(data is from 2017)
- PoW is much more energy-expensive on a mobile CPU than on a GPU cluster.
- unless we make PoW prohibitively expensive for normal users, spammers won't be slowed / stopped by it.
- spammers will either distribute PoW to their botnet, where you will be paying for the PoW, or rent some gigahashes.
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from https://rumorscity.com/2014/01/07/how-to-mine-litecoin-with-android/:
> I managed to pull around 5.6KHash/sec on my Nexus 7 with all four threads. -
So we are limited to ~20 KHashes for a reasonable "add contact" operation delay of 4 seconds, ~5KHashes (1 second) for sending a single chat message
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you can rent 500MH/s for three hours for ~3USD: https://www.miningrigrentals.com/rigs/scrypt
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at 500MH/s and 5KH/message, you can send 100'000 spam messages a second
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After three hours, you get one billion spam messages, for 3 dollars.