Basic Understanding Regarding Digital Signatures and Attacks

Sometimes we get lot of messages or files from our friends or may be relatives. Have we ever thought whether our friends or relative has sent this or not ??🥴

Usually no, but Don't brood Digital signature helps us to verify that it is your friend or not. 

Digital Signature is mathematical model or scheme that is significant for verifying the authenticity of your message. 

Lets us understand digital signature with the help of Generic Model (as shown in Fig. 1).


Fig.1 : Generic Model of Digital Signature Process

Properties
Message Authentication protects who exchange two parties who exchange message from any third party. However, it does not protect the two parties against each other. 

Suppose Jammu sends an authenticated message to Una. Please consider the following disputes that could arises.
A. Una may forge a different message and claim that it came from mandi. Una would simply have to create a message and append an authenticated code using the key that Jammu and Una share. 

B. Jammu can deny sending the message because it is possible for Una to forge a message, there is no way to prove that Jammu did in fact send the message. 

Now in above cases, it is a concern of matter. So we need authentication. The most charming solution to this problem is the digital signature,. The digital signature has the following properties. 

1. Must verify the author and the data and time of signature. 
2. Must authenticate the contents at the time of the signature. 
3. Must be verifiable by third parties, to resolve disputes. 

Attacks and Forgeries 

Before starting this, Please consider 'A' denotes the user whose signature method is being attacked, and 'C' denotes the attacker. 

1. Key Only Attack : A only knows C public key. 
2. Known Message Attack : C is given access to a set of messages and their signatures. 
3. Generic Chose Message Attack : C chooses a list of message before attempting to breaks A signature scheme, independent of A's  public key. C then obtains from  A valid signature for the chose message. The attack is generic because it does not depend on A's public key, the same attack is used against everyone. 
4. Directed Chosen Message Attack : Similar to generic attack, except that the list of message to be signed is chosen after C knows A's public key but before any signature are seen. 
5. Adaptive Chosen Message Attack : C is allowed to use A as an oracle😓. This means that C may request from A signatures of messages that depend on previously obtained message signature pairs. 

Now defines success at breaking a signature scheme as an outcome in which C can do any of the following with a non negligible probability., 

Total Break : C determines A's private key. 
Universal Forgery :  C finds an efficient signing algorithm that provides an equivalent way of constructing signatures on arbitrary messages. 
Selective Forgery : C forges a signature for a particular message chosen by C.
Existential Forgery : C forges a signature for at least one message. C has no control over the message. Consequently, this forgery may only be a minor nuisance to A. 

Digital Signature Requirements 

1. The signature must be a bit pattern that depends on the message being signed. 
2. The signature must have some information unique to the sender to prevent both forgery and denial. 
3. It must be relatively easy to produce the digital signature. 
4. It must be relatively easy to recognize easy to recognize and verify the digital signature. 
5. It must be computationally infeasible to forge a digital signature, either by constructing a new message for an existing digital signature or by constructing a fraudulent digital signature for a given message. 
6. It must be practical to retain a copy of the digital signature in storage. 
 
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%3CmxGraphModel%3E%3Croot%3E%3CmxCell%20id%3D%220%22%2F%3E%3CmxCell%20id%3D%221%22%20parent%3D%220%22%2F%3E%3CmxCell%20id%3D%222%22%20value%3D%22Bob's%20Private%20Key%22%20style%3D%22text%3Bhtml%3D1%3BstrokeColor%3Dnone%3BfillColor%3Dnone%3Balign%3Dcenter%3BverticalAlign%3Dmiddle%3BwhiteSpace%3Dwrap%3Brounded%3D0%3BfontSize%3D14%3B%22%20vertex%3D%221%22%20parent%3D%221%22%3E%3CmxGeometry%20x%3D%22-40%22%20y%3D%22240%22%20width%3D%2280%22%20height%3D%2260%22%20as%3D%22geometry%22%2F%3E%3C%2FmxCell%3E%3C%2Froot%3E%3C%2FmxGraphModel%3ESimilar 

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