What is a cipher?
A cipher is the encryption algorithm itself: the well-defined recipe that, together with a secret key, transforms readable plaintext into unreadable ciphertext, and that can run the process in reverse with the right key. The algorithm is publicly known; it is the key that is secret. That is a fundamental principle of modern encryption (Kerckhoffs's principle): security must never depend on keeping the method itself hidden.
From Caesar to AES
The oldest ciphers were simple substitutions: Caesar merely shifted the alphabet three places. The Enigma machine of the Second World War was an electromechanical cipher, which the Allies broke with enormous consequences for the course of the war. History shows the pattern that still holds: ciphers that were secure yesterday get broken tomorrow, which is why algorithms are continuously replaced. Modern examples of retired ciphers are DES, RC4 and 3DES: all once standards, today banned in any serious configuration.
Symmetric and asymmetric ciphers
Modern ciphers fall into two main groups:
- Symmetric ciphers use the same key for encryption and decryption. They are divided into block ciphers such as AES, which encrypt data in fixed blocks, and stream ciphers such as ChaCha20, which encrypt a continuous stream. They are fast and protect the data itself. See symmetric encryption.
- Asymmetric ciphers such as RSA and elliptic curves work with a key pair and solve the key exchange and signature problem. See asymmetric encryption.
In practice they are combined: asymmetric for key exchange, symmetric for the content itself.
Cipher suites: where it gets concrete
In day-to-day operations you most often meet the word in the form cipher suite: the package of algorithms a TLS connection negotiates (key exchange, encryption and integrity protection). Old cipher suites with weak algorithms are among the most frequent findings in vulnerability scans and penetration tests. They open the door to downgrade attacks, where an attacker forces the connection onto something that can be broken. Good practice is to disable outdated protocols and suites on servers, mail systems and network equipment, and to test the configuration regularly.
How MI Support IT can help
We harden customers' systems so that only modern, secure ciphers are accepted (on web servers, mail, VPN and internal equipment) and keep the configuration up to date as the standards move. This is part of our ongoing IT security services and the security reviews we run for our customers. Contact us if you want to know whether your systems still speak outdated encryption.