Learn about data security, encryption, hashing and encoding in our knowledge base.

Ciphertext-only Attack

Understanding and Preventing this Cryptographic Threat


In the world of cryptography, a ciphertext-only attack (COA) is a type of attack where an attacker only has access to the ciphertext, or the encoded version of a message, and attempts to determine the plaintext, or the original message. This type of attack is considered one of the most basic forms of cryptanalysis and is used to test the strength of encryption algorithms. In this article, we will cover the major aspects of COAs, including how they work, how to prevent them, and a practical example for better understanding.

What is a Ciphertext-only Attack?

A ciphertext-only attack occurs when an attacker is able to access the encoded version of a message, but does not have access to the decryption key or any other information about the encryption algorithm used. The attacker's goal is to determine the plaintext, or the original message, by analyzing the ciphertext and making educated guesses about the encryption method used.

How do Ciphertext-only Attacks Work?

There are several methods that attackers may use to conduct a COA. Some of the most common methods include:

  • Frequency analysis: This method involves analyzing the frequency of letters or other characters in the ciphertext and comparing it to known letter frequency in a language.
  • Known-plaintext attack: This method involves comparing the ciphertext to a known plaintext and attempting to determine the encryption method used.
  • Brute-force attack: This method involves trying every possible decryption key until the plaintext is found.

Preventing Ciphertext-only Attacks

There are several ways to prevent COAs and make them more difficult to carry out. Some of the most effective methods include:

  • Using a strong encryption algorithm: The stronger the encryption algorithm used, the more difficult it will be for an attacker to determine the plaintext.
  • Using a long encryption key: The longer the encryption key used, the more possible keys there are to try, making a brute-force attack more difficult.
  • Using a one-time pad: A one-time pad is a type of encryption that uses a unique key for each message, making a known-plaintext attack ineffective.

Practical Example:

Let's say an attacker intercepts a ciphertext message that is encrypted using the Caesar Cipher. The ciphertext message is "DWWDFN", which is not understandable. The attacker has no information about the encryption key or any other information. By using frequency analysis, the attacker can determine that the most common letter in the ciphertext is "D", which is probably representing the most common letter in the language, "E". This allows the attacker to guess that the encryption key is a shift of one letter. By shifting each letter in the ciphertext by one, the attacker can determine that the original plaintext message is "EVVEGO".

In this example, we can see how a simple encryption method like the Caesar Cipher is vulnerable to a ciphertext-only attack. However, by using a stronger encryption algorithm and a longer encryption key, the attacker would have a much more difficult time determining the plaintext.


Ciphertext-only attacks are a basic form of cryptanalysis that can be used to test the strength of encryption algorithms. By understanding how COAs work, how to prevent them, and a practical example, we can better protect our information and keep it secure.