Xctf attack and defense world crypto advanced area

0x01. Enter the environment , Download the attachment

Compressed package for topic , Contains a txt file , Pictured ：

0x02. Problem analysis

0x02_1. Virgilian code

What is the virgilian code ？ Virginia password is an encryption method based on Caesar password , It will all of Caesar's code 25 Sort the displacement into a table , Together with the original letter sequence 26 Line and 26 The alphabet of the column . in addition , The Virginia password must have a key , This key consists of letters , At least one , Up to the number of clear text letters .¹

for example , We have the following information ：

Plaintext ：I’ve got it.
secret key ：ok

You can get the ciphertext ：
Ciphertext ：W’fs qcd wd.

The operation is as follows , First look at a password form ：

First , The length of the key should be the same as that of the plaintext , If less than the plaintext length , Then repeat the splicing until the same . In this case , The plaintext length is 8 Letters （ Non letters are ignored ）, The key will be completed by the program as “okokokok”, Then the first letter of the plaintext is “I”, The first letter of the key is “o”, Find... In the table “I” Column and “o” Lines intersect , Letter “W” It's the first letter of the ciphertext ; Empathy ,“v” Column and “k” The line intersection letter is “F”;“e” Column and “o” The line intersection letter is “S”……

Be careful ：

• The Virginia code encrypts only letters , Case insensitive , If non alphabetic characters appear in the text, they will be retained as they are .
• If you enter multiple lines of text , Each line is encrypted separately .

0x03. Problem analysis

0x03_1. Restore secret key

Here refer to the boss's wp, Get a lot of inspiration .https://blog.csdn.net/weixin_45530599/article/details/108112665

Pictured ： You can see that there is such time data , And the characters and numbers should meet the corresponding length , It is easy to think of in English between…and… Usage of , And the character length just matches . that , Ciphertext frxnimp Corresponding to plaintext between, Check the password table , Find the column header by column ‘b’, Then find the encrypted character in the corresponding column ‘f’, Then you can find the ciphertext characters at the beginning of the line ‘e’, And so on , You can get encrypted ciphertext characters ‘enereicqvi’

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1. https://www.qqxiuzi.cn/bianma/weijiniyamima.php ︎