Cryptography notes

Catalog

The basic definition ：

Evaluation criteria for passwords ：

Text ：

Symmetric cipher algorithm （Symmetric-key Algorithm）

Asymmetric cipher algorithm （Asymmetric-key Algorithm）

The principle of double key encryption is as follows ：

Abstract algorithm （Digest Algorithm）

Conclusion ：

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The basic definition ：

Cryptography not only studies the mathematical algorithms of encryption and decryption . More time , Cryptography studies the strategy of protecting information security , We can call it “ agreement ”. Based on the existing mathematical model , We often ignore the specific mathematical implementation methods , Instead, focus on the security measures that can be built with the help of these mathematical tools .

As long as the decipherer can't afford it （ Time , Human or other resources ） Next, complete the decipherment , We can think that this password is very secure

Evaluation criteria for passwords ：

1. Security ： Practical safety , Randomness , reliability , Other safety factors

2. cost ： Patent requirements , Computational efficiency , Storage space efficiency

3. Algorithm and execution features ： flexibility , Conciseness

Text ：

There are three main types of cryptographic algorithms ： Symmetric cipher algorithm 、 Asymmetric cipher algorithm 、 Abstract algorithm .

Symmetric cipher algorithm （Symmetric-key Algorithm）

Symmetric algorithm   It refers to the same cryptographic algorithm of encryption key and decryption key , Also known as   Secret key algorithm   or   Single key algorithm  .

The algorithm is further divided into   Block cipher algorithm （Block cipher）  and   Stream cipher algorithm （Stream cipher） .

• Block cipher algorithm
  • Also known as block encryption algorithm
  • Encryption step one ： Split the plaintext into N A fixed length plaintext block
  • Encryption step two ： Encrypt each plaintext block with the same secret key and algorithm to get N A ciphertext block of equal length
  • Encryption step three ： And then N Ciphertext blocks are combined in order to get ciphertext
  
  
• Stream cipher algorithm
  • Also known as sequence cipher algorithm
  • Only encrypt at a time \ Decrypt one bit or one byte plaintext

In the case of single key encryption , There is only one key , So the key storage becomes very important . Once the key is leaked , The password is cracked .

Asymmetric cipher algorithm （Asymmetric-key Algorithm）

Asymmetric algorithm   It refers to the different cryptographic algorithms between encryption key and decryption key , Also known as   Public cryptographic algorithms   or   Public key algorithm , The algorithm uses a secret key （ Open ） To encrypt , With another secret key （ Private ） Decrypt .

The principle of double key encryption is as follows ：

a) Public key and private key are one-to-one correspondence , With a public key, there must be a corresponding 、 Unique private key , And vice versa .

b) be-all （ Public key , Private key ） Yeah, it's different .

c) Using public key can unlock the information encrypted by private key , And vice versa , namely , Public keys are used to encrypt information , The private key is used for digital signature .

d) It should be relatively easy to generate public key and private key at the same time , But extrapolate the private key from the public key , It should be difficult or impossible .

Because anyone can generate their own （ Public key , Private key ） Yes , So in order to prevent someone from spreading forged public key to cheat trust , You need a reliable third-party organization to generate certified （ Public key , Private key ） Yes . at present , The world's leading digital service certification provider is located in California, USA Verisign company , Its main business is distribution RSA digital certificate .

at present , The general single key encryption algorithm is AES, The general double key encryption algorithm is RSA, Both originated in the last century 70 years .

Abstract algorithm （Digest Algorithm）

Abstract algorithm   It compresses the data into summaries , Make the data smaller , Fix the data point format （ Data of any length is hashed , Will output a fixed length of data , be called “ Hash value ”） Also known as   Hash function  、  hash function  、  Hash function  、 One way function   etc. .

The output data of fixed length generated by the algorithm is called   The summary is worth  、  Hash value   or   Hash value  , The algorithm has no secret key .

Abstract algorithm   It is usually used to determine the integrity of data , That is, hash the data and compare it   The summary is worth   Is it consistent .

Algorithms are mainly divided into three categories ：MD（Message Digest, Message digest algorithm ）、SHA-1（Secure Hash Algorithm, Secure hash algorithm ） and MAC（Message Authentication Code, Message authentication code algorithm ）; Another national security standard SM3 Also belongs to the summary algorithm .

The original text has been slightly revised , The summaries are very different .

Conclusion ：

We have already explained the classic RSA Asymmetric cryptographic algorithms and SHA256 Abstract algorithm , In the future, I will publish a separate article to explain AES Symmetric cipher algorithm .

in my opinion , What's really interesting about cryptography is ： We want to protect our privacy on the Internet . For one thousand years , People have been trying , Explore , But the technology of the past has not been able to achieve a high degree of privacy , Electronic technology is expected to become a reality . It eclipses all classical codes （20 In the late 20th century , Like Caesar , Virginia ,ENIGMA）. But at the same time , It will also fall into the code of pen and paper and mind games , Rise to a level beyond human control by means of machines . Let all the bad guys who want to pry into people's privacy be out of reach

Put trust in the sun , Hide the secret in the consensus .