Encryption in the 19th century

Question

Heinrich is a scientist and a scholar, one of the greatest minds of the Austrian-Hungarian Empire around 1880. He studied mathematics, physics and engineering. He patented a few of his ideas and became very rich.

He writes a diary with all his observations, experiments and plans. Some of his ideas and inventions are potentially dangerous for humanity and he knows that. This is why he keeps his diary encrypted in order to protect his thoughts in case the diary is stolen.

He arrives at his lab in the morning and wants to continue his work. A few pages of the diary need to be decrypted - Heinrich needs to read last pages in order to refresh his yesterday's ideas and plans. Decryption of 10 pages shouldn't take more than one hour and it shouldn't require anything more than just paper and ink. He has to encrypt new pages and destroy all the plaintext paper trace in the evening.

What encryption schemes were known in the 19th century?

What encryption would Heinrich use?

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I tag this question [military], because I think encryption was mainly used in army those days and military procedures may be a valuable source of knowledge.

Answer 1

Something like a Vigenère cipher with a very long keyword. If the keyword is long enough, it approaches a "reused one-time pad". While reusing an one-time pad is a bad thing, proper exploits require a lot of computer power that might not be available to the attacker.

Answer 2

Invent his own language - although known before, late 19th century is when the constructed languages took off for real. Add an unusual alphabet of his own design, and everything is set. Though it could be "decrypted" with a lot of effort, especially if there are pictures or an occasional word from a natural language, it's still rather improbable.

Though this method is susceptible to the Rubber hose cryptanalysis, so if he suspects the adversary is capable of this kind of attack, he might not bother and use a simple substitution cipher to distract casual readers.

Answer 3

It seems to me that the Playfair cipher would be the obvious choice.

It was invented in 1854, and at the time quite a few people believed it to be unbreakable (and, in fact, it wasn't broken until around 1920, if memory serves).

If you want to get slightly more elaborate, there's a slightly modification of Playfair called two-square, which adds a little difficulty in carrying out the encryption/decryption, but adds a fair amount of extra security.

Both Playfair and two-square operate on pairs of characters (digraphs) instead single characters. This makes frequency analysis substantially more difficult. You have to look at frequencies of 676 possible letter pairs, rather than the 26 individual letters for most typical substitution ciphers (e.g., Vigenere).

In fact, Playfair was strong enough that it was used fairly heavily through WW I, and even continued to be used as a tactical cipher in WW II. With WW II technology, it still took a few hours to break it, so if you were encrypting something like "charge in 24 minutes" it was still secure. In 1880, it was still pretty widely viewed as simply unbreakable (and, as already noted, did remain unbroken for another 40 years or so).

Answer 4

Build a Machine Cipher

A machine cypher, similar to what would become the Enigma machine, is not out of the realm of possibility in 1880, especially to a billionaire inventor.

Enigma relies on rotors moving in such a way as to form a varying electrical circuit. The varying electrical circuits provide the cipher text after the operator types the plain-text into a keyboard. The machine illuminates the cipher key for each character as it is typed.

The light bulb was invented in 1879 by Edison, the first commerical keyboard produced in 1870. Babbage's mechanical difference engine was first built in 1859. So the various components were present at the cutting edge of technology. It is not unreasonable that Edison, or your Herr Heinrich, or any of Jules Verne's polymaths could have created a personal enigma-style cipher.

While Enigma was not unbreakable, the computing resources to break it were generally not available in the 1880s. Heinrich' secrets would be safest with an encryption machine of his own design.

Answer 5

The state of the art in detective fiction of the 19th century was the simple substitution cipher, as seen in Poe's The Gold-Bug (1843) and Doyle's The Adventure of the Dancing Men (1903). According to Wikipedia, the state of the art in real life at the time was the Vigenère cipher — le chiffre indéchiffrable — until it was in fact déchiffred circa mid-century.

Another cipher that would be mechanically plausible for the 19th century, but anachronistic in its level of sophistication, would be the Solitaire (a.k.a. Pontifex) cipher devised by Bruce Schneier and Neal Stephenson for Stephenson's novel Cryptonomicon. It uses a deck of playing cards to produce a stream of pseudorandom values, which can then be used as a non-repeating "keystream" in the same way as the Vigenère cipher uses the letters of its key word over and over.

