Account Code

Every Miden Account is essentially a smart contract. The Code defines the account's functions, which can be invoked through both note scripts and transaction scripts. Key characteristics include:

• Mutable access: Only the Account's own functions can modify its storage and vault. All state changes — such as updating storage slots or transferring assets — must occur through these functions.
• Function commitment: Each function can be called by its MAST root. The root represents the underlying code tree as a 32-byte commitment. This ensures integrity which means a function's behavior cannot change without changing the MAST root.
• Asset creation: Faucet Accounts can create assets.

An account's code is typically the result of merging multiple account components.

Authentication

Authenticating a transaction, and therefore the changes to the account, is done with an authentication procedure. Every account's code must provide exactly one authentication procedure. It is automatically called during the transaction epilogue, i.e. after all note scripts and the transaction script have been executed.

Such an authentication procedure typically inspects the transaction and then decides whether a signature is required to authenticate the changes. It does this by:

• checking which account procedures have been called
  • Example: Authentication is required if the distribute procedure was called but not if burn was called.
• inspecting the account delta.
  • Example: Authentication is required if a cryptographic key in storage was updated.
  • Example: Authentication is required if an asset was removed from the vault.
• checking whether notes have been consumed.
• checking whether notes have been created.

Recall that an account's nonce must be incremented whenever its state changes. Only authentication procedures are allowed to do so, to prevent accidental or unintended authorization of state changes.

Procedure tracking

The authentication procedure can base its authentication decision on whether a specific account procedure was called during the transaction (procedure tracking). A procedure is considered "tracked" only if it invokes account-restricted kernel APIs (procedures that are only allowed to be called from the account context, e.g. exec.faucet::mint). Procedures that execute only local instructions (e.g., a noop push.0 drop) will not be marked as tracked.