HSN Chief Scientist Daniel Sun |
On Traditional Encryption Methods
Daniel Sun: Chief Scientist of HSN. Computer Science MSc, data security expert, won many national and provincial awards in CUMCM (Contemporary Undergraduate Mathematical Contest in Modeling), graduate student tutor, published numerous academic papers during the master’s study. After graduation, he worked in Huawei and VenusTech in R&D and management of cryptographic and communication systems. Has rich experience in product development and blockchain management, led the development of many systems successfully and widely used in commerce.
Since ancient times, people have been paying attention to privacy security, especially during war, and to prevent information leakage, people used all kinds of methods. Generally speaking, these methods can be divided into two categories: substitution and replacement.
I.Substitution
The method of Substitution contains Single table substituting password, caser password, Playfair password, Hill password and Vigenere password.
- Single table substituting is to replace a specific character in plaintext with another fixed character for encryption purposes.
- Playfair password first embeds the key into a certain n * m matrix, and then arranges the letters that do not appear in the key in order. When encrypting, the plaintext is filled in, and then the same row circulates to the right, the same column circulates downward, and other letters are processed diagonally. This encryption is widely used in World War I and World War II.
- Caser password is used to perform addition and Residual processing to characters.
- Hill cipher uses encryption matrix and decryption matrix to encrypt and decrypt the plaintext. We all know that, with the increase of information obtained from ciphertext of plaintext, the matrix can be restored, so it is also not very safe.
- The Vigenere password is encrypted with multiple tables, which is expanded on the basis of a single table.
II.Replacement
Replacement methods include fence technology and rotary machine.
Fence technology arranges plaintext in one way and reads it in another way, while decryption is the opposite.
The rotary machine encrypts each letter by using the rotary wheel and performs the reverse operation when decrypting.
In modern times, in the history of the Anti-Japanese War of China, Zhou Enlai and others invented a way of encryption: the two sides first agreed on a book. When they separated, for encryption purpose, they would agree on which pages and lines of words to be used as the key and the information to be transmitted. Thus, even if other people got the ciphertext, they could not solve it without the book, page and line information. It is equivalent to one-time encryption, which is the highest level of security in communication, as long as the book is not found.
Another well-known method is that, since radio information transmission is easy to be eavesdropped, in order to ensure safety, people had come up with an excellent way: using specific languages with strong local dialects for transmission, such as Minnan and Zhejiang dialects, and more importantly, multi dialect switching will be carried out during the conversation process, which need special training, and ordinary people cannot understand it at all.
III.What’s the Reason？
But we know that all these traditional encryption methods are no longer used anymore. What’s the reason?
In 1976, a revolutionary book “Several new directions of password security” appeared in the security discipline. In this book, the previous encryption algorithms were systematically summarized, and new security theories were put forward. Why are the traditional encryption algorithms no longer used in modern times? The reason is all human languages have statistical characteristics.
An obvious example is that, in our input method called Pinyin (the official romanization system for Standard Chinese), the Chinese characters with the same inputs output in a priority order. The most often used characters are put first. The frequency of each letter and word in English is different. If there is no confusion, the frequency of the word before encryption is the same after encryption. According to the frequency, the corresponding letter and word can be inferred.
Here’s another interesting example: in the early days, when Dr. Hu Shi, a famous Chinese historian, read the classic book “A dream of Red Mansions”, he found that the writing style of the first 80 chapters was quite different from that of the last 40 chapters, so he doubted that the last 40 chapters were not written by Cao Xueqin, the recognized author of the book. Later, this matter was always in dispute, and he could not provide convincing evidence for it.
In the 1980s, Mr. Li Xianping and his students from Fudan University began to count the function words of each Chapter in the book. The function words were counted because a writer’s writing style is completely determined by the function words he is accustomed to using. After these function words were counted out, they were further classified. In addition, other articles written by Cao were also counted out and cluster analysis were applied. The results show that, the first 80 chapters were written by Cao, but the last 40 chapters were not written by Gao E alone, which proves the power of mathematics from a certain aspect.
In the Second World War, most of the password deciphering was based on statistics.
Based on the two examples mentioned above, it’s not hard to understand that the traditional encryption methods are not safe.
Modern security theory has developed several decades since it was put forward. What’s the difference between both of them, the traditional and modern encryption method? We will talk about that in the next article.
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