Question

Suppose that an intruder has an encrypted message as well as the decrypted version of that message. Can the intruder mount a ciphertext-only attack, a known-plaintext attack, or a chosen-plaintext attack?

Short answer

The intruder can mount a known-plaintext attack.

Step-by-step solution

Understanding Attack Types

In cryptography, different types of attacks are used to break encryption. A ciphertext-only attack happens when the attacker only has access to the encrypted message. In a known-plaintext attack, the attacker has access to both the encrypted message (ciphertext) and the unencrypted message (plaintext). A chosen-plaintext attack involves the attacker selecting arbitrary plaintexts to be encrypted and then studying the result (ciphertext).

Analyzing the Given Scenario

The problem states that the intruder has both the encrypted message (ciphertext) and the decrypted version of that message (plaintext). This situation fits the description of a known-plaintext attack since the attacker knows the plaintext corresponding to the ciphertext.

Determining Possible Attacks

Given that the intruder has both the plaintext and the ciphertext, the intruder can attempt a known-plaintext attack as they have the needed information to potentially deduce the secret key or encryption pattern. In this scenario, the other two types of attacks (ciphertext-only and chosen-plaintext attacks) are not applicable because they require different conditions to be met.

Key concepts

Ciphertext-Only Attack

In a ciphertext-only attack, imagine an eavesdropper who only manages to intercept encrypted messages without any additional context or known plaintexts. This is one of the most challenging forms of attack for the intruder. The attacker relies solely on the encrypted data to try to crack the encryption. There is no direct insight into what the message originally said. To overcome this limitation, attackers may use statistical methods or patterns to make educated guesses about the key. This is why strong encryption algorithms are designed to produce ciphertext that appears random, making such attacks very difficult.

• The attacker has only the ciphertext.
• Relies on statistical analysis to deduce information.
• This attack assumes no prior information about the plaintext.

For example, if a simple substitution cipher was used to encrypt a long text, an attacker might spot frequent letters in the ciphertext, suggesting what letter those frequent symbols could represent. Despite its difficulty, success in ciphertext-only attacks can compromise entire encryption systems.

Known-Plaintext Attack

In a known-plaintext attack, the attacker has access to both encrypted data (ciphertext) and some pieces of the original data (plaintext). This dual access provides a strategic advantage compared to ciphertext-only attacks. The goal here is to use the known relationship between plaintext and ciphertext to uncover the encryption key or pattern used. By understanding how specific plaintext is transformed into ciphertext, attackers can infer how the encryption works, potentially revealing the secret key.

• Attacker knows both plaintext and corresponding ciphertext.
• Valuable for reverse-engineering the encryption method.
• Data breaches often lead to known-plaintext scenarios.

For instance, if a hacker intercepts a sensitive email along with its encrypted version, they can analyze how each word or phrase changes upon encryption. This analysis can significantly aid in devising a strategy to decrypt future messages encrypted with the same method.

Chosen-Plaintext Attack

A chosen-plaintext attack takes things a step further by allowing the attacker to select the exact plaintext for encryption and then study the resulting ciphertext. By having control over what gets encrypted, the attacker can conduct experiments to deduce the encryption key or system. This attack is particularly effective against systems that don't change their keys very frequently or have predictable patterns.

• Attackers choose specific plaintexts for encryption.
• Watch the resulting ciphertext for insights into the encryption.
• Offers the most control over experimental conditions.

For example, if a system encrypts known text at predictable intervals, attackers can select specific inputs, predict outputs, and use this information to crack the encryption. This method exploits any weaknesses in the cryptographic algorithm when it fails to produce adequately random or unpredictable ciphertext.