312-50V9 Question #154: Real Exam Question with Answer & Explanation

The correct answer is D: Sender's private key. In a digital signature scheme, the sender encrypts the message digest using their own private key, allowing any recipient to verify authenticity by decrypting with the sender's public key.

Question

For messages sent through an insecure channel, a properly implemented digital signature gives the receiver reason to believe the message was sent by the claimed sender. While using a digital signature, the message digest is encrypted with which key?

Options

ASender's public key
BReceiver's private key
CReceiver's public key
DSender's private key

Explanation

In a digital signature scheme, the sender encrypts the message digest using their own private key, allowing any recipient to verify authenticity by decrypting with the sender's public key.

Common mistakes.

A. Encrypting the digest with the sender's public key would be cryptographically useless for authentication because anyone possesses the public key and could therefore forge the signature.
B. The receiver's private key is used for decrypting messages sent to the receiver, not for verifying a sender's digital signature - using it here would mix up confidentiality and authentication operations.
C. Encrypting the message digest with the receiver's public key describes asymmetric message encryption for confidentiality, not a digital signature - it would mean only the receiver could decrypt it but would not prove who sent it.

Concept tested. Digital signature key usage and message digest signing

Reference. https://csrc.nist.gov/publications/detail/fips/186/5/final

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