From the earliest days of human communication, people have sought to conceal messages and protect sensitive information from prying eyes. The study of analyzing and breaking these secret codes is known as cryptanalysis – and it has a long, fascinating history full of brilliant minds, world-changing events, and a constant battle of wits between code-makers and code-breakers.
In this in-depth article, we‘ll explore the complete history of cryptanalysis from ancient times to the modern digital age. We‘ll look at the most important people, techniques and events in code-breaking, with a special focus on methods that exploit weaknesses and vulnerabilities in cryptosystems. As a digital technology expert, I‘ll highlight the impact of computers and provide unique insights into the future of this ever-evolving field. Let‘s dive in!
Cryptanalysis Fundamentals
Cryptanalysis is the flip side of the better-known practice of cryptography, which is the art of making codes and ciphers. While cryptographers work to develop secure encryption systems, cryptanalysts look for flaws and weaknesses that allow breaking that encryption. The two disciplines are in constant tension yet utterly interdependent. Cryptanalysis exposes problems that cryptographers then work to fix in the next generation of ciphers, in a never-ending cycle of code-breaking and code-making.
The basic objective of cryptanalysis is to find non-random patterns or distinguishing characteristics in encrypted data that provide clues about the original unencrypted information (called the "plaintext") or the secret encryption key. Cryptanalysts use a wide variety of mathematical, statistical and logical tools to detect this non-randomness.
The specific cryptanalytic methods used depend on how much information the code-breaker has to work with. The main categories of cryptanalytic attacks are:
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Ciphertext-only – The cryptanalyst has access only to encrypted messages and must work from those alone. This is the most challenging scenario, relying heavily on detecting non-randomness in the ciphertext.
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Known-plaintext – The cryptanalyst has some plaintext messages and their corresponding encrypted versions. This allows deriving clues about the encryption algorithm and key through statistical comparison. The more plaintext-ciphertext pairs, the more powerful the attack.
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Chosen-plaintext – The cryptanalyst can choose the plaintext to be encrypted and analyze the resulting ciphertext. Carefully crafted plaintext can maximize the exposure of weaknesses. A variation is the adaptive chosen-plaintext attack, where the cryptanalyst makes a series of plaintext choices informed by the previous results.
These broad categories encompass dozens of individual cryptanalytic techniques targeting different aspects of cryptosystems. Some try to leverage brute computational power to defeat encryption, while others look for subtle mathematical flaws or backdoors in the physical implementation. We‘ll examine several of the most important techniques as we trace the history of cryptanalysis.
Early Origins
The practice of secret writing dates back to the very dawn of literacy, with evidence of deliberately obscured hieroglyphics in ancient Egypt as early as 1900 BC. These early encryption attempts used simple letter substitutions or transpositions that seem trivial to break from a modern perspective – but they sometimes remained secure for hundreds of years in an era with mass illiteracy and no systematic study of cryptanalysis.
The first known treatise on cryptanalysis was written by an Arab polymath named al-Kindi in the 9th century AD. Among other techniques, al-Kindi pioneered the use of frequency analysis – studying the relative prevalence of different ciphertext letters and comparing those frequencies to the known statistics of the plaintext language [1]. For instance, if a particular ciphertext letter appears about 12% of the time (roughly the frequency of ‘e‘ in standard English), there‘s a good chance it represents the plaintext ‘e‘. With enough ciphertext, these statistical regularities can be used to reverse simple substitution ciphers letter by letter.
Frequency analysis remained the foundation of cryptanalysis for the next thousand years. Rudimentary ciphers like the Caesar shift and monoalphabetic substitution were routinely broken by Renaissance scholars and cipher secretaries in the courts of Europe. However, encryption systems grew increasingly complex to resist frequency-based attacks. Nomenclators added lists of codewords for common names and phrases, while homophonic substitution used multiple cipher letters to represent a single plaintext character. This foreshadowed the eternal back-and-forth of cryptology, with cipher-makers and code-breakers constantly spurring each other to greater heights.
Cryptanalysis Mechanized
The true golden age of manual cryptography and cryptanalysis came in the early 20th century with the mechanization of encryption. Rather than shuffling letters with quill and parchment, encryption could now be performed with typewriter-like machines using rotors, electrical circuits and gears. This allowed for far more complex ciphers resistant to traditional cryptanalytic methods.
The most famous of these new gadgets was the German Enigma machine, whose cracking by Allied code-breakers played a decisive role in World War II. Enigma had a series of 3-5 rotors that scrambled each plaintext letter with a different substitution, making simple frequency analysis impossible. It also had a plugboard that added a further layer of substitution. Together, these components could be configured in over 150 million million million possible ways [2]. Enigma seemed unbreakable to its creators.
However, Polish and later British cryptanalysts led by the brilliant mathematician Alan Turing found several clever ways to reduce the staggering number of possible Enigma configurations. They exploited operator mistakes, quirks of German language and protocol, and flaws in the Enigma design itself (like the fact that no letter could ever encrypt to itself). Turing also developed a device called the Bombe that could check Enigma settings against a piece of suspected plaintext.
At the peak of this effort, over 10,000 people worked at Bletchley Park, the secret British cryptanalysis facility [3]. They decoded up to 7,000 Enigma messages per day, providing crucial intelligence about German military operations [4]. The Bletchley Park team built some of the world‘s first programmable digital computers, the Colossus series, to assist with cryptanalysis. This groundbreaking work foreshadowed the key role computing would play in the future of cryptology.
