Ciphertext

Ciphertext is the result of turning regular information, called plaintext, into a secret form using a special process known as encryption. This is done using an algorithm, which is like a set of steps or a recipe, called a cipher. When information is turned into ciphertext, it looks like a mix of symbols that no one can read without the right key to unlock it.

The purpose of ciphertext is to keep important information safe from people who might try to look at it without permission. By changing the original information into something unreadable, it helps prevent hacking and the loss of sensitive data.

To get the original information back from ciphertext, a process called decryption is used. This is the opposite of encryption and requires the correct key or method to turn the secret symbols back into readable text. It’s important to know that ciphertext is different from codetext, which comes from a code system rather than a cipher.

Conceptual underpinnings

Imagine Alice wants to send a secret message to her friend Bob. She starts with her original message, called plaintext. To keep it safe, she uses a special method called an encryption cipher and a secret code called a key. This changes her message into something called ciphertext. Ciphertext looks like random symbols and can’t be read by anyone else.

When Bob gets the ciphertext, he uses another special method and the same secret key to change it back into the original message. This way, only Bob can read what Alice sent, keeping their conversation private.

Types of ciphers

Main article: Cipher

The history of cryptography is very old. Cryptography is about hiding messages. Long ago, people hid messages by hand, but now machines are used.

Historical ciphers

Old ways of hiding messages are called classical ciphers. Some examples are:

Substitution cipher: swapping letters in the message with other letters (for example, Caesar cipher and one-time pad)
- Polyalphabetic substitution cipher: using many different swaps (for example, Vigenère cipher and Enigma machine)
- Polygraphic substitution cipher: swapping groups of letters (for example, Playfair cipher)
Transposition cipher: moving letters to new places in the message (for example, rail fence cipher)

These old methods are not used much today because they are easy to solve.

Modern ciphers

New ways of hiding messages are safer. They are made to keep the secret key safe, even if someone knows part of the hidden message. Modern methods have two types:

Private-key cryptography (symmetric key algorithm): one secret key is shared to hide and uncover the message
Public-key cryptography (asymmetric key algorithm): two keys are used, one for hiding and one for uncovering

With a symmetric key algorithm (like DES and AES), the same key is shared by the sender and receiver. The sender uses it to hide the message, and the receiver uses it to uncover it. These can work on blocks of data or on a stream of data.

With an asymmetric key algorithm (like RSA), there are two keys: a public key anyone can use to hide a message, and a private key only the receiver uses to uncover it.

Cryptanalysis

Main article: Cryptanalysis

The Zimmermann Telegram decrypted into plaintext (and translated into English).

Cryptanalysis is the study of finding hidden messages without the special key used to lock them. It looks at how the locking system works and finds ways to unlock the message.

There are different ways to try to unlock these messages, depending on what information is known. The locked message, called ciphertext, is often the easiest to get and is very important in these attempts.

Famous ciphertexts

Main article: List of ciphertexts

The Shugborough inscription, England