What Is a Block Cipher?

The mathematical model of block cipher is to encode a sequence of numbers (referred to as plaintext numbers) after the plaintext message is encoded and divided into groups of length n (can be regarded as a vector of length n ), and each group is separately in the key. Controlled conversion into a sequence of output digits (referred to as ciphertext digits) of equal length.

The study of modern block ciphers began in the mid 1970s and has more than 40 years of history. During this period, people have achieved fruitful research results in this field.

For block ciphers, in early research, it was basically conducted around DES, and some similar algorithms were introduced, such as: LOKI, FEAL, GOST, etc. In the 1990s, people

In general, the study of block ciphers includes three aspects: the design principles of block ciphers, the security analysis of block ciphers, and the statistics of block ciphers.

Diffusion and confusion are the main factors affecting password security. The purpose of diffusion is to allow a single number in the plaintext to affect multiple numbers in the ciphertext, so that the statistical characteristics of the plaintext disappear in the ciphertext, which is equivalent to

The collection of AES has set off a new upsurge in block cipher research. The 15 AES candidate algorithms reflect the current level of block cipher design and can be said to be a confluence of research results in recent years. The overall structure used by block ciphers can be divided into Feistel structures (such as CAST-256, DEAL, DFC / E2, etc.), SP networks (such as Safer +, Serpent, etc.) and other cryptographic structures (such as Frog and HPC). The Feistel structure is widely known due to the publication of DES and has been adopted by many block ciphers. The biggest advantage of the Feistel structure is that it is easy to ensure that encryption and decryption are similar, which is especially important in implementation. It is more difficult for SP networks to do this, but the diffusion characteristics of SP networks are better. In the existing block ciphers, all basic operations include XOR, addition, subtraction, table lookup, multiplication, and data-dependent loops. The table lookup operation provides the security foundation of DES. Careful selection of S-boxes can better resist linear and differential cryptanalysis, providing good data and

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Block cipher advantages

Plain text information has good scalability, sensitivity to insertion, does not require key synchronization, strong applicability, and is suitable as an encryption standard.

Block cipher disadvantages

Encryption is slow and errors spread and propagate.

Block ciphers convert fixed-length plaintext blocks into equal-length ciphertexts, a process under the control of a secret key. Use inverse transform and same key to achieve decryption. For many current block ciphers, the block size is 64 bits, but this is likely to increase.

Plaintext messages are often much longer than a specific packet size and use different techniques or operations. Examples of such methods are: Electronic Code Book (ECB), Cipher Block Link (CBC), or Cipher Feedback (CFB). ECB uses the same key to simply encrypt each plaintext block one by one; in the CBC method, each plaintext block is XORed with the previous ciphertext block before encryption, which adds complexity The degree can make certain attacks more difficult to implement. The "output feedback" method (OFB) is similar to the CBC method, but the amount of "exclusive OR" is independently generated. CBC is widely used, for example in DES (qv) implementations, and various approaches are discussed in depth in corresponding books on the technical aspects of cryptography. Please note: The general weakness of your own cryptosystem is to use some of the public algorithms in a simple form, rather than in a specific way that provides additional protection.

Iterative block ciphers are those whose encryption process has multiple cycles, thus improving security. In each loop, an appropriate transformation can be applied by using a special function to derive a subkey from the initial key. This additional computing requirement necessarily affects the speed at which encryption can be managed, so there is a balance between security needs and execution speed. There is no free lunch in the world, and so is cryptography; as elsewhere, part of the skill in applying the appropriate methods is derived from understanding the trade-offs that need to be made and how they relate to the balance of needs.

Block ciphers include DES, IDEA, SAFER, Blowfish, and Skipjack the last is the algorithm used in the US National Security Agency (NSA) limiter chip. ^[3]