GATE IT 2004 | Question: 25

Question

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

• A. Sender encrypts using receiver's public key
• B. Sender encrypts using his own public key
• C. Receiver decrypts using sender's public key
• D. Receiver decrypts using his own public key

Comments

" to send a secret message to a receive "

 

thats why (c) is wrong!

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This topic is not in syllabus of GATE 2021 as it comes under network security: basics of public key and private key cryptography

https://gatecse.in/gate-2021-syllabus-including-changes/

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Out of syllabus

Answer 1

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: $A$

Comments

Superb explanation

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Yes ,this is called confidentiality.

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this is really mind blowing explanation

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You explained it really well. @Soumya29

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Brilliant

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just perfect explanation

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Source: https://en.wikipedia.org/wiki/Public-key_cryptography

Answer 2

A) Sender encrypts using receiver's public key

Comments

why not option B as public key is common or same to both parties so we can also says that Sender encrypts using his own public key

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@rishu_darkshadow No,we cannot say that Sender encrypts using his own public key.

Let us suppose that we have A as sender and B as receiver, and say both have a pair of private and public keys i.e (K_A^- , K_A⁺) for A & (K_B^-, K_B⁺) for B. Also only A knows about K_A^- and only B knows about K_B^-  ,(being private to individual A & B),rest the whole world knows about public key of both A & B, including the intruder, if any. Now as per your doubt if we use K_A⁺ for encryption of message 'm', we need K_A^- to decrypt it, which as mentioned will be known only to A. Since only K_A^- and K_A⁺ together can open a message.So since B doesn't know A's private key, he won't be able to decrypt the message. But by choosing K_B⁺ for encryption, B can definitely decrypt it since he has his own K_B^- for it. So concluding, A(Sender) does need K_B⁺ (Receiver's public key) to encrypt the message. Hence A.

Answer 3

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

Answer 4

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 5

If Alice wants to send data to Bob

Then you must be clear about how to send data to receiver

If sender sends data with encrypting receiver public key which receiver has shared, no one knows actually what is the private key of receiver. So receiver only can decode it with its private key.

This way the secret message will be shared by the sender to the receiver

Answer 6

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