Suppose Alice and Bob achieve some agreement and want to make it official, thus

Question

Suppose Alice and Bob achieve some agreement and want to make it official, thus Alice write down the agreement as a file m and encrypts m using Bob’s public key pk to get c. Alice sends c to Bob, trying to convince B that c is the commitment of Alice about the agreement. Would this method work? Explain why.

If Alice further uses a message authentication code, and generate a tag ? on c (using a shared key k). Now Alice sends both c and ? to Bob. Can Bob now be convinced that Alice already committed to the agreement? Explain why.

Explanation / Answer

First method

The first methos will work.Bob can decrypt C using his private key and Bob can read what is the agreement sent by Alice.

Explanation :   

Cryptography is a technology used to keep a communicaion secret between the communicating peoples in the presense of a third party. Nobody else can understand what is the message.

So the purpose of cryptography is converting plain text message into some unreadable form which is called cypher text. and send to receiver.once the cypher text is reached at receiver, receiver will convert the cypher text into plain text which is the original text. The process of converting plain text to cypher text at the sender side is called Encryption.The process of converting cypher text back into the plain text at receiver side is called Decryption.

plain text : it is the original message.

Cypher text : it is the encrypted message.

   Cryptography is two types, symmetric cryptography and Assymmetric cryptography. here in the above given question assymmetric cryptography is used. so i will answer this question based on that.

assymmetric cryptography

  In this technology there will be a different set of public key and private key pairs for sender and receiver. this public key will be announced to public, therefore everybody knows their public key each other. but everyone will keep their private key very secret, therefore one can know only his or her private key.

two methods are there to encrypt a message,

When Alice sends a message m to Bob, :   

Alice encrypted the agreement using the public key of Bob to C. while receiveing C at Bob, he can decrypt C into the agreement sent by alice using his private key. nobody else can read C in the middle because that person do not know the private key of Bob,so that the person can't decrypt it into original message.since this is a correct way of sending message. this will work fine.

If Alice further uses a message authentication code :

in the second method ? on c is generated using a shared key. this is a method of Symmetric cryptography.

Explanation :

Symmetric cryptography

here a message is encrypted using a shared key. this key will be shared between sender and receiver. sender will send this key along with the encrypted message to receiver.therefore any person who comes in middle can get both the key and encrypted message and the person can decrypt it using the key, this person can also make changes to the original message. in such case the message may be correpted while receiving it at receiver side. it is not a safe way of sending message.

in this method alice generate ? on C using a shared key. so while alice sending both ? and C to Bob, bob can decrypt C using the shared key and bob can read what is sent by Alice. therefore bob can be convicted that it is already committed to the agreement. but if any person comes in middle and make any changes to C will affect badly in this situation.there is a chance for C to be correpted before receiving at Bob.

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