The example you posted above isn't related to people who use mixers, that user was 'applying for a signature campaign' that happens to be for a mixing service, it's very likely that he doesn't use mixers, which is fine, but my statement was for those who actually use mixers, I doubt that the % of people who actually mix their coins don't understand what private key or how to generate one using a wallet.

Also, that conclusion should be read in context, when you use Whirlwind you have to deal with a private key one way or the other, you either generate it using the front end of Whirlwind's website, or using another wallet like Electrum or even a mobile wallet, the end result is the same (you get a private key), but the possibility of losing it is a lot higher when you rely on your memory to help you save the private key before you close the browser or hit "next".

Furthermore, when the user copies the private key from Whirlwind website, they are most likely going to save it as plain text on their computer, while if they generate a private key using a proper wallet, that private key will be encrypted, you don't have to save, and even if you lose your computer you still have access to the private key assuming you have backed up your wallet.

So now even if we were to assume that the majority of people who use mixers DON'T know how to safely generate a private key, they are still better of take that risk than risk 1- not saving their private key generated on browser 2- losing that private key 3- the key is exposed in plain text.


But you said you have the private key, if you do, you can sign a message, just because the wallet you use doesn't allow you to use that, it means nothing, check this https://reinproject.org/bitcoin-signature-tool/#sign , repo > https://github.com/weex/bitcoin-signature-tool

I have not checked the code but you can easily find some trusted tools which you could use offline to sign a message, you could also import that private key to a proper wallet and then sign the message.