GATE CS 2016 Set 1 Q52 — Concept of Layering: OSI and TCP/IP Protocol Stacks

A digital signature is used to provide authenticity, integrity, and non-repudiation. The process for sender $B$ to send a digitally signed message $m$ to receiver $A$ is as follows:

1. Generate Message Digest: $B$ first computes a cryptographic hash of the message, denoted as $H(m)$.
2. Sign the Digest: To prove their identity, $B$ encrypts the message digest $H(m)$ using their own private key, $K_B^-$. Since only $B$ possesses $K_B^-$, only $B$ could have created this signature: $$\text{Signature} = K_B^-(H(m))$$
3. Transmit: $B$ sends the original message $m$ along with the signature to $A$: $$\left\{m, K_B^-(H(m))\right\}$$

Verification at Receiver $A$:

Upon receiving the package, $A$ decrypts the signature using $B$'s public key $K_B^+$ to get $H(m)$, and independently computes the hash of the received message $m$. If both match, the signature is verified.

Why other options are incorrect:

• Option A: Uses $B$'s public key $K_B^+$. Anyone can access $B$'s public key, so this does not provide authenticity.
• Option C: Uses $A$'s private key $K_A^-$, which is secret to $A$. $B$ does not have access to $K_A^-$.
• Option D: Encrypts the message with $A$'s public key $K_A^+$. This provides confidentiality (only $A$ can read it) but does not serve as a digital signature from $B$.