Accounts

Accounts are basic building blocks representing a user or an autonomous smart contract.

For smart contracts the go-to solution is account-based state. Miden supports expressive smart contracts via a Turing-complete language and the use of accounts.

In Miden, an account is an entity which holds assets and defines rules about how to transfer these assets.

Account design¶

In Miden every account is a smart contract. The diagram below illustrates the basic components of an account.

Key to diagram

Account ID: A unique identifier for an account. This does not change throughout its lifetime.
Storage: User-defined data which can be stored in an account.
Nonce: A counter which increments whenever the account state changes.
Vault: A collection of assets stored in an account.
Code: A collection of functions which define the external interface for an account.

Account ID¶

A ~63 bits long identifier for the account ID (one field element felt).

The four most significant bits specify the account type - regular or faucet - and the account-storage-modes - public or private.

Account data storage¶

Storage for user-defined data that is composed of two components.

Storage slots: Users have access to 254 slots. Per slot, users can store a word (~32 bytes). Currently, we reserve slot 0 for the account owner’s key and slot 255 for information about the storage layout.
Storage maps: Users requiring additional storage can use a StorageMap. A storage map is a key-value store where both keys and values are words (~32 bytes). A single storage map can store huge amounts of data (e.g., up to \(2^{256}\) words per map). The roots of the storage maps are stored in storage slots. So, a single account can contain up to 254 storage maps.

As described below, accounts can be stored off-chain (private) and on-chain (public). Accounts that store huge amounts of data, as it is possible using storage maps, are better designed as off-chain accounts.

Nonce¶

A counter which increments whenever the account state changes.

Nonce values must be strictly monotonically increasing and increment by any value smaller than 2^32 for every account update.

Vault¶

An asset container for an account.

An account vault can contain an unlimited number of assets. The assets are stored in a sparse Merkle tree as follows:

For fungible assets, the index of a node is defined by the issuing faucet ID, and the value of the node is the asset itself. Thus, for any fungible asset there will be only one node in the tree.
For non-fungible assets, the index is defined by the asset itself, and the asset is also the value of the node.

An account vault can be reduced to a single hash which is the root of the sparse Merkle tree.

Code¶

The interface for accounts. In Miden every account is a smart contract. It has an interface that exposes functions that can be called by note scripts and transaction scripts. Users cannot call those functions directly.

Functions exposed by the account have the following properties:

Functions are actually roots of Miden program MASTs (i.e., a 32-byte hash). Thus, the function identifier is a commitment to the code which is executed when a function is invoked.
Only account functions have mutable access to an account’s storage and vault. Therefore, the only way to modify an account’s internal state is through one of the account’s functions.
Account functions can take parameters and can create new notes.

Note

Since code in Miden is expressed as MAST, every function is a commitment to the underlying code. The code cannot change unnoticed to the user because its hash would change. Behind any MAST root there can only be 256 functions.

Example account code¶

Currently, Miden provides two standard implementations for account code.

Basic user account¶

There is a standard for a basic user account. It exposes three functions via its interface.

Basic user account code

  use.miden::contracts::wallets::basic->basic_wallet
  use.miden::contracts::auth::basic

  export.basic_wallet::receive_asset
  export.basic_wallet::send_asset
  export.basic::auth_tx_rpo_falcon512

Note scripts or transaction scripts can call receive_asset and send_asset.

Transaction scripts can also call auth_tx_rpo_falcon512 and authenticate the transaction.

Warning

Without correct authentication, i.e. knowing the correct private key, a note cannot successfully invoke receive_asset or send_asset.

Basic fungible faucet (faucet for fungible assets)¶

There is also a standard for a basic fungible faucet.

Fungible faucet code

#! Distributes freshly minted fungible assets to the provided recipient.
#!
#! ...
export.distribute
    # get max supply of this faucet. We assume it is stored at pos 3 of slot 1
    push.METADATA_SLOT exec.account::get_item drop drop drop
    # => [max_supply, amount, tag, note_type, RECIPIENT, ...]

    # get total issuance of this faucet so far and add amount to be minted
    exec.faucet::get_total_issuance
    # => [total_issuance, max_supply, amount, tag, note_type RECIPIENT, ...]

