Bob wants to ensure that Alice can check whether his message has been tampered with. He creates a checksum of the message and encrypts it using asymmetric cryptography. What key does Bob use to…

Question

Options

• AAlice's private key
• BAlice's public key
• CHis own private key
• DHis own public key

• A
  11% (4)
• B
  6% (2)
• C
  81% (29)
• D
  3% (1)

Explanation

Why C is Correct: Bob encrypts the checksum (digital signature) with his own private key because only he possesses it, proving the message originated from him. Alice can then decrypt it using Bob's public key - if decryption succeeds and the checksum matches, she knows the message hasn't been tampered with and was genuinely sent by Bob.

Why the Distractors are Wrong:

• A (Alice's private key): Bob doesn't have access to Alice's private key - it's private to Alice alone.
• B (Alice's public key): Encrypting with Alice's public key achieves confidentiality (only Alice can decrypt), not integrity/authentication - anyone could have sent it.
• D (His own public key): If Bob used his own public key, anyone could decrypt it (since public keys are openly available), but more critically, it wouldn't prove Bob sent it, since anyone could encrypt with his public key.

Memory Tip: Think of it this way - "You sign with what only YOU have." A digital signature uses the sender's private key to prove identity and integrity. Private = personal proof of origin. If the goal were secrecy, you'd use the recipient's public key instead.

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