What is an authentication value (Authentikationswert), and how does it differ from encryption?

An authentication value (like a MAC tag) is a short fixed-length tag appended to a message to verify integrity and origin — unlike encryption, it doesn't hide the message content.

Authentication value:

• Input: message $m$ + key $K$
• Output: short tag $t = h(K, m)$ (e.g., 256 bits regardless of message length)
• Message is sent in plaintext alongside the tag
• Anyone can read the message; only key holders can verify/generate the tag

Encryption:

• Input: message $m$ + key $K$
• Output: ciphertext $c$ (same length as $m$ or longer)
• Message content is hidden
• Only key holders can recover the plaintext

Key difference: Authentication protects integrity without secrecy. Encryption protects secrecy without integrity. In practice, you usually need both (authenticated encryption).

When you might want authentication without encryption:

• Software update verification (everyone should be able to read the update, but must verify it's genuine)
• Financial transactions in regulated systems (auditors need to see the data, but it must be tamper-proof)