How do you measure the difficulty and success rate of a ciphertext-only attack?

1 Encryption algorithm

The encryption algorithm is the set of rules and steps that transforms the plaintext into the ciphertext and vice versa. The strength and security of the encryption algorithm depends on how well it resists various types of attacks, such as brute force, frequency analysis, differential cryptanalysis, linear cryptanalysis, and so on. Some encryption algorithms are more vulnerable to certain attacks than others, depending on their design, implementation, and configuration. For example, a simple substitution cipher can be easily broken by frequency analysis, while a one-time pad cipher is theoretically unbreakable by any ciphertext-only attack.

2 Key size and randomness

The key size and randomness are also important factors that affect the difficulty and success rate of a ciphertext-only attack. The key size is the number of bits or characters that make up the secret key used to encrypt and decrypt the message. The larger the key size, the more possible keys there are, and the harder it is for the attacker to guess or exhaust them. The randomness is the degree of unpredictability and variation in the key generation and selection process. The more random the key, the less likely it is for the attacker to find any patterns or correlations in the ciphertext.

3 Ciphertext length and quality

The ciphertext length and quality are also relevant factors that influence the difficulty and success rate of a ciphertext-only attack. The ciphertext length is the number of bits or characters that make up the encrypted message. The longer the ciphertext, the more information and clues it may provide to the attacker, but also the more computation and resources it may require to analyze it. The ciphertext quality is the measure of how well the ciphertext conceals the plaintext and the key. The higher the ciphertext quality, the more uniform and random it looks, and the less it reveals any statistical or structural features of the encryption algorithm or the plaintext.

4 Attack strategy and tools

The attack strategy and tools are the methods and techniques that the attacker uses to perform the ciphertext-only attack. The attack strategy is the plan and approach that the attacker follows to try to break the encryption and recover the plaintext or the key. The attack strategy may depend on the type and characteristics of the encryption algorithm, the key size and randomness, the ciphertext length and quality, and the attacker's goals and resources. The attack tools are the software and hardware that the attacker employs to execute the attack strategy and analyze the ciphertext. The attack tools may include various cryptanalysis tools, such as frequency analyzers, cipher solvers, key crackers, and so on.

5 Success criteria and metrics

The success criteria and metrics are the standards and measures that the attacker uses to evaluate the outcome and effectiveness of the ciphertext-only attack. The success criteria are the objectives and expectations that the attacker sets for the attack, such as recovering the whole plaintext, part of the plaintext, or the key. The success criteria may vary depending on the attacker's motivation and purpose, such as espionage, sabotage, or curiosity. The success metrics are the indicators and values that the attacker calculates to quantify and compare the success of the attack, such as accuracy, confidence, speed, cost, or difficulty.

6 Difficulty and success rate

The difficulty and success rate of a ciphertext-only attack are the overall results and implications of the factors and methods discussed above. The difficulty is the level of challenge and complexity that the attacker faces to perform the attack and achieve the success criteria. The difficulty may depend on the combination and interaction of the encryption algorithm, the key size and randomness, the ciphertext length and quality, and the attack strategy and tools. The success rate is the probability and frequency that the attacker reaches the success criteria and metrics. The success rate may depend on the quality and quantity of the ciphertext, the effectiveness and efficiency of the attack strategy and tools, and the reliability and validity of the success metrics.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?