Getting started with CTF-Cryptography Basics

• Opening

The Association has received an increasing number of new students and has also broadcast some basic courses. I wonder if you can understand them all. I do not know how to learn information security at the beginning. The main type of competition we compete for is CTF (cross the fire (O? V ?) Mmm 2333 is a joke .... Okay, it's the battle of the flag. This is an entry for students with 0 bases. Please try it out ~~

• CTF Introduction

CTF (Capture the Flag) is a form of competition in the field of network security that involves technical competition among network security technicians. CTF originated from the defcon global hackers' conference in 1996, in order to replace previous hackers' technical competition by launching real attacks to each other. So far, it has become a popular competition in the world's network security circle. In 2013, more than 50 international CTF events were held globally. As the birthplace of the CTF competition, defcon ctf has become the CTF competition with the highest level of technology and influence in the world, similar to the "World Cup" in the CTF arena ". (I really want to go to DEFCON.Second (second) second )〃.... Cough)

CTF is a popular form of information security competition. Its English name can be translated into "flag winning" or "flag winning ". The general process is that participating teams conduct defense and defense, program analysis, and other forms, take the lead to get a string or other content in a certain format from the competition environment provided by the organizer (the format is generally flag {xxxxxx}), and submit it to the organizer to win the score. For convenience, we call this content "flag ".

In general, this kind of competition is to find the key string (FLAG) from the program or data through technical means (pwn reverse analysis decode, etc.) and submit it correctly... (● Success? ●●)

• After a long time, I entered the topic below.
• (We recommend that you start with cryptography when you first import CTF. The following describes how to use it ):
• Classical Cryptography:
• Caesar password (shift password): encrypted by the partial digits of the plaintext in the alphabet

For example, when the offset is 3 bits, A corresponds to D, B Corresponds to E, C corresponds to F, and so on...

This is easy to understand. I will not detail it here.

When the offset is 13 characters, the Caesar password is also called the rotary password (rot13): the plaintext is encrypted to obtain the ciphertext, and then the ciphertext is encrypted to obtain the plaintext (because the offset is 13 characters, a total of 26 letters, encryption will return to the plain text). In CTF, the keyword eye will contain words such as rotation, maneuver, and thirteen kicks.

• The Morse code expresses 26 letters and numbers 0-9 in the "." and "-" arrangement.
• The Playfair password is in a 5x5 table, with 26 letters (I and j are regarded as a grid) filled with 25 grids. If you have a key, first enter the key in a non-repeated order, and then enter other letters that are not in the key in 26 alphabetic order. For example, the key is crazy dog.

The rules for plaintext encryption are as follows: 1. If P1 P2 is in the same row, the corresponding ciphertext C1 C2 is the right Letter of P1 P2. The first column is regarded as the right side of the last column. For example, according to the previous table, CT corresponds to DC2. If P1 P2 is in the same column, the corresponding ciphertext C1 C2 is a letter that is close to P1 P2. The first row is considered below the last row. 3 if P1 P2 is not in the same row and is not in the same column, C1 C2 is the other two-byte Letter of the rectangle determined by P1 P2 (as for horizontal replacement or vertical replacement, you must make an appointment in advance, or try it on your own ). For example, in the preceding table, wh corresponds to ku or UK. According to the preceding table, the plaintext Where there is life, there is hope. it can be sorted as follows: Wh er et he re is li fe th er EI sh op ex and then ciphertext: ku yo XD ol Oy pl fk dl Fu yo LG ln ng ly converts the ciphertext to uppercase, and then several letters are arranged in a group. For example, the five groups are kuyox doloy plfkd lfuyo lglnn. The reverse decryption process is OK ~

• The barrier password divides the plaintext to be encrypted into N groups, and then connects the 1st characters in each group to form an irregular line. However, there is a hidden rule in the fence password itself, that is, there are usually not many letters that make up the fence. (Generally no more than 30, that is, one or two sentences)
• Generally, the two-column fence password is commonly used.
  For example, plain text: There is a cipher
  Remove the space and change it to thereisacipher.
  Two groups.
  Extract the first letter: teescpe
  The second letter is hriaihr.
  Connected together: teescpehriaihr

 

• The Virginia password is an encryption algorithm that uses a series of Caesar passwords to form the password alphabet. It is a simple form of Multi-Table passwords.

• To generate a password, use the table method. This table, as shown in figure 1, contains the 26-line alphabet. Each row is obtained by an offset from the first row to the left. The specific line of alphabet used for compilation is based on the key and will be constantly changed during the process.

  For example, assume that the plaintext is: attackatdawn selects a keyword and repeats to obtain the key. If the keyword is Lemon, the key is lemonlemonle for the first letter A in the plaintext, the first letter of the corresponding key, L, is encrypted using the L-line alphabet in the table to obtain the first letter of the ciphertext. Similarly, the second letter in plaintext is t, and the corresponding Row E is encrypted in the table to obtain the second letter X in ciphertext. Similarly, the decryption process of the plaintext: attackatdawn key: lemonlemonle ciphertext: lxfopvefrnhr is opposite to that of encryption. For example, according to the L-line alphabet corresponding to the first letter l of the key, it is found that the first letter l in the ciphertext is located in column A, so the first letter in the plain text is. The second letter e of the key corresponds to the E-line alphabet, and the second ciphertext letter X is located in the T column of this line, so the second plaintext letter is T. And so on.
• At present, I 'd like to introduce so many classical passwords first.

Getting started with CTF-Cryptography Basics