Practice Safe Signing
Are you holding some cryptocurrency secured by a paper wallet in a safe deposit box? Good for you! That’s an excellent way to keep it safe.
But then your currency splits. Last week that piece of paper was worth 100 FooCoins, and this week it is worth 100 FooCoins and 100 BarCoins.
If you think one side of the split is a terrible idea, doomed to fail, you might be tempted to go get your paper wallet, “sweep” the coins into a wallet that supports the bad coin, and move them to an exchange to cash out (or maybe buy more of the good coin).
Great! I don’t give investment advice. But I will encourage you to sweep the “good” coins, first, and move them to a new wallet. Don’t be lazy and just write “BadCoins swept Nov 11, 2017” on the paper wallet and put it back in the vault.
Because sooner or later I think somebody will create a BadCoin (or a wallet) with a transaction signature algorithm designed to leak your private keys in the signature data that is broadcast to the world. A NewCoin will launch starting with the ledger of an OldCoin and a spiffy new signature scheme “for full replay protection” or “to be more quantum resistant” or “to be more scalable” or “with 150% more cowbell”. OldCoin holders will rush to sell it to get some free money… and a week or three later they’ll lose much more than they gained as their OldCoins are spent out from under them.
Unless you have the technical ability to audit the code yourself for subliminal signature channels, every time you use your private key(s) to sign a transaction or message, you are implicitly trusting the software (or hardware) developers– even if you jump through hoops and sign transactions offline.
So go ahead and dump those BadCoins… but if there is a chance the BadCoin developers are sophisticated and evil, move your GoodCoins first.