Information Content Estimation

[Cathal Garvey (Phone)]

Hey all,
Am mulling over a federated datastore for zero-knowledge web applications, using hashcash as a "commitment" price for otherwise gratis data storage. All very straightforward, but:
Zero knowledge is as much for host protection as client protection. Hosts don't WANT plaintext.

Short of stupidly CPU-intensive stuff like letter counting, UTF8 decoding, etc, how might a server verify it's receiving encrypted data? I was thinking a function that estimates apparent entropy and rejects anything that doesn't look random enough to be encrypted, what such functions are available, fast, widely implemented?
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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