Understanding a known-plaintext attack

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

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

For instance, if “CRYPTO” is encoded as “XUZZA,” knowing this pair could enable the attacker to decode other parts of the message that are also encrypted with the same substitution key. This demonstrates how, with some encryption algorithms, even a tiny amount of knowledge can result in broader decryption.

This kind of attack uses a flaw in encryption techniques that make it possible to identify patterns or connections produced between the plaintext and the ciphertext. If not correctly prevented, known-plaintext attacks can jeopardize the security of encryption systems.

Visual representation of a known-plaintext attack

Two common methods to exploit plaintext and its corresponding encrypted form to uncover encryption keys include frequency analysis and pattern matching. The frequency analysis method uses straightforward encryption methods with one-to-one letter or symbol substitutions. Attackers can figure out the key or unlock the rest of the communication by comparing the frequency of occurrence of particular letters or patterns in the known plaintext and the associated ciphertext.

Attackers can spot trends when the same plaintext produces the same ciphertext in the pattern-matching method. They can recognize the encryption algorithm and decrypt the entire message by identifying patterns in the encrypted text and comparing them to known patterns in the plaintext.

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How does a known-plaintext attack work?

In a KPA, the attacker can learn vital details about the encryption method by analyzing how particular chunks of the plaintext are converted into ciphertext using the same encryption key or algorithm.

The attack involves the following steps:

Collecting known pairs

The attacker accumulates pairs of the original plaintext and the associated encrypted ciphertext acquired through various techniques, such as intercepted communications or data leaks.

Analyzing patterns

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

Deriving a key or algorithm

The attacker tries to determine crucial encryption elements, such as the encryption key, algorithm or other process parameters, based on the patterns they have noticed. They can independently replicate the encryption process thanks to this deduction.

Decrypting other data

The attacker can decrypt other encrypted material that employs the same encryption algorithm by using the deduced key or algorithm. This procedure may leak confidential information or jeopardize the encryption system’s security.

Chosen-plaintext attacks vs. known-plaintext attacks

Chosen-plaintext attacks involve adversaries selecting plaintext and analyzing the corresponding ciphertext, whereas known-plaintext attacks occur when attackers possess partial knowledge of the plaintext. 

Understanding the differences between these two cryptographic attacks is vital for effective cryptographic defense strategies.

Chosen-plaintext attack vs. Known-plaintext attack

Frequency analysis concentrates on examining the occurrence of letters or symbols to determine encryption algorithms, unlike classic cryptanalysis, which examines ciphertext for patterns and flaws.

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How to protect against a known-plaintext attack?

To safeguard against known-plaintext attacks, adopt strong encryption algorithms, manage encryption keys securely, use unique keys per session, and add randomness to encryption processes to enhance protection against attacks.

Choose encryption algorithms that can withstand known-plaintext attacks by using strong encryption techniques. By preventing patterns in the plaintext from correlating with patterns in the ciphertext, modern cryptographic algorithms like the Advanced Encryption Standard (AES) are made to survive such attacks. AES is a widely used symmetric encryption algorithm known for its security and efficiency.

How does the AES algorithm works

Securely manage encryption keys to avoid unauthorized access. Use secure key repositories, rotate your keys frequently and use strong key generation techniques. Additionally, avoid encrypting discrete, predictable chunks of data. To stop an attacker from using known pairs, encrypt the entire message or file.

Moreover, use various keys for various sessions and endeavors. The impact of a known-plaintext attack is reduced because each session will use a different encryption key. Also, maintain the most recent versions of your systems, libraries and encryption software. Security fixes that repair vulnerabilities are frequently included in updates.

Before encrypting the plaintext of the data, add a cryptographic salt — a random value — to it. This makes each encryption unique, even when encrypting the same plaintext multiple times. In addition, avoid encryption methods that are known to be vulnerable to known-plaintext attacks. That said, perform proper due diligence when selecting encryption algorithms.