> ## Documentation Index
> Fetch the complete documentation index at: https://trust-link-tsn.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# TIN Layer: Transfer Identity Number and Route Discovery

> TIN is the 10-digit Transfer Identity Number issued by the Transfer Identity Protocol. It resolves payment routing without exposing a wallet address.

**In plain English:** TIN is a portable 10-digit payment identity. A sender can pay a TIN without knowing the recipient's wallet address. TIN resolves to routing metadata and the TCAP relationship, not to a private key. The root wallet is the TIN authority.

## What TIN is (and is not)

| Wallet address                               | TIN                                                        |
| -------------------------------------------- | ---------------------------------------------------------- |
| Public cryptographic account identifier      | Payment identity and route-discovery identifier            |
| Identifies a public key or account authority | Resolves permitted payment routing metadata                |
| May own token accounts                       | Does not itself hold tokens or act as a private key        |
| Usually exposed directly                     | Binds to the TCAP relationship and encrypted snapshot path |

TIN is not a username, bank account number, or legal identity. Any name, business, phone, or verification context is optional enrichment and is not payment authorization.

## Registrar program

<Note>**Status: Live on Devnet.**</Note>

The TIN Registrar exposes deterministic PDA helpers rather than a large instruction surface:

| Helper                     | Seeds                    |
| -------------------------- | ------------------------ |
| `identity_pda`             | `identity_seed: [u8;32]` |
| `registry_pda`             | `tin: u64`               |
| `escrow_pda`               | `tin, escrow_id`         |
| `vault_pda`                | `tin, escrow_id`         |
| `global_state_pda`         | (singleton)              |
| `platform_registry_pda`    | (singleton)              |
| `tin_mutation_staging_pda` | mutation-scoped          |

## Resolution flow

1. Sender supplies a 10-digit TIN.
2. TIN service resolves the active public routing metadata.
3. TSN SDK resolves the GPRU / TCAP relationship commitment and policy.
4. Recipient route is bound into the signed plan commitment.
5. If local private derivation is required, the owning wallet gives a fresh approval on that device.

Resolution must not return plaintext seeds, child private keys, or an unrestricted wallet-to-person mapping. Registrars apply non-enumeration, rate limiting, revocation, and state-version checks.

## Record classes

| Class              | Examples                                                          | Boundary                                            |
| ------------------ | ----------------------------------------------------------------- | --------------------------------------------------- |
| Public             | TIN number, display label, active status, route version           | Returned for resolution subject to anti-enumeration |
| Integrity metadata | Owner-key commitment, route commitment, state version             | Public verification material, not private authority |
| Encrypted          | Privacy-receiving-root metadata, snapshot refs, recovery material | Ciphertext; unlock requires owner wallet approval   |
| Off-chain app data | Optional profile or notification references                       | Stored under the app's own access policy            |

## Ownership and recovery

The root wallet is the TIN authority. It can approve access from any device. Encrypted TIN material is never sent in plaintext to a backend, Node, Receiver, or Cranker. Older device-bound envelopes require a one-time upgrade from a device that can already unlock them; new and upgraded TINs use the wallet-owned envelope model.

## Related

<CardGroup cols={2}>
  <Card title="TSN" icon="network-wired" href="/layers/tsn">
    Settlement coordination and epoch treasury.
  </Card>

  <Card title="GPRU" icon="route" href="/layers/gpru">
    Non-custodial authorization and routing.
  </Card>

  <Card title="TCAP" icon="lock" href="/layers/tcap">
    Private balance accounting.
  </Card>

  <Card title="Architecture" icon="diagram-project" href="/how-it-works/architecture">
    End-to-end sequence diagram.
  </Card>
</CardGroup>