Chapter V - message authentication and digital signature

1. Message authentication ： The process by which the message receiver verifies that the received message is from a trusted source and has not been tampered with , Used to prevent active attacks , Including camouflage 、 Content modification 、 Sequential modification 、 Timing modification .
   Purpose ： Ensure the authenticity and integrity of the message , Enable the receiver to identify the source of the message 、 Authenticity of content and validity of time .
   Message authentication model ： A triple （K,T,V）
   
   Authentication function ： Three types of ： Message encryption function 、 Message authentication code 、 Hash function
2. Hash function ：h = H（M）
   Input ： Messages of any length M
   Output ： Fixed length hash value H（M）
   One way function ： Forward calculation is easy , Reverse calculation is difficult
   news M Functions of all bits of ： Any change in one or more bits in the message will cause the hash value to change
   Hash Function general structure ： Almost all hash Function USES ： Send the original message M Divide into blocks of fixed length ; The last piece is filled ; Set the initial value CV0
   Compression function f, CVi=f(CVi-1,Yi-1); the last one CVi by hash value .
3. digital signature ： Authentication techniques to prevent source or destination denial
   Put forward the reason ： Message authentication protects the data exchange between communication parties from being infringed by a third party , But there is no guarantee that both parties will cheat each other .
   The basic requirements ： Can be physically inseparable from the signed document ; The signer cannot deny his signature ; Signatures cannot be forged ; Easy to verify
   classification ： It can be divided into direct digital signature and arbitration digital signature ; It can be divided into unconditional digital signature and computationally secure digital signature ; It can be divided into one-time digital signature and multiple digital signature
4. Arbitration digital signature ： The sender sends the signed message to the arbiter first , The arbiter tests the message and its signature , To check its source and content , Then time stamp the message , And send it to the receiver together with the instruction of arbitration verification .
   requirement ： The arbiter plays a key and sensitive role , All participants must have great confidence in the normal operation of this arbitration mechanism
5. Blind signature steps
   (1) The message sender first blinds the message
   (2) Let the signer sign the blind message
   (3) The message owner removes the blind factor from the signature , Get the signer's signature on the original message