Knowing the message, can a signature be derived from an address or pubkey?

Question

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?

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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 0x30755ed65396facf86c53e6217c52b4daebe72aa4941d89635409de4c9c7f9466d4e9aaec7977f05e923889b33c0d0dd27d7226b6e6f56ce737465c5cfd04be400,

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.

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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).

Thanks!

Answer 1

Formulating an answer of some sorts based on the comments.

So in general hashes can't be reversed. Once you calculate hash(A) = B there is no way to calculate A if you know B.

However if A is something short enough (for example a phonenumber) or something guessable (for example your first name) one can simply try all combinations of A and apply the hash function to that and see if it matches B. That can be done fully offline so it can be done in an efficient manner. This is a brute force way of finding A.

If I understood correctly you want to do further calculations by using B as private key for a public address so something like public_address = getAddr(B) where getAddr includes hashing and elliptic curve stuff.

So if someone wanted to hack your private key they would basically still need to guess A. Or just guess the private key - whichever is easier. So security wise I don't see a big difference here. Do note that there may be other attack vectors based on how you store the different values and so on.