Limiting transaction spam through hashcash

[Noitev]

Since bitcoin is based on hashcash, why is it so hard to simply make transactions using hashcash for sending btc? You can limit transaction spam by simply adding a nonce and hashing it. Then if the txid is lower than another target, it is accepted, otherwise it is rejected. Since It'll be so hard to make this compatible with previous versions of btc, I suggest it is used only for microtransactions. so if the amount is under 0.005 btc or whatever, make it require a hashcash signature.

[streblo]

This is already implemented: the txn fee IS hashcash, insofar as bitcoins are hashcash.

[Etlase2]

Adding the nonce also requires adding bandwidth and storage to already wasteful transaction sizes. Although storing the nonce might not be necessary, the bandwidth overhead is still a concern.