# GATE2004-IT-25

8.8k views

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
1
Why not option C  ?

Plz explain.
5
@ashwina since the sender's public key is known to everyone (including receiver) , then anyone can decrypt it then the message becomes insecure. So c is wrong

Correct me if I'm wrong?
7
@ashwina

option (c) would be valid if it is the case of a digital signature.
0
" to send a secret message to a receive "

thats why (c) is wrong!
0
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/

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
2
Superb explanation
0
Yes ,this is called confidentiality.
0
this is really mind blowing explanation
0
You explained it really well. @Soumya29
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Brilliant
0
just perfect explanation
0 A) Sender encrypts using receiver's public key

0

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

0

@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 (KA- , KA+) for A & (KB-, KB+) for B. Also only A knows about KA- and only B knows about KB-  ,(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 KA+ for encryption of message 'm', we need KA- to decrypt it, which as mentioned will be known only to A. Since only KA- and KA+ 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 KB+ for encryption, B can definitely decrypt it since he has his own KB- for it. So concluding, A(Senderdoes need KB+ (Receiver's public key) to encrypt the message. Hence A.

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

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