What Is Ciphertext?

Date First Published: 24th May 2023

Topic: Cybersecurity

Subtopic: Security Mechanisms & Technologies

Article Type: Computer Terms & Definitions

Difficulty: Medium

Difficulty Level: 6/10

CONTENTS

Types Of Ciphers
Ciphertext Attacks

Learn more about what ciphertext is in this article.

Ciphertext is text that has been scrambled into random numbers, letters, and symbols by a cipher in order to make it unreadable if intercepted by anyone. Each time data is encrypted, it is converted from plaintext to ciphertext. When decryption takes place after the recipient receives it, it is converted back from ciphertext to plaintext.

Note:

Do not confuse 'cipher' with 'ciphertext'. The term 'cipher' is the name for the algorithm to perform encryption, whilst ciphertext is the text that has been encrypted by a cipher.

Types Of Ciphers

Ciphers come in different types, including:

Substitution ciphers - Also known as monoalphabetic substitution, this replaces bits, characters, or character blocks with different bits, characters, or character blocks to produce ciphertext. Units of plaintext are replaced with the ciphertext in a defined way. The unit may be single letters, pairs of letters, triplets of letters, and mixes of different units.
Transposition ciphers - Unlike substitution ciphers, the letters of the plaintext are rearranged in a different order, but the letters themselves stay the same according to a specific algorithm.
Polygraphic ciphers - With polygraphic ciphers, one letter is substituted for another with two or more groups of letters.
Permutation ciphers - With permutation ciphers, the plaintext characters stay the same but the positions and order are rearranged so that the ciphertext is a permutation of the ciphertext.

Ciphertext Attacks

Cryptanalysis refers to methods designed to gain access to the contents of encrypted messages, even if the cryptographic key is unknown. This is carried out using attacks and requires an understanding of how the system works and finding a secret key. Below are five examples of ciphertext attacks:

Ciphertext-only - The cryptanalyst only has access to a specific set of ciphertext. However, they don't have access to the corresponding plaintext. This attack succeeds when the corresponding plaintext can be determined from a specific set of ciphertext and sometimes the encryption key.
Known-plaintext - The attacker has access to both the plaintext and its encrypted version (ciphertext). This can be used to further reveal information about the private keys and code books.
Chosen-plaintext attack - The attacker finds a way to trick someone who knows the secret key into decrypting any ciphertext and sending it back. The cryptanalyst can gather information by obtaining the decryptions of chosen ciphertexts. The goal of this attack is to get information that reduces the security of the encryption method.
Chosen-ciphertext attack - This is when a cryptanalyst chooses a piece of ciphertext and tries to obtain the corresponding plaintexts. They can gather information by obtaining the decryptions of the chosen ciphertexts. Using these pieces of information, the cryptanalyst can find out the hidden secret key used for encryption.
Related-key attack - This is when the attacker can observe a cipher operating under different keys whose values the attacker doesn't know. Some mathematical relationship connecting the key is known to the attacker.