A special private key that can be used to make a TX to a certain wallet ONLY

Question

I want to be able to achive this:

• a private key or its substitute (P) of a user wallet is stored on a server

• there're 100s of user wallets there, so there're 100s of (P) too

• server isn't a safe environment, as it's known

• there's also a main, admin wallet (M), in cold, local storage

• (P) must only be used to send outbound TXs to (M). Not anywhere else

This can't be achived with a normal private key, right? But how, if at all, it can be archived? Multisignature wallets?

The goal is automate withdrawals from the user wallets on a server, to make the process safer.

I don't consider any third-party solutions.

Answer 1

Your objectives are valid and you've correctly identified storing private keys on a server as a security risk.

A more appropriate solution might be to write a simple solana program (smart contract) whose data structures include a single PDA escrow account per withdrawal wallet. Your on-chain logic can then ensure that the withdrawals can only be withdrawn from a particular escrow into a single, well-defined target wallet.

Pretty much all DEXes and custodial NFT staking platforms do something similar in their contracts when handling their customers' tokens.

Rather than storing a bunch of private keys (bit of an anti-pattern in web3), you will save yourself a lot of headache and operational overhead by going down the solana program route.

Unless you're operating a CEX and are able to deal with all the safety requirements associated with a custodial solution, an on-chain program is probably a more appropriate solution.