# Core types

Corda provides a large standard library of data types used to represent the Data model previously described. In addition, there are a series of helper libraries which provide date manipulation, maths and cryptography functions.

## State and References

State objects contain mutable data which we would expect to evolve over the lifetime of a contract.

A reference to a state in the ledger (whether it has been consumed or not) is represented with a `StateRef` object. If the state ref has been looked up from storage, you will have a `StateAndRef` which is simply a `StateRef` plus the data.

The `ContractState` type is an interface that all states must implement. A `TransactionState` is a simple container for a `ContractState` (the custom data used by a contract program) and additional platform-level state information, such as the notary pointer (see Consensus and notaries).

A number of interfaces then extend `ContractState`, representing standardised functionality for common kinds of state such as:

> `OwnableState` A state which has an owner (represented as a `CompositeKey`, discussed later). Exposes the owner and a function for replacing the owner e.g. when an asset is sold.

> `SchedulableState` A state to indicate whether there is some activity to be performed at some future point in time with respect to this contract, what that activity is and at what point in time it should be initiated.

## NamedByHash and UniqueIdentifier

Things which are identified by their hash, like transactions and attachments, should implement the `NamedByHash` interface which standardises how the ID is extracted. Note that a hash is not a globally unique identifier: it is always a derivative summary of the contents of the underlying data. Sometimes this isn’t what you want: two deals that have exactly the same parameters and which are made simultaneously but which are logically different can’t be identified by hash because their contents would be identical. Instead you would use `UniqueIdentifier`. This is a combination of a (Java) `UUID` representing a globally unique 128 bit random number, and an arbitrary string which can be paired with it. For instance the string may represent an existing “weak” (not guaranteed unique) identifier for convenience purposes.

## Transaction lifecycle types

The `WireTransaction` class contains the core of a transaction without signatures, and with references to attachments in place of the attachments themselves (see also Data model). Once signed these are encapsulated in the `SignedTransaction` class. For processing a transaction (i.e. to verify it) a `SignedTransaction` is then converted to a `LedgerTransaction`, which involves verifying the signatures and associating them to the relevant command(s), and resolving the attachment references to the attachments. Commands with valid signatures are encapsulated in the `AuthenticatedObject` type.

Note

A `LedgerTransaction` has not necessarily had its contract code executed, and thus could be contract-invalid (but not signature-invalid). You can use the `verify` method as shown below to validate the contracts.

When constructing a new transaction from scratch, you use `TransactionBuilder`, which is a mutable transaction that can be signed once its construction is complete. This builder class should be used to create the initial transaction representation (before signature, before verification). It is intended to be passed around code that may edit it by adding new states/commands. Then once the states and commands are right, this class can be used as a holding bucket to gather signatures from multiple parties. It is typical for contract classes to expose helper methods that can contribute to a `TransactionBuilder`. Once a transaction has been constructed using the builders `toWireTransaction` or `toSignedTransaction` function, it shared with other participants using the Flow framework.

Here’s an example of building a transaction that creates an issuance of bananas (note that bananas are not a real contract type in the library):

    val notaryToUse: Party = ...
    val txb = TransactionBuilder(notary = notaryToUse).withItems(BananaState(Amount(20, Bananas), fromCountry = "Elbonia"))
    txb.signWith(myKey)
    txb.setTime(Instant.now(), notaryToUse, 30.seconds)
    // We must disable the check for sufficient signatures, because this transaction is not yet notarised.
    val stx = txb.toSignedTransaction(checkSufficientSignatures = false)
    // Alternatively, let's just check it verifies pretending it was fully signed. To do this, we get
    // a WireTransaction, which is what the SignedTransaction wraps. Thus by verifying that directly we
    // skip signature checking.
    txb.toWireTransaction().toLedgerTransaction(services).verify()

In a unit test, you would typically use a freshly created `MockServices` object, or more realistically, you would write your tests using the domain specific language for writing tests.

## Party and CompositeKey

Entities using the network are called parties. Parties can sign structures using keys, and a party may have many keys under their control.

Parties can be represented either in full (including name) or pseudonymously, using the `Party` or `AnonymousParty` classes respectively. For example, in a transaction sent to your node as part of a chain of custody it is important you can convince yourself of the transaction’s validity, but equally important that you don’t learn anything about who was involved in that transaction. In these cases `AnonymousParty` should be used, which contains a composite public key without any identifying information about who owns it. In contrast, for internal processing where extended details of a party are required, the `Party` class should be used. The identity service provides functionality for resolving anonymous parties to full parties.

An `AuthenticatedObject` represents an object (like a command) that has been signed by a set of parties.

Note

These types are provisional and will change significantly in future as the identity framework becomes more fleshed out.

## Multi-signature support

Corda supports scenarios where more than one key or party is required to authorise a state object transition, for example: “Either the CEO or 3 out of 5 of his assistants need to provide signatures”.

### Composite Keys

This is achieved by public key composition, using a tree data structure `CompositeKey`. A `CompositeKey` is a tree that stores the cryptographic public key primitives in its leaves and the composition logic in the intermediary nodes. Every intermediary node specifies a threshold of how many child signatures it requires.

An illustration of an “either Alice and Bob, or Charlie” composite key:

To allow further flexibility, each child node can have an associated custom weight (the default is 1). The threshold then specifies the minimum total weight of all children required. Our previous example can also be expressed as:

### Verification

Signature verification is performed in two stages:

> 1. Given a list of signatures, each signature is verified against the expected content.
> 2. The public keys corresponding to the signatures are matched against the leaves of the composite key tree in question, and the total combined weight of all children is calculated for every intermediary node. If all thresholds are satisfied, the composite key requirement is considered to be met.

## Date support

There are a number of supporting interfaces and classes for use by contracts which deal with dates (especially in the context of deadlines). As contract negotiation typically deals with deadlines in terms such as “overnight”, “T+3”, etc., it’s desirable to allow conversion of these terms to their equivalent deadline. `Tenor` models the interval before a deadline, such as 3 days, etc., while `DateRollConvention` describes how deadlines are modified to take into account bank holidays or other events that modify normal working days.

Calculating the rollover of a deadline based on working days requires information on the bank holidays involved (and where a contract’s parties are in different countries, for example, this can involve multiple separate sets of bank holidays). The `BusinessCalendar` class models these calendars of business holidays; currently it loads these from files on disk, but in future this is likely to involve reference data oracles in order to ensure consensus on the dates used.

## Cryptography and maths support

The `SecureHash` class represents a secure hash of unknown algorithm. We currently define only a single subclass, `SecureHash.SHA256`. There are utility methods to create them, parse them and so on.

We also provide some mathematical utilities, in particular a set of interpolators and classes for working with splines. These can be found in the maths package.