In asymmetric encryption, whose key does the sender use, and why?

The sender uses the receiver's public key to encrypt — because only the receiver's corresponding private key can decrypt it.

The logic:

• Goal: Only Bob should be able to read the message
• Bob's private key is known only to Bob
• Bob's public key is available to everyone
• Therefore: Encrypt with Bob's public key → Only Bob can decrypt with his private key

$$c = E(K_{pub,B}, m) \quad \Rightarrow \quad m = D(K_{priv,B}, c)$$

Common mistake: Students often confuse this with digital signatures, where the key usage is reversed:

• Encryption: Use receiver's public key (for confidentiality)
• Signing: Use sender's private key (for authentication)

Analogy: Think of encryption as a mailbox — anyone can drop a letter in (public key = mail slot), but only the owner can open it (private key = mailbox key).