Cryptography -- the mode of block cipher

Block ciphers and stream ciphers

Cryptographic algorithms can be divided into block ciphers and stream ciphers .

1. Group password ： A class of cryptographic algorithms that can only process a piece of data of a specific length at a time . The number of bits of a packet is called the packet length .DES The packet length of is 64 The bit ,DES You can only encrypt at one time 64 The plaintext of bits , And generate 64 Ciphertext of bit . Treat in groups , There is no need to record the progress of encryption through internal state .
2. Stream code ： A class of cryptographic algorithms for continuous data processing . Generally speaking, with 1 The bit 、8 Bit or 32 Encryption and decryption in bits . Encrypt and decrypt a string of data streams , Need to maintain internal state . One time password belongs to stream password .

Block cipher mode

Due to the limited processing length of block cipher , So when processing long plaintext data , The block cipher algorithm needs to be iterated , The iterative method is called block cipher pattern . The main models are ：

1. ECB Pattern ： Electronic codebook mode .
   ECB The result of encryption of plaintext group is directly regarded as ciphertext Group . Plaintext grouping and ciphertext grouping are one-to-one correspondence . shortcoming ： Attackers can manipulate plaintext without breaking passwords （ Manipulate ciphertext order , Because plaintext grouping and ciphertext grouping correspond one by one ）.
   
   

2. CBC Pattern ： Password group link mode .
   CBC The pattern first groups the plaintext with the previous ciphertext XOR operation , And then encryption . There is a difference between plaintext group and ciphertext group XOR And cryptographic algorithm .
   
   

3. CFB Pattern ： Ciphertext feedback mode .（ The ciphertext group is fed back to the cryptographic algorithm ）
   CFB The mode will send the previous ciphertext packet back to the input of the cryptographic algorithm , Proceed again XOR operation . There is only XOR operation .
   
   

4. OFB Pattern ： Output feedback mode .（ The input of the cryptographic algorithm is the output of the previous cryptographic algorithm ）
   OFB The output of the pattern cipher algorithm will be fed back to the input of the cipher algorithm . Through the output of plaintext grouping and cryptographic algorithm XOR Generate cryptographic grouping .
   
   

5. CTR Pattern ： counter mode .
   CTR Mode is a stream cipher that generates a key stream by encrypting the counter accumulated step by step . The final ciphertext packet is a bit sequence obtained by encrypting the counter , Group with plaintext XOR obtain .