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The mysterious art of encryption: How did Arab scholars uncover the secrets of ciphers in the 9th century?
In the 9th century Arab world, as science and art were on the rise, a polymath (versatile scholar) named al-Kindi first pioneered the new field of cryptography. He wrote the Manuscript on the Decoding of Ciphered Messages, a work that marked the birth of modern cryptography, particularly in the reshaping of cryptanalysis. Al-Kindi's work not only summarized existing knowledge, but also introduced the concept of frequency analysis, a technique that is still used today to crack classical codes.
Frequency analysis is a method of understanding and deciphering encrypted messages by studying how often letters or groups of letters appear in ciphertext.
The basis of frequency analysis is that the frequency of occurrence of certain letters or letter groups in each language shows a certain regularity. For example, in English, the letters E, T, A, and O are the most common, while Z, Q, X, and J are relatively uncommon. Therefore, cryptographers can infer the corresponding plaintext letters by counting the frequency of occurrence of letters in the ciphertext. In al-Kindi's time, this technique was an effective means of deciphering encrypted messages, especially in simple substitution ciphers.
Application of Frequency Analysis in Simple Substitution Ciphers
In a simple substitution cipher, each letter of the plaintext is replaced with another letter, and this replacement is consistent for every letter. For example, if all E's are changed to X's, then X's that appear frequently in the ciphertext may represent E's. Generally speaking, cryptographers first need to calculate the frequency of letters in the ciphertext and then map them based on the guessed plaintext letters. In general, X appears more often than other letters, suggesting that it may correspond to e in the plaintext, and is less likely to be the less common letters Z or Q.
Simple substitution ciphers can be cracked using frequency analysis, where common letter combinations reveal the structure of the cipher.
A deeper analysis could also consider the frequency of letter pairs (bigrams) and three-letter combinations (transliterations). For example, in English, the letters Q and U often appear together, although Q by itself is rare. Such similar characteristics led to more evidence and speculation within Al-Kindi's frequency analysis at the time.
Specific case analysis
Suppose an analyst named Eve intercepts a ciphertext and knows that the ciphertext is encrypted using a simple substitution cipher. Eve was able to easily identify that the letter I was the most common, XL was the most common combination of letters, and XLI was the most common three-letter combination by counting the frequencies of the letters in the ciphertext. This strongly suggests that X corresponds to the letter t, L to the letter h, and I to the letter e.
Relying solely on the frequency of letter appearance, Eve was able to gradually infer the content hidden in the entire ciphertext.
As the speculation continues, Eve can get more words. For example, "Rtate" could be "state," which would lead her to infer that R corresponds to the letter s. By piecing the pieces together like this, she might eventually be able to decipher the entire ciphertext and get the gist of the text. This process requires not only skill but also patience and keen insight.
History and Applications
"Manuscript on Decoding Coded Messages" is the first known document on frequency analysis, which aroused widespread research enthusiasm in the scientific community at that time. Over time, this technology quickly spread to Europe, and by the Renaissance, a complete theoretical framework and practical manual had been formed. In 1474, Cicco Simonetta of Italy wrote a manual on Latin and Italian decryption, which provided a theoretical basis for later cryptographers.
Over the following centuries, countless cryptographic techniques were influenced by frequency analysis, which became an important tool in cryptography.
However, with the development of cryptographic technology, cryptographers began to develop more complex alternatives, such as homophone substitution and polyalphabetic substitution. Although these technologies enhance the security of encryption, they also make the decryption process more cumbersome. Furthermore, rotor machines of the early 20th century such as the Enigma machine were almost completely resistant to cracking using frequency analysis, and decoding these complex encryption techniques would require more advanced and comprehensive analysis techniques.
Nowadays, with the rapid development of computer technology, the process of frequency analysis has become more efficient, and computers can complete operations that previously required a lot of manpower in a few seconds. This makes classical cryptography almost useless for modern data protection. However, the principle of frequency analysis still provides inspiration for the new generation of cryptographic technology, promoting the progress and development of cryptography.
Did this innovation by Arab scholars truly change our understanding and application of cryptography? Does this mean that ancient wisdom can still demonstrate its amazing durability and effectiveness when faced with the challenges of modern technology?
