From Julius Caesar to modern satellites — the art of protecting information in space
Cryptography (from the Greek kryptós = hidden + graphía = writing) is the science of transforming information so that only whoever holds the key can read it. But it goes far beyond hiding messages: it guarantees four fundamental properties of secure communications.
Radio signals spread in all directions: anyone with an antenna can intercept them. Without cryptography, anyone could read the telemetry, forge commands or tamper with the scientific data of a satellite.
Julius Caesar used a simple letter shift for his military communications. With a shift of 3: A→D, B→E, Z→C. It is the "Hello World" of cryptography and the oldest known system.
Only 25 possible keys. A computer tries them all in microseconds. It was secure only because enemies did not know the concept of encryption existed.
The German Enigma machine had more than 1023 possible settings. Breaking it, by Alan Turing and others at Bletchley Park with the first computing machines, shortened the war by years and helped give birth to modern computing.
Rivest, Shamir and Adleman invented the first practical public-key system. It relies on the fact that multiplying two huge primes is trivial, but factoring the result would take millions of years even for the most powerful computers.
The Advanced Encryption Standard is the worldwide standard for symmetric encryption. Your bank uses it, so do WhatsApp, the HTTPS of this website and military satellites. With a 256-bit key, trying every combination would take longer than the age of the universe.
Type any message and experiment with the oldest cipher in history. The shift is the "key". ROT-13 (shift 13) is its own inverse.
Sender and receiver share the same secret key. It is very fast and efficient for encrypting large volumes of data. The challenge is how to exchange the key securely.
Each user has two mathematically related keys: a public one (which can be shared with everyone) and a private one (kept secret). What is encrypted with one can only be decrypted with the other.
Many satellites only accept commands that carry a valid digital signature or authentication code. The satellite checks it against the key stored in its memory; if it does not match, the command is ignored. Amateur radio satellites cannot encrypt their content, but they can authenticate their telecommands.
A hash function turns any amount of data into a fixed-length string. It is a one-way operation: easy to compute, practically impossible to reverse. Change a single bit and the hash changes completely (avalanche effect).
Modern cryptography combines asymmetric and symmetric methods: it uses the (slow) asymmetric one only to exchange a session key, and then the (fast) symmetric one to encrypt the data. That is how HTTPS and TLS work, and how secure satellite communications work.
The ground station and the satellite exchange their public keys and agree on an AES session key using Diffie-Hellman.
All further communication uses AES-256 with the session key. Very fast and secure.
Each command carries a signature made with the operator's private key. The satellite verifies it before executing anything.
Satellites pose cryptographic challenges that do not exist in ground communications:
