[jcoglan]

Possibly, although it depends on the lifetime of the link and the size of the tag (e.g. SHA-1 is 40 chars, each case requires mean of 8 guesses, so whole thing only requires 320 requests).

Better to have resistance as baked into your crypto as you can, rather than relying on a firewall further up the stack. If the data itself is resilient, no implementation will be able to defeat it efficiently.