Encryption is the conversion of information, called plaintext, into a ciphertext (also called cipher or keytext). A secret key is used, which may only be known to authorized persons. With knowledge of the key, the ciphertext can be converted back into the plaintext, which is called decryption.
Encryption is used to maintain the secrecy of information, for example in order to be able to transmit military messages secretly or to protect written messages or stored data against unauthorized “reading”. The science of “secret writing” is called cryptography, derived from ancient Greek κρυπτός kryptós. Its origins lie in antiquity, possibly even further, because secrecy has always been an important concern for humans.
In the modern era, the need for encryption methods that are easy to use and at the same time sufficiently secure increased due to the invention of telegraphy, especially radio telegraphy, and the hard disk drive (HD) for computers. Until the end of the 19th century, almost only manual methods were known for encryption – apart from simple devices such as encryption disks. With the introduction of the electric typewriter and the teletype at the beginning of the 20th century, more and more machine methods for encryption came into use, such as rotor cipher machines. These promised easier handling and improved cryptographic security. Today, encryption is usually done with the help of encryption software or dedicated hardware.
---
Encrypt, Keys, Decrypt and Deciphering
Encrypt
By encrypting, the original “open wording” of a text, the “plaintext”, is converted into an incomprehensible string of characters, the “ciphertext”. The technical terms plaintext and ciphertext have grown historically and can be interpreted much more broadly today. In addition to text messages, all other types of information can be encrypted, such as voice messages, images, videos or the source code of computer programs. The cryptographic principles remain the same.
Keys
The crucial parameter in encryption is the “key”. The good choice of a key and its secrecy are important prerequisites for keeping the secret. In the case of encoding, the codebook represents the key. In the case of most classical and also some modern methods of encryption, it is a password (also: password, keyword, code word or code word, solution, password or slogan from Italian la parola “the word”; English password). In many modern methods, such as e-mail encryption, the user is now relieved of the choice of a key. This is automatically generated without the user noticing. This also eliminates the “human factor”, namely the often too careless choice of an insecure password – because it is too short and easy to guess.
Decrypt
The reverse step to encryption is decryption. The authorized recipient, the “decryptor”, recovers the plaintext from the ciphertext using the secret key known to him. In the case of symmetric encryption methods, which have been known only for a long time, this is identical to that used by the encryptor. The decryption process, on the other hand, is usually similar, but not necessarily identical to the encryption. If it does, as in the case of the German key machine “Enigma” from the Second World War, then it is called involutional encryption.
The key is agreed in advance between the two communication partners, for example by means of a secret key board that both have, or in another secret way, for example by courier. In modern asymmetric methods, there are two different keys, one for encryption (“public key”) and another needed for decryption (“private key”). If the private key is lost, the ciphertext can no longer be decrypted. If the key falls into the hands of a stranger, third parties can also read the ciphertext, so the secret is no longer kept.
Deciphering
Linguistically separate from decryption is the concept of “decipherment”. Decipherment is the art of extracting the secret message from the ciphertext without being in possession of the key. This is the job of a cryptanalyst, often referred to as a “codebreaker”. Ideally, it will not be possible to decipher because the encryption method is sufficiently “strong”. It is then referred to as “unbreakable” or at least “cryptographically strong”. In contrast to “strong encryption”, “weak encryption” can be broken with reasonable effort using cryptanalytic methods without prior knowledge of the key.
The field of work that deals with the deciphering of ciphertexts is cryptanalysis. In addition to cryptography, it is the second subfield of cryptology. Cryptanalysis is not only used for the unauthorized decipherment of secret messages, but also deals with the “(un)breakability” of encryptions, i.e. the testing of the security of encryption methods against unauthorized decipherment.
Most encryption methods are pragmatically secure at best, which means that no practical way to decipher them has been found in their cryptanalysis. The longer a method has been publicly known and the more widespread it is in use, the more confidence can be placed in security, because it can be assumed that many capable cryptologists have examined it independently of each other and that any weakness that may exist would have been found and published.
There are methods whose safety can be proven assuming the validity of certain mathematical conjectures. For example, for the RSA cryptosystem, it can be shown that a user’s private key can be efficiently computed from their public key if and only if a large number (on the order of a few hundred decimal places) can be efficiently decomposed into its prime factors.
