2024-07-08
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When you need to encrypt plaintext of any length, you need to iterate the encryption in groups. There are many modes of block ciphers. If the mode is not selected properly, the confidentiality of the plaintext cannot be fully guaranteed.
Block Cipher: Divide the plaintext data into blocks of fixed length (usually 128 bits or 64 bits) and then encrypt them block by block. Each block is called a block.
Stream Cipher：An encryption algorithm that processes data streams continuously, encrypting bit by bit or byte by byte, suitable for data stream encryption.
Mode is an iterative algorithm for block ciphers
For example, the ECB mode in the DES algorithm we often use. The ECB mode is a method of dividing the plaintext into multiple groups and encrypting them one by one. The ECB algorithm has a small amount of calculation and a fast encryption and decryption speed, but it has now been proven that ECB is an unreliable mode. So don't use the ECB mode in formal occasions.
ECB groups the plaintext and then directly encrypts it to produce ciphertext groups
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In ECB mode, plaintext and ciphertext are one-to-one corresponding. The same plaintext can always get the same ciphertext. So although the plaintext cannot be directly derived from the ciphertext, it is possible to carry out targeted attacks based on the characteristics of the ciphertext.
For example: Bob sent a transfer message to Alice:
分组1 = Bob的银行账号
分组2 = Alice的银行账号
分组3 = 转账金额
Although Eve cannot directly modify the account number and amount (because there is no decrypted data), Eve can launch an attack by exchanging group 1 and group 2.
分组1 = Alice的银行账号
分组2 = Bob的银行账号
分组3 = 转账金额
This attack is successful because ECB does not hide plaintext information.
Ciphertext Group Chaining Mode
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The core of CBC is to use the previous ciphertext group as a variable in the next plaintext encryption. In this way, even if the plaintext is the same, different results will appear after the ciphertext XOR operation. This achieves the purpose of obfuscating plaintext information.
But I wonder if you have noticed how the first plaintext block is processed? When processing the first plaintext block, the XOR is 0. Then the ciphertext block A degenerates into ECB mode. In order to avoid this situation, we need to artificially add an initial ciphertext block, which is called IV component.
Application Scenario：
An improvement to CBC mode
In CFB mode, the plaintext blocks and ciphertext blocks are simply XORed. At this point, the output of the cryptographic algorithm has some of the meaning of a one-time pad.
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Output feedback mode. The output of the cryptographic algorithm is fed back into the input of the cryptographic algorithm.
OFB does not encrypt plaintext through a cryptographic algorithm, but generates ciphertext by XORing the plaintext blocks with the output of the cryptographic algorithm. Therefore, OFB and CFB are somewhat similar.
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I have devoted myself to the research of technology for more than 30 years. I am proficient in various languages such as Java, Linux, JavaScript, PHP, CSS, etc. I have made many contributions in the field of open source. I have established a developer documentation site to share some problems in technology development for everyone to read.
