The cryptography can be roughly not divided into two, ie expressions and replacement, and of course there are more complex methods combined. The letter is unchanged in the translocation, the location change is changed; the alphabet change in the replacement is constant. The first document that will replace the password for military use is the "Gao Lu" of Caesar. Caesar describes how he will send secretly to Sichzero that is homoked and endangered. Among them, the Roman alphabet is replaced by Greek letters, making the enemy could not understand information. Su Tonnius wrote a detailed description of the replacement password used by Caesar in the "Caesal" written in the Bive II. Caesar only simply replaces the third letter after each letter in the information in the alphabet. This password is usually called Caesar shift password, or simply says that Caescite. Although Su Tonnius only mentions the passage of the three positions, it is obvious that we can use from 1 to 25 positions, so in order to make the password have higher security, the single letter replaces the password Appeared. Such as: 明 码 表 c v v 密 q q q 明 明 明 文 文 密 c c 明 明 明 明 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 文 序 序 序 序 序 序 序 序 序 是 序 序 是 是 序 序 序 序 序 是 序 序 序 是 是 序 序 序Billion billion billions, we have more than 4 × 1027 password tables. Crack is very difficult. How to crack the single letter replacement password included in Caesar password? METHODS: Alphabetical Analysis Although we don't know who is discovered that alphabetics can be used to crack passwords. But the 9th century scientist Al Jindi did the earliest description of the technology in "Manuscript on Deciphering Encryption Information". "If we know the language used by encryption information, the method of deciphering this encrypted information is to identify an additional article written in the same language, about one paper long, then calculate the frequency of occurrence of each letter. We labeled the number of frequencies in the first frequency. The second standard of the frequency is 2, the third standard is No. 3, and it is pushed until all letters in the number of sample articles. Then we observe the secrets that need to deciphery, the same Classify the letters, find the highest frequency of frequencies, and all use the highest frequency alphabet in the sample article. The second high-frequency letter is replaced by the second high-frequency alphabet, and the third is replaced by 3. The letters have been replaced by the alphabet in the sample. "Taking English as an example, first we build a frequency table of each letter in alphabets with a certain length of ordinary article. In analyzing the alphabetics in the ciphertext, the comparison can be broken.
Although the design of the cirler later has improved the previous envelope, for example, the introduction of empty symbols, etc., the purpose is to break the normal letter frequencies. However, small improvements have not been able to cover a huge defect of mono-alphabetically replacement. By the 16th century, the best password is enabled by most encrypted information at the time. Limitations: The text may be seriously deviated from the standard frequency, and the article is less than 100 letters, then the decryption of it will be more difficult. And not all articles apply standard frequency: In 1969, French writer Georgis Peray wrote a 200-page novel "Escape", which has a word containing letters E. More amazing is that the British novelists and those who have successfully translated "escape" into English, and there is no letter E. Aldel is named "Vacuum". If this book encrypts with a single password table, the frequency analysis is very difficult.
A new set of cryptographic systems is established by the stunger de Vigenere in the 16th century. Its password is no longer encrypted with a password, but 26 different password tables are used. The biggest advantage of this cipher table is that it can restrain frequency analysis, thereby providing better security.
