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29 votes
29 votes

A sender is employing public key cryptography to send a secret message to a receiver. Which one of the following statements is TRUE?

  1. Sender encrypts using receiver's public key
  2. Sender encrypts using his own public key
  3. Receiver decrypts using sender's public key
  4. Receiver decrypts using his own public key

5 Answers

Best answer
87 votes
87 votes

In public key cryptography, both sender and receiver generate a pair of keys - Public key and Private key. Public keys are known globally.

Suppose $A$ is sender and $B$ is the receiver.

So, $A$ has $3$ keys:

1. Public key of $A$ (Everyone knows)              2. Private key of $A$(only $A$ knows)                 3. Public key of $B$(Everyone knows)

And $B$ also has $3$ keys:

1. Public key of B (Everyone knows)              2. Private key of B(only B knows)                 3. Public key of A(Everyone knows)

  • Anything that is encrypted using public key of A can be decrypted only using private key of A. 
  • Anything that is encrypted using private key of A can be decrypted only using public key of A.   
  • Anything that is encrypted using public key of B can be decrypted only using private key of B. 
  • Anything that is encrypted using private key of B can be decrypted only using public key of B.   

Now $A$ wants to send a secret message to $B$.

So, for encryption: A has following $3$ options:

  1. Public key of A (Everyone knows):
    So, for decryption $B$ needs - Private key of $A$ - only $A$ knows it. So, $B$ will not be able to decrypt it. 
  2.  Private key of $A$ (only $A$ knows) 
    So, for decryption $B$ needs - Public key of $A$ - Everyone knows it. So everyone can decrypt it. So it is of no use. 
  3. Public key of $B$ (Everyone knows):
    So, for decryption $B$ needs - Private key of $B$ - only $B$ knows it. So, only $B$ will able to decrypt it (That's what we want)

So, for providing Security:

Sender encrypts using receiver's public key and Receiver decrypts using his own private key.

 

Correct Answer: $A$

edited by
15 votes
15 votes

A) Sender encrypts using receiver's public key

0 votes
0 votes
Answer : A. Sender would use receivers Public key to encript the msg, now no one can attach the msg. Receiver will just decrypt using his private key. FYI- C would have been the Answer if digital signal verification is used.
Answer:

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