Question

In Public Key Cryptography (RSA), if 'A' wants to send a confidential message to 'B', 'A' should encrypt the message using:

• A. A's Private Key
• B. A's Public Key
• C. B's Public Key
• D. B's Private Key

Hint:

To remember the rule: "Encryption is for the Recipient." You always use the recipient's public key to ensure privacy.

Answer 1

The Correct Option is C

Concept:
In asymmetric encryption systems like RSA, the core principle for achieving Confidentiality (ensuring only the intended recipient can read the message) is to use the recipient's public key.

Step 1: The Logic of Encryption.
If 'A' wants to ensure that only 'B' can read a message:
• 'A' must encrypt the message with a key that can only be reversed by 'B's secret key.
• In RSA, a message encrypted with a Public Key can only be decrypted with the corresponding Private Key.
• Since 'B' is the only person who possesses 'B's Private Key, 'A' should use 'B's Public Key for encryption.

Step 2: Evaluating the alternatives.

• A's Private Key: If 'A' uses this, anyone with 'A's public key (which is everyone) can decrypt it. This provides Authentication (Digital Signature), not confidentiality.
• A's Public Key: If 'A' uses this, 'A' is the only person who can decrypt the message. This is useless for sending data to 'B'.
• B's Private Key: 'A' does not have access to this key; only 'B' does.

Step 3: The RSA Workflow.
The standard workflow for secure communication is: 1. Sender 'A' gets Recipient 'B's Public Key. 2. 'A' encrypts the message with 'B's Public Key. 3. 'A' sends the cipher text to 'B'. 4. 'B' decrypts the cipher text using 'B's Private Key.