Plz explain.

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+26 votes

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

- Sender encrypts using receiver's public key
- Sender encrypts using his own public key
- Receiver decrypts using sender's public key
- Receiver decrypts using his own public key

+71 votes

Best answer

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:

- 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. - 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. - 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: $A$

+15 votes

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 **(K _{A}^{-} , K_{A}^{+}) for A** &

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