The most effective way for your eccentric inventor to protect his enciphered message would be to make it hard for the attacker to tell what is the message and what isn't. For example, instead of encrypting "SECRETFORMULA" to "OOAUAIOIAUOUEAIUOIIIUAIEAA", he could encrypt it to "motor acquaint voilá hum you're radium soil lithium nailed array". This would be very time-consuming to encrypt, although I guess not too bad if you're good at free-associating words quickly. Your genius inventor might even be able to disguise the cipher as a plausible-sounding diary entry! In general, this is known as steganography, and state-of-the-art methods for the 19th century would have included acrostics and the Cardan grille. With either of those methods, especially the Cardan grille, the inventor could read and write essentially at full speed.

Answer 6

I don't think our great scientist would really spend one hour every day to decrypt yesterday's annotations, and probably even more time to encrypt today's.

This is a diary where he will be writing lots of things, and he will need some agility both writing and reading so it doesn't delay his investigations.

He would probably use just a Caesar cipher¹, additionally replacing some keywords with different words².

He will end up being quite proficient with caesar substitutions, so he needs very little effort to read and write his diary. He may prepare in the morning the key translating today's alphabet (or use a mechanical wheel that can be reused) and write directly using that as a hint, and wouldn't need to use an intermediate plaintext sheet at all for reading.

This doesn't preclude that he may use more complex encryption methods for really dangerous inventions, after he completes his experiments and they are no longer unrealistic fantasies.

¹ It may be enhanced by using a different offset (key) each day, eg. calculated performing some operations on the month, day of week and moon phase.

² Thus, a formula becomes nonsense without knowing what the actual elements are.

Answer 7

One approach to making encryption much harder to crack, at the risk of having mistakes in encryption garble a message unrecoverably, is to use movable tiles to keep track of the encryption mapping and rearrange them during the encryption process. For example, when using the Playfair cipher, one could say that after processing each pair of letters, one would swap within the matrix the plaintext version of the first character and the ciphertext version of the second. Digraph frequency analysis would generally go out the window, since the scrambling method would split up which letters shared rows and columns with other letters. The effort required to keep track of the continuously-changing key would not be unreasonable, but a character would need to be very careful to avoid making mistakes.

Answer 8

As far as mathematical options go, the best ones I know have already by suggested. I will suggest a different option, used by the Spartans. The Scytale. Basically you wind your medium around a rod and write along the rod. When the medium is unwound you get nothing but shuffled letters. The description in Wikipedia makes this most basic implementation appear easily breakable. While I am no expert, I suppose a few twists could be added:

Don't align letters with stripes.

If you always place your letters on the stripe perfectly, you have nothing but reshuffled letters. This is easily breakable, by statistical analysis of the letter frequency. Instead use very, very thin stripes, so that mere dots are found on each, offering no clue to which letter they belong. This might be difficult to reproduce by hand, but Heinrich will let a machine do the winding for him. Very neatly and reproducibly.

Don't use a simple rod

In the event that the stripes are found, the cryptanalyst will have the patience to test all kinds of rod diameters. Therefore, don't use a rod of fixed diameter, but rather a complicated shape, similar to what you get by spinning an elaborate key around the long axis, or the ones found here.

Disguise the data as everyday objects

Unfortunately, I do not know the shape and size of 19th century sewing spools, so I will take today's situation as a guide. If I had enough trusted technicians, I would use sewing yarn, or something resembling it as a medium. I would manufacture a machine typing on the yarn, while it is of course wound around the intricately shaped rod. It would type in tiny letters, wind and unwind the yarn for me and reading would be easy with a magnifying glass. When storing the data, I would wind it on an ordinary spool. I would take care to leave some yarn blank, so the outside of the yarn (when on an ordinary spool) would be blank and inauspicious.

If my typing/printing technique is good enough, I can even hide my yarn inside Gobelins, carpets, clothes, for longer travel.

Problems

Yarn has an axis of rotation. To avoid that becoming a problem, you must have the ink seep through the yarn. That might be difficult to achieve uniformly. And you must leave blank layers, to avoid damaging layers below.

Answer 9

Solitaire cipher is strong (modern) cipher that is hard to break even using computers. However @o.m. answer would be more possible as Vinegre cipher was already known in 19th century.