> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cloak.ag/llms.txt
> Use this file to discover all available pages before exploring further.

# Core concepts

> Current primitives: UTXOs, fees, viewing keys, scanner, and root retries

## Canonical transaction path

The active runtime path is UTXO transactions through:

* `transact` for deposit/withdraw/transfer
* `swapWithChange` / `swapUtxo` for swap flows
* SDK-managed submit and status lifecycle

Note-model and UTXO-model methods are both part of v1, with UTXO-first integrations recommended for most apps.

## Commitments and Merkle tree

Each deposit/output creates a Poseidon commitment and appends it to the on-chain Merkle tree.

* Height: `32`
* Root history: `100` recent roots

Proof generation needs:

* Leaf index
* Path elements/indices
* A root still present in root history

## Nullifiers

Spending creates a nullifier. The program rejects a reused nullifier.

This is the core double-spend guard for both note and UTXO flows.

## Stale-root retries

If your proof root is no longer in history, transactions can fail with `RootNotFound` (`0x1001`).

The SDK includes retry paths that:

* fetch fresh Merkle data
* regenerate proof
* resubmit

## Fee model: gross, fee, net

Fees apply only when value leaves the pool (`externalAmount < 0`); deposits and
shielded→shielded transfers are fee-free.

For SOL withdrawals/swaps:

* `gross` = absolute public withdrawal amount
* `fee` = `5_000_000` lamports fixed + `gross * 3 / 1000`
* `net` = `gross - fee`

For SPL (USDC/USDT) withdrawals:

* `fee` = `gross * 3 / 1000` only — no fixed component, charged in the withdrawn token

Minimum deposit: `10_000_000` lamports (`0.01 SOL`, SOL pool only). See [Fee Model](/protocol/fee-model).

## Proof data shapes

* UTXO transact and swap requests use `264`-byte public inputs:
  `root + publicAmount + extDataHash + mint + nullifiers + commitments + chainNoteHash`.
* Proof bytes are always `256` bytes (Groth16).

## Viewing key registration

* SDK flows enforce viewing-key registration by default before protocol transactions.
* Registration is a signed wallet challenge bound to a 32-byte viewing key (`nk`).
* Registration is cached in-process to avoid repeat calls.

## Chain-native scanner and cache model

* `scanTransactions` reads chain transactions directly from RPC.
* It decrypts compact chain notes with viewing key `nk` and verifies `chainNoteHash` integrity.
* It computes per-tx `gross`, `fee`, `netAmount`, and running balance.
* Client apps can store encrypted report snapshots locally and expose explicit cache clear + rescan.

## Related pages

* UTXO details: [UTXO Transactions](/sdk/utxo-transactions)
* Protocol rules: [Shield Pool Program](/protocol/shield-pool)
* Runtime flow: [Transaction Flows](/platform/transaction-flows)