cosmos-sdk/x/auth/spec/02_state.md

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# State

## Accounts

Accounts contain authentication information for a uniquely identified external user of an SDK blockchain,
including public key, address, and account number / sequence number for replay protection. For efficiency,
since account balances must also be fetched to pay fees, account structs also store the balance of a user
as `sdk.Coins`.

Accounts are exposed externally as an interface, and stored internally as
either a base account or vesting account. Module clients wishing to add more
account types may do so.

* `0x01 | Address -> ProtocolBuffer(account)`

### Account Interface

The account interface exposes methods to read and write standard account information.
Note that all of these methods operate on an account struct confirming to the
interface - in order to write the account to the store, the account keeper will
need to be used.

```go
// AccountI is an interface used to store coins at a given address within state.
// It presumes a notion of sequence numbers for replay protection,
// a notion of account numbers for replay protection for previously pruned accounts,
// and a pubkey for authentication purposes.
//
// Many complex conditions can be used in the concrete struct which implements AccountI.
type AccountI interface {
	proto.Message

	GetAddress() sdk.AccAddress
	SetAddress(sdk.AccAddress) error // errors if already set.

	GetPubKey() crypto.PubKey // can return nil.
	SetPubKey(crypto.PubKey) error

	GetAccountNumber() uint64
	SetAccountNumber(uint64) error

	GetSequence() uint64
	SetSequence(uint64) error

	// Ensure that account implements stringer
	String() string
}
```

#### Base Account

A base account is the simplest and most common account type, which just stores all requisite
fields directly in a struct.

```protobuf
// BaseAccount defines a base account type. It contains all the necessary fields
// for basic account functionality. Any custom account type should extend this
// type for additional functionality (e.g. vesting).
message BaseAccount {
  string address = 1;
  google.protobuf.Any pub_key = 2;
  uint64 account_number = 3;
  uint64 sequence       = 4;
}
```

### Vesting Account

See [Vesting](05_vesting.md).