Encryption and encoding resolution

Classical cryptography

Originated in Ancient Wars ： In the war , In case of ⽌ Important information leaked after the letter was intercepted ,⼈ They began to read books
Xinjin ⾏ encryption .

Shift encryption

Like a password stick , send ⽤ The strips of cloth wound around ⽊ On the stick ⽅ In this way, we can write letters ⾏ encryption
encryption algorithm ： Write after winding secret key ：⽊ Great ruler ⼨

Alternative encryption

Make by the rules ⽤ Different ⽂ Words to replace the original ⽂ Words come in ⾏ encryption .

Modern cryptography

1. You can encrypt anything ⼆ Hexadecimal data
2. ⾮ With the advent of symmetric encryption, cryptography has become more and more popular ⼴ General ⽤ Way ： digital signature

Symmetric encryption

1. Communication double ⽅ send ⽤ Same as ⼀ Key , send ⽤ Encryption algorithm with key to encrypt , Decrypt with ⽤ The encryption process
   The complete inverse of the process with the key to enter ⾏ Decrypt .
2. characteristic ： Encryption is decryption using the same key , But encryption and decryption algorithms are different , Decryption algorithm is the inverse operation of encryption algorithm .
3. Algorithm ：DES（56 A key , The key is too short ⽽ Gradually abandoned ⽤）、AES（128 position 、192 position 、256 A key ,
   Now it's the most current ⾏）
4. Symmetric encryption functions ： Protect communications , prevent ⽌ Information is not secure ⽹ After the network was intercepted , Information is ⼈ Read or tamper with .
5. Symmetric encryption cracking risk ： Because the encryption and decryption keys are the same , For example, on a landing site , Once the key is intercepted , And then the crazy registration operation of the website , You can get a lot of original texts - The ciphertext is right , At the same time, there is the original data after decryption , So there is a certain probability that you can try out the key content , To crack the key , This is man in the middle attack . This probability can only be reduced by optimizing the encryption algorithm .
6. The disadvantages of symmetric encryption ： As mentioned above , The key cannot be transmitted over an insecure network , It's easy to leak the key , But the Internet is often insecure .
7. Symmetric encryption process diagram :
   

⾮ Symmetric encryption

1. principle ：⾮ Symmetric encryption keys are one to two （ Public and private keys ）, send ⽤ The public key enters into the data ⾏ Encrypts get encrypted ⽂; send ⽤ The private key enters the data ⾏ Decrypt the original data . The diagram is as follows ：
   

2. advantage ： As shown in the figure above , send ⽤⾮ Symmetric encrypted communication , Can be in untrustworthy ⽹ There will be two ⽅ Pass the public key of to ⽅, Before data transmission , The sender uses the other party's public key to encrypt the original data , After sending it , The receiver uses its private key to decrypt .

3. characteristic ： The encryption key and decryption key of asymmetric encryption are different , But the encryption algorithm is the same .

4. effect ： The public key and private key of asymmetric encryption are mutually solvable , therefore ⾮ Symmetric encryption can also respond to ⽤ Digital signature technology .

Signature and authentication ：

① The sender and receiver hold their own private key and the other party's public key respectively as follows ：
②. The sender uses its own private key and encryption algorithm to sign the original data , And then send it to the receiver along with the original data , The receiver uses the sender's public key to decrypt the signature data to get the original data , Compare and verify with the original data .

③ This actually sends the original data twice , The usual practice is that the original data cannot be transmitted on the network , There is a risk of leakage , Instead, we do the original data once before sending it hash Algorithm （ Irreversible ） Generate summary , Then use your private key to hash After the signature of the data digest , Then put the signed abstract on the original data hash After the summary sent out . The receiver uses the sender's public key to decrypt the signed digest to get the original data hash Later data , And then it's sent to you hash After the original data comparison , To verify the consistency . That is to say, the original data is not transmitted directly , It's not encrypted , It's about delivering it hash value , The receiver does not save the original data （ Similar to the server does not save the password , Just save the password hash value ）

5. Asymmetric encryption algorithm ：RSA（ can ⽤ For encryption and signature ）、DSA（ only ⽤ In signature , But faster ）
6. ⾮ Advantages and disadvantages of symmetric encryption ： It can be in an unsafe place ⽹ The key is transmitted over the network , Because it's a public key , But the computation is complicated , Performance will be much worse than symmetric encryption .
7. Crack the risk ： Common passwords may be collected in rainbow tables .

