Introduction

Block cipher is a method of encryption as well as decryption. The process runs on the plain text blocks instead of working on every direct text bit individually. Every block comes in the same size, and these do not contain a fixed number of bits. Ciphertext that gets generated has several blocks that are the same as the plaintext block number. The block cipher also makes use of the exact key both for encryption as well as decryption.

  1. What is Block Cipher?
  2. Principles of Block Cipher
  3. Block Cipher Modes

1. What is Block Cipher?

In the block cipher method of encryption, the plain text is divided into blocks of a fixed size. Every block consists of the same amount of bits. At one time, the block cipher will operate on a particular block of the plain text only. It then applies the key onto it to make the corresponding ciphertext block.

In decryption, too, only one ciphertext block is operated to form the corresponding text. The perfect example of this is the DES or the data encryption standard. In the DES method, the algorithm of decryption will be the same as is used in the encryption method.

In every block, the block cipher will operate on the entire league and is operated at one time. This is instead of operating on one bit at one time. The method of operating one bit at one time is time-consuming, and since block cipher is the cryptographic algorithm based on the computer, it must stay fast. This is why since the block cipher operates on a block of the bit simultaneously, it makes it much faster than the stream cipher.

The method was fast, but this had a limitation as it generated the ciphertext for a repeated pattern of text in the plain text format. The limitation was, however, resolved when the chaining of the block cipher was implemented.

2. Principles of Block Cipher

The design of the block cipher is based on the three principles, which are

  • Number of rounds
  • Design of function F
  • Key schedule algorithm

A) Number of Rounds

In this, the number of rounds will judge the block cipher algorithm’s strength. It is thought that if the number of rounds is more, it makes it difficult for the cryptanalysis to break into the algorithm. Even in case, the function F stays weak, then the number of rounds makes it challenging to get into the algorithm and break it.

B) Design of Function F

The block cipher design principles F has to be designed to make it impossible for any cryptanalysis to unscramble and substitute it. It is the non-linearity criteria that strengthen the F function. If F is not linear, then it makes it even more painful to crack into it. In designing the process, it needs to get confirmed that there is a property of great avalanche.

This will state that when there is a change of one bit of the input, this change will be reflected in the output bits. F has to be designed so that it has a criterion of F bit independence that suggests that the output bit should change independently. This is if there are any changes in the input bit.

C) Key Schedule Algorithm

This suggests that the schedule key should be able to confirm the strict effect of avalanche and the criterion of bit independence.

3. Block Cipher Modes

The block cipher operation modes are divided into five essential parts and are defined by the NIST. These modes of operation help in enhancing the algorithm such that there could be a wide application range that could be adapted to use the encryption of block cipher.

These modes are

  • Electronic Code Book Mode
  • Cipher Block Chaining Mode
  • Cipher Feedback Mode
  • Output Feedback Mode
  • Counter Mode

A) Electronic Feedback Mode

The electronic feedback mode is the easiest of the block cipher operation mode. In this mode, the plain text gets divided into blocks. Each of the blocks is 64-bit. Every block gets encrypted one at each time to form the cipher block. A particular key is used to encrypt every block.

When the receiver receives a message, then the ciphertext gets divided into 64-bit blocks, and every block gets decrypted independently at one time to get the bare text block that corresponds to it. The method is used to encrypt small messages where the probability of the test repeating is rare.

B) Cipher Block Chaining Mode

This overcomes the ECB limitations. Here, even if the plain text has many repeating blocks, its encryption does not produce a similar cipher block. This is done by adding chaining to it, which gets the ciphertext block obtained depending on the current and any previous plain text block input.

C) Cipher Feedback Mode

Every application may not have been designed to operate on data blocks. Some of them could be a bit or character-oriented too. This is where the cipher feedback mode comes into the picture. It operates on the smaller units instead of operating on blocks.

D) Output Feedback Mode

The output feedback mode operates in encrypted IV, which gets fed into the plain text block encryption. The mode also operates on the block of bits.

E) Counter Mode

The counter mode does not contain any mechanism of feedback. Nothing gets fed from the last step into the next step. Instead, a sequence of numbers is used, called a counter that gets inputted into the encryption function with the key. When a bare text block gets encrypted, it causes the counter value to be incremented by 1.

Conclusion

So, this is everything about the block cipher, the block cipher modes of operation, its principles, and its mode. The block cipher will take a block that consists of the bits in plaintext and then generate the block of ciphertext bits. Both of them would usually be of the same size. The block size stays fixed, and the choice of the block’s size does not directly affect the strength of the scheme. Instead, the power of the cipher is related to the length of the key

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