How to protect our users' privacy on-chain?

Ultimate Digits links users' crypto wallets to their mobile numbers, enabling asset (crypto/NFTs) transfers using just the recipient's number. For the privacy-conscious, we provide virtual Ethereum mobile numbers (as NFTs) starting with the +999 prefix.

Here's an example to consider: One challenge Ultimate Digits aims to address revolves around privacy concerns associated with this convenience.

Specifically, when a user, let's call him Bob, links his mobile number to his cryptocurrency wallet through Ultimate Digits, he might not want everyone in his contact list—who has saved his number—to access or view his wallet's holdings and transactions.

Ideally, only individuals with whom Bob engages in transactions should be able to see his direct wallet address. Therefore, we're tasked with developing a solution that effectively hides users' wallet addresses from being accessible or visible to all their contacts, ensuring that wallet addresses are only revealed during actual transactions.

We've come up with a solution called "Salting and Hashing":

• Suppose a user has the mobile number "11234567890" and their Ethereum address (used as salt) is "0xabc...123".
• The last 4 digits (Ultimate Digits) are 7890.
• Salted value becomes 7890abc...123.
• Hash the salted value with keccak256 to get UltimateDigitsHash.
• Store this hash on-chain.

More details here: https://docs.google.com/document/d/1L7yeGCIOwUUoxLRMBPLzjrGcHg7ETSoXb4hNtWriAQM/edit?usp=sharing

Vote on whether we should implement this approach or brainstorm alternatives.