# Salting & Hashing

**Ultimate Digits Hashing Rule**

*Preliminary Steps*

1. Normalize the data, remove any spaces, and strip any non-numeric characters from the mobile number.
2. Ensure the mobile number has a consistent format. Since we’re targeting international numbers, we always use the full international version. E.g., +1 123 456 7890 becomes 11234567890. 

*Ultimate Digits Extraction*

Extract the last n digits from the mobile number where n is a predetermined value (for example, 4). This is referred to as the 'Ultimate Digits'.

*Salting*

Combine the 'Ultimate Digits' with a user-specific salt. The salt can be derived from the user's crypto address or another unique user-specific identifier (randomized salting pattern). The salt ensures even if two users have the same 'Ultimate Digits', their hashes will differ.

`SaltedValue = Ultimate Digits + Salt`

*Hashing*

We use a cryptographic hash function to hash the SaltedValue. We use the keccak256 hash function (standard for Ethereum).

`UltimateDigitsHash = keccak256(SaltedValue)`

*Storing on the Blockchain*

Store the UltimateDigitsHash on the blockchain. This value can be used for verification without revealing the actual mobile number.

**Verification**

1. When a user wants to verify or change their number, they'll undergo the same process to generate a new hash.
2. The newly generated hash is then compared with the one on-chain to confirm a match. 

**Implementation Example**

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