Which technique provides sender authentication and message integrity by encrypting the hash with the sender's private key?

• A. Hash Function
• B. Digital Signature
• C. Public Key Infrastructure
• D. Message Authentication Code

Answer: (B)

Digital signatures enable sender authentication and message integrity by signing a hash of the message with the sender’s private key. First, a hash of the message is created to produce a compact digest. That digest is then encrypted with the sender’s private key to form the signature. When the message is received, the recipient uses the sender’s public key to decrypt the signature and obtain the hash, and independently hashes the received message to produce a fresh digest. If the two digests match, the recipient can be confident that the signer held the private key (authentication) and that the message has not been altered (integrity). This also supports non-repudiation, since only the private key holder could have produced that signature.

The other options don’t fit as well: a hash function alone only assures integrity without proving who created it. Public Key Infrastructure is the framework for managing keys and certificates, not the act of signing itself. A Message Authentication Code relies on a shared secret to verify integrity and authenticity between known parties, but it does not provide verification by someone who doesn’t share that secret or non-repudiation.