    # compute maximum amount that can be minted, max_mint_amount = max_supply - total_issuance
    sub
    # => [max_supply - total_issuance, amount, tag, note_type, RECIPIENT, ...]

    # check that amount =< max_supply - total_issuance, fails if otherwise
    dup.1 gte assert.err=ERR_BASIC_FUNGIBLE_MAX_SUPPLY_OVERFLOW
    # => [asset, tag, note_type, RECIPIENT, ...]

    # creating the asset
    exec.asset::create_fungible_asset
    # => [ASSET, tag, note_type, RECIPIENT, ...]

    # mint the asset; this is needed to satisfy asset preservation logic.
    exec.faucet::mint
    # => [ASSET, tag, note_type, RECIPIENT, ...]

    # store and drop the ASSET
    mem_storew.3 dropw
    # => [tag, note_type, RECIPIENT, ...]

    # create a note containing the asset
    exec.tx::create_note
    # => [note_ptr, ZERO, ZERO, ...]

    # store and drop the ASSET
    padw mem_loadw.3 movup.4 exec.tx::add_asset_to_note
    # => [note_ptr, ASSET, ZERO, ...]
end

#! Burns fungible assets.
#!
#! ...
export.burn
    # burning the asset
    exec.faucet::burn
    # => [ASSET]

    # increments the nonce (anyone should be able to call that function)
    push.1 exec.account::incr_nonce

    # clear the stack
    padw swapw dropw
    # => [...]
end

The contract exposes two functions distribute and burn.

The first function distribute can only be called by the faucet owner, otherwise it fails. As inputs, the function expects everything that is needed to create a note containing the freshly minted asset, i.e., amount, metadata, and recipient.

The second function burn burns the tokens that are contained in a note and can be called by anyone.

Difference between burn and distribute

The burn procedure exposes exec.account::incr_nonce, so by calling burn the nonce of the executing account gets increased by 1 and the transaction will pass the epilogue check. The distribute procedure does not expose that. That means the executing user needs to call basic::auth_tx_rpo_falcon512 which requires the private key.*

Account creation¶

For an account to exist it must be present in the account database kept on the Miden node(s).

However, new accounts can be created locally by users using the Miden client. The process is as follows:

Alice creates a new account ID (according to the account types) using the Miden client.
Alice’s Miden client asks the Miden node to check if the new ID already exists.
Alice shares the ID with Bob (eg. when Alice wants to receive funds).
Bob executes a transaction and creates a note that contains an asset for Alice.
Alice consumes Bob’s note to receive the asset in a transaction.
Depending on the account storage mode (private vs. public) and transaction type (local vs. network) the operator eventually receives the new account ID and - if the transaction is correct - adds the ID to the account database.

A user can create an account in one of the following manners:

Use the Miden client as a wallet.
Use the Miden base builtin functions for wallet creation: basic wallet, fungible faucet

Account types¶

There are two basic account types in Miden: Regular accounts and faucets. Only faucets can mint new assets. Regular accounts can be mutable or immutable, which simply means that it is possible to change the account code after creation.

Type and mutability is encoded in the most significant bits of the account’s ID.

	Basic mutable	Basic immutable	Fungible faucet	Non-fungible faucet
Description	For most users, e.g. a wallet. Code changes allowed, including public API.	For most smart contracts. Once deployed code is immutable.	Users can issue fungible assets and customize them.	Users can issue non-fungible assets and customize them.
Code updatability	yes	no	no	no
Most significant bits	`00`	`01`	`10`	`11`

Public and private accounts¶

Users can decide whether to keep their accounts private or public at account creation. The account ID encodes this preference on the third and fourth most significant bit.

Accounts with public state: The actual state is stored on-chain. This is similar to how accounts work in public blockchains, like Ethereum. Smart contracts that depend on public shared state should be stored public on Miden, e.g., DEX contract.
Accounts with private state: Only the hash of the account is stored on-chain. Users who want to stay private, and manage their own data, should choose this option. Users who want to interact with private accounts need to know the account’s interface.