Julius Caesar used a substitution cipher (now called a Caesar cipher) for sensitive private and military correspondence. The cipher involves shifting all of the letters in a message in one direction a secret number of times, wrapping around if necessary. This appears to have done well for Caesar, but could he have done better? The cipher itself is quite trivially breakable.
What if Julius Caesar had been given access to a one-time pad, or OTP? An OTP is an encryption technique that is unbreakable when used correctly. The key must be the same size as the message and completely random, but if it is, it provides information-theoretic security. The way it works is simple. The pad is the same size as the message to be encrypted. You add the position of each letter in the pad (1 for A, 2 for B, etc) to the position of each letter in the message, modulo the size of the alphabet. An example encryption of "HELLO" using pad "XMCKL" from Wikipedia:
H E L L O message 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) message + 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = 30 16 13 21 25 message + key = 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) (message + key) mod 26 E Q N V Z → ciphertext E Q N V Z ciphertext 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) ciphertext - 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = -19 4 11 11 14 ciphertext – key = 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) ciphertext – key (mod 26) H E L L O → message
A random pad can be trivially generated by flipping a coin to determine which letter is present. The pad is delivered securely to the correspondents. It is one of the few concepts in cryptography that are completely and provably unbreakable. How would the Roman Empire's role in history change if:
- Julius Caesar and his private correspondents knew of the technique?
- the concept of the OTP itself was widely known throughout the empire?
In practice, the OTP would allow the Roman Empire to communicate with perfect secrecy. I could very well be overestimating how important this was for them, but it seems to me like it would lead to a rather large change given that more secret messages could be sent between two parties without needing to trust the messenger. It could have major ramifications.