Alice needs to send a confidential document to her coworker, Bryan. Their company has public key infrastructure set up. Therefore, Alice both encrypts the message and digitally signs it. Alice uses __

Question

Options

• ABryan's public key; Bryan's public key
• BAlice's public key; Alice's public key
• CBryan's private key; Alice's public key
• DBryan's public key; Alice's public key

• A
  7% (2)
• B
  3% (1)
• C
  3% (1)
• D
  86% (25)

Explanation

PKI Encryption & Digital Signatures

Option D is correct because in PKI, encryption and digital signatures use different key pairs for different purposes: Alice encrypts the message using Bryan's public key (ensuring only Bryan can decrypt it with his private key), and Bryan verifies Alice's digital signature using Alice's public key (confirming the message truly came from Alice, since only she holds her private key to sign it).

Options A and B are wrong because using Bryan's public key twice (A) ignores the signature verification process entirely, and using Alice's public key twice (B) would mean Alice is encrypting to herself, making it unreadable by Bryan. Option C is wrong because Bryan's private key is never shared - it's secret by definition - so Alice could never use it to encrypt.

Memory Tip: Think of it in two steps - "Who receives it? Use their public key to lock it" (Bryan receives → Bryan's public key encrypts), and "Who sent it? Use their public key to verify it" (Alice sent → Alice's public key confirms her signature). The rule is simple: public keys verify and unlock identity; private keys sign and decrypt.

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