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  • Writer's pictureRACHIT YADAV

Guide to Cryptanalysis: Learn the Art of Breaking Codes

What is Cryptanalysis?

Cryptanalysis is the field of studying a cryptographic system, learning to decipher and understand hidden messages without having the original decryption key. Cryptanalysis involves observing the properties of encrypted messages and discovering weaknesses and vulnerabilities in the encryption protocol that can be exploited to reveal the original contents.

The terms cryptography and cryptanalysis are closely linked and often even confused. Cryptography is the practice of hiding or encoding information so that only its intended recipient(s) will be able to understand it. Cryptography is related to other fields, such as steganography, which attempts to hide information “in plain sight” (disguising not only the message but also the fact that there is a hidden message, to begin with). On the other hand, cryptanalysis attempts to decode the messages that have been encoded using cryptography. Cryptanalysis and cryptography, therefore, play complementary roles: cryptography turns plaintext information into ciphertext, while cryptanalysis seeks to convert this ciphertext back into plaintext (Bone, 2023).

Cryptanalysis plays a crucial role in evaluating the security of cryptographic systems. In general, the more difficult it is to crack a cryptographic system using cryptanalysis, the more secure the system is.

How Does Cryptanalysis Work?

Cryptanalysis uses a wide range of tools, techniques, and methodologies to decode encrypted messages. These include:

  • Mathematical analysis: The use of mathematical principles and algorithms to find weaknesses in a cryptographic system. This may involve using mathematical properties to find certain patterns or relationships in the encrypted message and detect vulnerabilities in the encryption protocol itself.

  • Frequency analysis: The study of the frequency of different letters and symbols in an encrypted message. This technique is particularly effective against so-called “substitution ciphers,” in which each letter or symbol in the original message is simply replaced with another.

  • Pattern recognition: Identifying repetitive sequences or patterns within an encrypted message. Recurring patterns may correspond to common words or phrases (such as “the” or “and”), helping cryptanalysts partially or fully decrypt the message.

Cryptanalysis techniques vary depending on the type of cipher being used. As mentioned above, for example, basic substitution ciphers can be attacked by calculating the most common letters in the message and comparing the output with a list of the most frequent letters in English. Transposition ciphers are another cryptography method in which the letters of the message are rearranged without being changed. These ciphers are vulnerable to “anagramming” techniques: trying different permutations of letters and hunting for patterns or recognizable words in the results.

What Are the Types of Cryptanalysis?

Cryptanalysis is a tremendously rich and complex field with many different approaches. The types of cryptanalysis include:

  • Known-plaintext attack: In a known-plaintext attack (KPA), the cryptanalyst has access to pairs of messages in both their original and encrypted forms. This allows the attacker to analyze how the encryption algorithm works and produce a corresponding decryption algorithm.

  • Chosen-plaintext attack: A chosen-plaintext attack (CPA) is even more powerful than a KPA—the cryptanalyst can choose the plaintext and observe the corresponding ciphertext. This allows the attacker to gather more information about the algorithm’s behavior and potential weaknesses.

  • Differential cryptanalysis: In differential cryptanalysis, the cryptanalyst has access to pairs of messages that are closely related (for example, they differ only by one letter or bit), as well as their encrypted forms. This allows the attacker to examine how changes in the original text affect the algorithm’s ciphertext output.

For relatively basic ciphers, a so-called ”brute-force attack” may be enough to crack the code (Georgescu, 2023). In a brute-force attack, the attacker simply tries all possible cryptographic keys until the right combination is discovered.

The efficacy of brute-force attacks is highly dependent on the computational complexity of the encryption algorithm. The more complex the algorithm is and the more keys to analyze, the harder it will be to crack the code.

As computers become faster, however, cryptographic algorithms that were previously secure have now become more vulnerable. For example, organizations such as NIST have retired their use of SHA-1, one of the first widely used encryption algorithms, in favor of its more complex successors, SHA-2 and SHA-3 (NIST, 2022).

What Are the Challenges in Cryptanalysis?

Cryptanalysis is a dynamic and challenging area of study. Below are just a few of the major difficulties for today’s cryptanalysts:

  • Key size and algorithm complexity: The larger the key used to encrypt information, the higher the number of possible keys used in the encryption algorithm. This makes algorithms more complex and brute-force attacks more difficult or even impossible (at least on a human timescale).

  • Encryption protocols: Cryptanalysis focuses not only on the mathematical properties of the encryption algorithm but also on how the algorithm is implemented in real-world encryption protocols. Vulnerabilities in this implementation are often easier to attack than the algorithm itself.

  • Lack of KPA or CPA attacks: Known-plaintext and chosen-plaintext attacks are often the best-case scenarios for attackers seeking to understand an algorithm’s behavior. In the real world, however, cryptanalysts rarely have access to large amounts of this data — for example, they may only have ciphertext and not plaintext to analyze.

Organizations seeking to keep their information safe should follow a number of tips and best practices to make cryptanalysis harder for an attacker. For one, they should choose robust cryptographic algorithms that are computationally difficult to solve. In addition, they should store their encryption keys in a safe location using strong access control to prevent them from being compromised.

What Are the Ethical Considerations in Cryptanalysis?

Like many other topics in IT security, cryptanalysis comes with its own set of issues, controversies, and considerations. Would-be cryptanalysts need to obey ethical boundaries and responsibilities, following guidelines such as:

  • Getting authorization: Cryptanalysis should only be carried out with the target’s permission, which is a best practice observed in ethical hacking. Attempting to break encryption schemes without authorization is often considered illegal.

  • Privacy and data protection: Information is often encrypted because it is sensitive or confidential (such as personal data, healthcare records, or financial details). Cryptanalysts must preserve data privacy even when the encryption algorithm is successfully cracked.

  • Responsible disclosure: When cryptanalysts discover a weakness in a cryptographic system, this vulnerability should be appropriately reported as soon as possible. For example, responsible disclosure typically involves notifying the affected parties so they can discreetly fix the issue rather than making a public announcement.

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