The Computer Age
The end of World War II marked the beginning of the information age, with electronic computers rapidly increasing in power and prevalence. These developments completely transformed the nature of cryptography and by extension cryptanalysis. Computers enabled encryption schemes orders of magnitude more complex than previous manual or mechanical methods. At the same time, computers gave code-breakers the ability to automate attacks and test huge numbers of keys or plaintext-ciphertext combinations.
The postwar period also saw the rise of public academic research into cryptography, which had largely been restricted to military and intelligence organizations before. This sparked a wave of innovation in both cipher design and cryptanalysis. In the 1970s, IBM developed the first commercial block cipher, the Data Encryption Standard (DES), which was soon adopted by the U.S. government [5]. DES was designed to resist all known cryptanalytic attacks, but concerns about its 56-bit key size emerged almost immediately.
In 1977, the RSA public-key cryptosystem was invented, ushering in a new era of asymmetric cryptography. Public-key systems use a pair of keys, one public for encrypting messages and one private for decrypting. RSA relies on the difficulty of factoring very large prime numbers, as opposed to the symmetric single-key systems that use substitution and permutation. While public-key cryptography has many advantages, it is typically slower than symmetric encryption and is often used in combination for key exchange.
The 1990s saw the emergence of powerful new forms of cryptanalysis that focused not on the mathematics of the cipher but on weaknesses in its physical implementation. Eli Biham and Adi Shamir developed differential cryptanalysis, which analyzes how differences in plaintext input affect the resulting ciphertext output [6]. Around the same time, Mitsuru Matsui pioneered linear cryptanalysis, which looks for linear approximations to the complicated nonlinear functions used in block ciphers [7].
Side-channel attacks also emerged as a significant threat to deployed cryptosystems. These attacks exploit information leaked by the physical execution of the cipher, such as the time it takes, the power it consumes or even the sound it makes. The first major side-channel attack was demonstrated by Paul Kocher in 1996, showing that the time to perform private key operations in several public-key systems correlated with the individual key bits [8].
As the 20th century drew to a close, the rapid growth of the internet and electronic commerce made robust cryptography a matter of urgent economic and national security. The aging DES was too weak for the modern world, vulnerable to brute force by custom-built DES crackers. After a competition between 15 different designs, the U.S. National Institute of Standards and Technology (NIST) selected the Rijndael cipher as the new Advanced Encryption Standard (AES) in 2001 [9].
AES has proven to be a strong, efficient and flexible cipher that is still the standard today for government and commercial encryption. AES supports key sizes of 128, 192 or 256 bits, making brute-force guessing of the key computationally infeasible. Various cryptanalytic attacks on reduced-round versions of AES have been published, but none that threaten the full cipher. Even so, cryptographers are acutely aware of the long history of ciphers that seemed invincible eventually being defeated by clever cryptanalysis. Work continues on both attacking AES and developing newer, stronger ciphers.
Quantum Cryptanalysis
While AES is secure against traditional code-breaking, a paradigm-shifting new attack vector looms on the horizon in the form of quantum computing. Instead of classical bits that can be either 0 or 1, quantum computers operate on qubits that can exist in superpositions of 0 and 1 simultaneously. This allows quantum algorithms to solve certain problems exponentially faster than classical computers.
In 1994, mathematician Peter Shor demonstrated a quantum algorithm that can factor large numbers in polynomial time, rendering most current public-key cryptosystems like RSA vulnerable [10]. Other quantum algorithms like Grover‘s search can speed up brute force attacks on symmetric ciphers, effectively halving the key length. While large-scale quantum computers don‘t exist yet, their future development over the next few decades is a major threat to existing cryptography.
In response, the field of post-quantum cryptography has emerged to develop encryption systems resistant to quantum cryptanalysis. These systems rely on mathematical problems like lattices and error-correcting codes that are difficult even for quantum computers. NIST is currently running a competition to standardize post-quantum ciphers, with the first algorithms expected to be selected in the next few years [11].
Of course, post-quantum cryptography will inevitably give rise to post-quantum cryptanalysis seeking to break these new systems. Optimization of the cryptanalytic quantum algorithms is an active area of research, as is the hunt for new quantum algorithm techniques. Some researchers have even raised the possibility of "quantum-resistant quantum computing" – using quantum ciphers and quantum-based cryptanalysis in an endless quantum arms race [12].
Conclusion
From ancient frequency analysis to quantum algorithms, the history of cryptanalysis is a series of epochal battles between code-makers and code-breakers. Just as one side seems to gain the upper hand with a new technique, the other side works feverishly to counter it in a constant cycle of innovation. Today, cryptography is woven into the fabric of our digital lives, securing everything from bank transfers to military secrets. And wherever sensitive data is encrypted, cryptanalysts are working to uncover it by any means possible.
As a digital technology expert, I believe cryptanalysis in the coming decades will be dominated by the threat and promise of quantum computing. The potential for quantum computers to break current encryption is a ticking time bomb, but also an opportunity to fundamentally rethink how we secure information. Post-quantum cryptography is an exciting field full of new mathematical frontiers. At the same time, quantum cryptanalysis will push these new systems to their limits and expose any lurking weaknesses.
Regardless of the specific technologies, the core dynamic of cryptanalysis will remain the same as it has for centuries – a high-stakes race between those trying to hide information and those trying to uncover it. As long as secrets exist, someone will always try to crack them.