hash Algorithm

1. Definition ： Convert arbitrary data to specified ⼤⼩ Range （ It's usually very ⼩, for example 256 Bytes within ） The data of
2. effect ①： Extract summary information from data , So the main thing is ⽤ It's digital fingerprints .
3. effect ②： Fast only ⼀ Validation , for example Java Medium hashCode() ⽅ Law , Can quickly verify the inequality of two values .
4. effect ③： Data integrity verification . For example, from ⽹ Download ⽂ After that , adopt ⽐ Yes ⽂ Piece Hash value （ for example MD5、SHA1,SHA256 All belong to hash Algorithm , It's all irreversible ）, You can confirm the download ⽂ Is there any damage to the parts . If you download ⽂ Pieces of Hash Values and ⽂ It provides ⽅ Given Hash value ⼀ Cause , It proves that the downloaded ⽂ It's in good condition ⽆ It's a loss .
5. effect ④： Quick search , for example HashMap, Arrays are used internally , By calculation key Of HashCode() Direct positioning value The index , There's no need to traverse the entire array .
6. effect ⑤： Privacy protection , When important data has to be exposed , There's something you can choose to expose Hash value （ for example MD5）, To ensure the security of the original data . for example ⽹ When you log in at the station , You can just save ⽤ The password of the user Hash value , In each login verification, you only need to input ⼊ Of the code Hash Value and the... Stored in the database Hash It's worth it ⽐ That's right ,⽹ standing ⽆ Need to know ⽤ The password of the user . such , When ⽹ When station data is stolen ,⽤ It's not because ⾃⼰ Your password was stolen, causing other ⽹ The security of the station is also threatened , however hash The algorithm does not belong to encryption , Because it's a one-way irreversible operation . And it doesn't belong to coding , Because encoding corresponds to decoding , It's reversible .

Encoding and decoding

1. Definition ： To encode is to take data from ⼀ Convert one data format to another ⼀ Data formats . Decoding is the same . It's also reversible , There's coding, there's decoding .

Base64 code

1. Definition ： take ⼆ Hexadecimal data （ All the computer data is ⼆ Hexadecimal data ） Convert to by 64 The encoding algorithm for a string of characters ,
2. Algorithm ： Put the original data every 6 Bit correspondence Base 64 In the index table ⼀ Two characters arranged into ⼀ A string （ Each character 8 position ）,base64 The index table is as follows ：
   
3. Base64 Of ⽤ Way ： take ⼆ Binary data expands the way of storage and transmission . Ordinary strings go through Base64 The result of coding will become ⾁ The eye can't read .
4. Base64 The shortcomings of ： because ⾃ The principle of the body （6 Displacement 8 position ）, So every time Base64 After coding , The data will increase ⼤ about 1/3, So it will affect the storage and transmission performance . therefore Use base64 It is not efficient to encode and transmit pictures （ Because the volume increases ）, It won't be safe either （ because base64 It's coding , It's reversible , It can be decoded ）

variant ：Base58,⽐ Special currency envoy ⽤ The coding ⽅ type , Removed Base64 Number in “0”, word ⺟⼤ Write “O”, word ⺟⼤ Write
“I”, He Zi ⺟⼩ Write “l”, as well as “+” and “/” Symbol ,⽤ On ⽐ The representation of special currency address .
Base58 about Base64 The changes to the , The main ⽬ What's important is ⽤ The convenience of households . By removing the indistinguishable
character , bring Base58 about 「⼈⼯ Copy 」 more ⽅ then . in addition , Removed “+” “/” After the number one, let ⼤
Most software can ⽅ Double click to select

URL send ⽤ Percent sign code of

stay URL In the string of , Yes ⼀ Some No ⽤ For special ⽤ The reserved character of the path , send ⽤ Percent sign “%” Enter... For prefix
⾏ code , To avoid parsing errors .

Compression and decompression coding

Compress ： Put the data so that ⽤ The coding algorithm with more storage advantages is introduced ⾏ code .
decompression ： Decode compressed data back to its original form , With ⽅ So that ⽤
The purpose is to reduce costs ⼩ Data take up ⽤ Storage space .

serialize

Put the data object （⼀ It's in memory , for example JVM Objects in the ） The process of converting to a sequence of bytes . object
In program memory ⾥ The storage form is scattered （ Stored in different memory areas 、 And it's led by ⽤ Into the ⾏ even
Pick up ）, Through serialization, objects in memory can be converted to ⼀ A sequence of bytes , from ⽽ send ⽤ Byte array byte[] Isomorphism
Step in ⾏ Local storage or ⽹ Network transmission , Reassemble when needed （ Deserialization ） To make ⽤. The goal is to allow objects in memory to be stored and transferred .

serialize It's not coding , Encoding is the transfer of data from ⼀ Convert one data format to another ⼀ Data formats ;⽽ Serialization is the transfer of data from objects in memory （⽽ It's not a specific format ） Convert to a sequence of bytes