A difference between encryption keys and MACs is that MACs do not become weaker as the attacker gains more computing power, except by making bruteforcing the au

A difference between encryption keys and MACs is that MACs do not become weaker as the attacker gains more computing power, except by making bruteforcing the authentication key easier, which does not depend on the length of the tag. The attacker is limited by the amount of messages your system can process. Another difference is that old communication is not at risk. As soon as the client and server is upgraded, they are secure, as opposed to encryption where attackers may still recover sensitive information from things that were encrypted years ago. So it doesn't need the same degree of future proofing. So while a 80-bit encryption key is on the weak end of the spectrum, even 80 bits of security is pretty conservative for a MAC.