Golden Encyclopedia | What is known plaintext attack

Author: Jagjit Singh, Cointelegraph; Compiler: Songxue, Jinse Finance

1. Understand known plaintext attacks

A known-plaintext attack (KPA) is when a hacker uses a well-known pair of unencrypted and encrypted material to determine an algorithm or encryption key.

In a known-plaintext attack, the attacker has access to both the encrypted form of the data (ciphertext) and its corresponding plaintext copy of the original data (unencrypted form). An attacker attempts to determine the encryption key or algorithm by examining the relationship between plaintext and ciphertext.

For example, if "CRYPTO" is encoded as "XUZZA", knowing this pair could enable an attacker to decode other parts of a message that were also encrypted with the same replacement key. This suggests that with certain encryption algorithms, even a small amount of knowledge can enable broader decryption.

This attack exploits flaws in encryption techniques that can identify patterns or connections that arise between plaintext and ciphertext. Known-plaintext attacks can compromise the security of an encryption system if not properly prevented.

Two common methods of using plaintext and its corresponding encrypted form to reveal encryption keys include frequency analysis and pattern matching. The frequency analysis method uses a simple encryption method and one-to-one letter or symbol substitution. An attacker can figure out the key or unlock the rest of the communication by comparing the frequency of specific letters or patterns in the known plaintext and the associated ciphertext.

Attackers can spot trends when the same plaintext produces the same ciphertext in a pattern-matching approach. They can identify encryption algorithms and decrypt entire messages by identifying patterns in encrypted text and comparing them to known patterns in plaintext.

**Second, how to carry out the known plaintext attack? **

In KPA, an attacker can learn important details about an encryption method by analyzing how a particular block of plaintext is converted to ciphertext using the same encryption key or algorithm.

The attack involves the following steps:

collect known pairs

The attacker accumulates the original plaintext and the associated encrypted ciphertext obtained through various techniques such as interception of communications or data exfiltration.

Analysis mode

When plaintext is encrypted to create ciphertext, the attacker compares the schemas, modifications, and transformations that occur. To understand the operation of the encryption process, they search for regular relationships between known plaintext and ciphertext.

Derived Key or Algorithm

Attackers attempt to determine key cryptographic elements, such as encryption keys, algorithms, or other process parameters, based on the patterns they notice. Thanks to this derivation, they can independently replicate the encryption process.

Decrypt other data

An attacker can use the derived key or algorithm to decrypt other encrypted material using the same encryption algorithm. This process may reveal confidential information or compromise the security of the encryption system.

3. Chosen plaintext attack and known plaintext attack

A chosen-plaintext attack involves an adversary choosing a plaintext and analyzing the corresponding ciphertext, while a known-plaintext attack occurs when the attacker has partial knowledge of the plaintext.

Understanding the differences between these two encryption attacks is critical to an effective encryption defense strategy.

Frequency analysis focuses on examining the occurrence of letters or symbols to determine an encryption algorithm, unlike classical cryptanalysis, which examines ciphertext for patterns and flaws.

**4. How to prevent known plaintext attacks? **

To prevent known plaintext attacks, use a strong encryption algorithm, securely manage encryption keys, use a unique key for each session, and add randomness to the encryption process to enhance defense against attacks.

Choose an encryption algorithm that is resistant to known-plaintext attacks by using strong encryption techniques. Modern encryption algorithms such as the Advanced Encryption Standard (AES) defend against such attacks by preventing patterns in the plaintext from being associated with patterns in the ciphertext. AES is a widely used symmetric encryption algorithm known for its security and efficiency.

Securely manage encryption keys to avoid unauthorized access. Use a secure key repository, rotate keys frequently, and use strong key generation techniques. Also, avoid encrypting discrete, predictable chunks of data. To prevent attackers from using known pairs, encrypt the entire message or file.

Also, use different keys for different sessions and tasks. Since each session will use a different encryption key, the impact of known-plaintext attacks is reduced. Also, maintain the latest versions of systems, libraries, and encryption software. Security fixes that fix vulnerabilities are often included in updates.

Before encrypting the plaintext of the data, add an encryption salt (random value) to it. This makes each encryption unique, even if the same plaintext is encrypted multiple times. Also, avoid encryption methods that are known to be vulnerable to known-plaintext attacks. That said, do proper due diligence when choosing an encryption algorithm.