tl;dr if I were to sign an Ethereum message, eg.
web3.eth.personal.sign("Bitcoin", "SOME-ETH-ADDRESS"), would it be reasonably safe to use a 32-byte hash of the signature as the private key for a Bitcoin wallet?
Is it possible to calculate the signature hash of a signed message, using eg.
web3.eth.personal.sign("Hello world", "0x11f4d0A3c12e86B4b5F39B213F7E19D048276DAe", "test password!") taken from the web3 docs
which would return
if you DO NOT have the private key?
Specifically, if the message being signed is KNOWN, and the address and/or pubkey of the signer are KNOWN, can the signature hash be calculated by a 3rd-party.
The use-case is to use the signature (32-byte hashed) as the PRIVATE KEY for generating other addresses (non-Ethereum).
I saw this question, Can a message hash be recovered from a signature?, however, this line is a bit confusing:
It should be noted that while computing the message hash is infeasible, it is possible to check guesses, if the message was not salted before hashing (which it usually isn't).