrapped around a cylindrical baton in a helix. A message is written horizontally across the baton so that a single letter takes up the width of the strip. When unwound, the message becomes a jumble of letters. To read it, the strip must be wound around a baton of the same diameter as the original.
Invented by Italian scholar Leon Battista Alberti in the 15th century, this mechanism consists of two concentric discs that can be rotated about a shared axis. The discs are divided into equal partitions, and a letter of the alphabet is inscribed in each segment around the edge. The inner wheel is turned so that a predetermined letter (the “key”) corresponds to the letter “A” on the outer wheel. With this done, the message can be ciphered by looking at the letter on the outer circle and writing down the corresponding letter on the inner circle. The message can only be deciphered by someone who has an identical cipher wheel and knows the key letter. Someone with the Jeweler skill (p. B203) can engrave a cipher wheel onto metallic discs the size of large coins to enable easy concealment (+1 to Holdout). $80, neg.
Instead of attempting to disguise a message using a code or a cipher, it’s sometimes simpler to hide the whole thing!
Lemon juice, onion juice, and various other kitchen recipes produce perfectly good secret ink, which becomes visible when heated. Philo of Byzantium (280-220 B.C.) is the first to write about using a reagent to make the writing visible; his recipe uses crushed gallnuts in water, with vitriol as the reagent. Another example, described by Pliny, makes use of the juice of the spurge; rubbing ashes on the paper causes the juice to darken. One curious method involves writing on a boiled egg with a mixture of vinegar and alum; the shell absorbs the writing, and when it’s peeled off, the message is visible on the surface of the egg inside. To use invisible ink requires skill; too much pressure leaves visible indentations, too little results in not enough ink being applied. When a scribe uses improvised equipment or writes a long message in such ink, the GM should roll secretly against DX, or against DX-based Artist (Calligraphy) at +6. Failure means the recipient cannot read the whole message. Critical failure means that anyone who makes a Vision roll can see it – a fact the writer fails to notice. For more about legibility, see GURPS Low-Tech Companion 1. Per vial: $10, neg.
Some types of paper, such as rice paper, dissolve quickly in warm water, or can be swallowed. This makes them useful for secret messages. Once the recipient reads the message, he can quickly dispose of the evidence. Suitable rice paper was invented in China early in the Tang Dynasty (618-907 A.D.). Per sheet: $1, neg.