Eh. OK let's say instead of just SHA-256'ing the plaintext data to derive a key you do 50,000 bcrypt rounds. Then the client encrypts the plaintext, hashes the ciphertext, and sends the hash to the server. If it takes 0.5 s to generate a single bcrypt key, it would take about 1,500 years to find a single credit card number.
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Eh. OK let's say instead of just SHA-256'ing the plaintext data to derive a key you do 50,000 bcrypt rounds. Then the client encrypts the plaintext, hashes the ciphertext, and sends the hash to the server. If it takes 0.5 s to generate a single bcrypt key, it would take about 1,500 years to find a single credit card number.
Sure, but then it takes 0.5s per file to check whether the server and client are in sync, too.