What Are Soulbound Tokens?

A soulbound token is:

1. Non-transferable: Bound to the recipient only.
2. Exclusive: Represents something unique and personal.
3. Verifiable: On-chain proof of ownership and authenticity.

Example: A company could issue loyalty rewards as soulbound tokens that only the original customer can redeem, ensuring uniqueness and preventing resale.

ERC4671: The Non-Tradable Token Standard (NTT)

ERC4671 defines the framework for Non-Tradable Tokens (NTTs). Each contract represents a certificate authority that can issue or revoke tokens.

Key Features:

1. Unique Certificates
2. Multiple Tokens
3. Revocation
4. Verifiability .

IERC4671 Interface

ERC4671 is ERC165-compliant and includes:

Metadata & Identity

Each NTT contains:

1. Name
2. Symbol
3. TokenURI

Note: Metadata should avoid confidential data since URIs can be queried publicly.

Enumerable Features

ERC4671Enumerable lets contracts query:

1. emittedCount()
2. holdersCount()
3. tokenOfOwnerByIndex(owner, i)
4. tokenByIndex(i)

Delegation & Minting

Authorities can delegate minting rights:

1. delegate() / delegateBatch()
2. mint() / mintBatch()
3. issuerOf()

Consensus-Based Minting & Revocation

Some use-cases require multiple voters (multi-sig style):

1. voters()
2. approveMint(owner)
3. approveRevoke(tokenId)

Pull Mechanism

A token owner can migrate their NTT to a new address by signing a transfer authorization, ensuring recovery without making tokens tradable.

NTT Store

The NTT Store (IERC4671Store) allows users to expose their NTTs for third-party verification.

Soulbound Identity & Security

1. Non-transferable identity layer for Web3
2. Privacy concerns: URIs must not leak sensitive data
3. Solution: Restrict metadata access or keep confidential data off-chain

Conclusion: Soulbound Tokens (ERC4671 NTTs) introduce a secure, verifiable, and exclusive identity layer for Web3. They ensure digital assets remain personal, non-tradable, and tamper proof.