Account Linking

Account linking is a unique Flow concept that enables sharing ownership over accounts. In order to understand how we can achieve that we must first understand how accounts on Flow are accessed.

Accounts on flow can be accessed in Cadence through two types, PublicAccount and Account. As the name implies the PublicAccount type gives access to all public account information such as address, balance, storage capacity, etc., but doesn't allow changes to the account. The Account type (or more specifically, an entitled &Account) allows the same access as PublicAccount but also allows changes to the account, including adding/revoking account keys, managing the deployed contracts, as well as linking and publishing Capabilities.

Accessing Account

Accessing Account allows for modification to account storage, so it's essential to safeguard this access by mandating that transactions are signed by the account being accessed. Account entitlements enable for more granular access control over the specific parts of the account that can be accessed from within the signed transaction. A transaction can list multiple authorizing account it wants to access as part of the prepare section of the transaction. Read more about transaction signing in the transaction documentation.

Since access to the Account object enables state change, the idea of account ownership actually translates to the ability to access the underlying account. Traditionally, you might consider this the same as having key access on an account, but we'll see in just a minute how programmatic, ownership-level access is unlocked with Capabilities on Flow.

Account Capabilities

Before proceeding the reader will need a clear understanding of Cadence capabilities to follow this section. Advanced features such as Account Capabilities are powerful but if used incorrectly can put your app or users at risk.

Cadence allows the creation of Capabilities to delegate access to account storage, meaning any account obtaining a valid Ccapability to another account object in the storage can access it. This is a powerful feature on its own - accessing another account programmatically without the need for an active key on the accessible account. The access to the object can be limited when creating a Capability so only intended functions or fields can be accessed.

Account linking is made possible by the extension of Capabilities on the Account object itself. Similar to how storage capabilities allow access to a value stored in an account's storage, &Account Capabilities allow delegated access to the issuing Account. These Capabilities allow for access to key assignment, contract deployment, and other privileged actions on the delegating Account - effectively sharing ownership of the account without ever adding or sharing a key. This Capability can of course be revoked at any time by the delegating account.

Creating Account Links

When referring to 'account linking' we mean that an &Account Capability is created by the parent account and published to another account. The account owning the &Account Capability which was made available to another account is the child account. The account in possession of the Capability given by the child account becomes its parent account.

A link between two existing accounts on Flow can be created in two steps:

1. A child account creates an &Account Capability and publishes it to the parent account.
2. The parent account, claims that Capability and can access the child's account through it.

These two steps are implemented in Cadence as two transactions:

Create capability

The account B creates and publishes the &Account Capability to the account A at the address 0x01

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#allowAccountLinking

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transaction {

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prepare(signer: auth(IssueAccountCapabilityController, PublishInboxCapability) &Account) {

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// Issue a fully-entitled account capability

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let capability = signer.capabilities

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.account

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.issue<auth(Storage, Contracts, Keys, Inbox, Capabilities) &Account>()

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// Publish the capability for the specified recipient

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signer.inbox.publish(capability, name: "accountCapA", recipient: 0x1)

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}

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}

Claim capability

The account A claims the Capability published by account B.

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transaction {

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prepare(signer: auth(ClaimInboxCapability) &Account) {

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let capabilityName = "accountCapB"

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let providerAddress = 0x2

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// Claim the capability published by the account 0x2

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let capability = signer.inbox

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.claim<auth(Storage, Contracts, Keys, Inbox, Capabilities) &Account>(

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capabilityName,

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provider: providerAddress

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) ?? panic(

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"Capability with name ".concat(capabilityName)

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.concat(" from provider ").concat(providerAddress.toString())

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.concat(" not found")

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)

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// Simply borrowing an Account reference here for demonstration purposes

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let accountRef = capability.borrow()!

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}

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}

What is account linking most useful for?

Account linking was specifically designed to enable smooth and seamless custodial onboarding of users to your Flow based application without them first requiring a wallet to do so. This pattern overcomes both the technical hurdle, as well as user's reluctance to install a wallet, opening access to Flow applications to every user. Users can experience an app without any delay while still offering a path to self-sovreign ownership.

Naturally, users may expect to use their account with another application, or otherwise move assets stored in that account elsewhere - at minimum from their wallet. When an app initially leverages account linking, the app creates the account instead of the user and stores that user's specific state in the app-created account. At a later point, users can take ownership of the app account providing they possess a full Flow account, typically by installing a wallet app.

Account linking enables users to possess multiple linked child accounts from different apps. Complexities associated with accessing those child accounts are eliminated by abstracting access to them through the user's parent account.

info

Simply put, child accounts are accessed and can be treated as a seamless part of the parent account.

All assets in the app account can now jump the walled garden to play in the rest of the Flow ecosystem. The user does not need to rely on the custodial app to execute transactions moving assets from the child account as the parent account already has access to the assets in the child account.

This shared control over the digital items in the in-app account enables users to establish real ownership of the items beyond the context of the app, where they can use their parent account to view inventory, take the items to other apps in the ecosystem, such as a marketplace or a game.

Most importantly, users are able to do this without the need to transfer the digital items between accounts, making it seamless to continue using the original app while also enjoying their assets in other contexts.

Security Considerations

Account linking is a very powerful Cadence feature, and thus it must be treated with care. So far in this document, we've discussed account linking between two accounts we own, even if the child account is managed by a third-party application. But, we can't make the same trust assumptions about custodial accounts in the real world.

Creating an &Account Capability and publishing it to an account we don't own means we are giving that account full access to our account. This should be seen as an anti-pattern.

warning

Creating an &Account Capability and sharing it with third-party account effectually the same as giving that person your account's private keys.

Because unfiltered account linking can be dangerous, Flow introduces the HybridCustody contract that helps custodial applications regulate access while enabling parent accounts to manage their many child accounts and assets within them.

Hybrid Custody and Account Linking

Apps need assurances that their own resources are safe from malicious actors, so giving out full access might not be the form they want. Using hybrid custody contracts, the app still maintains control of their managed accounts, but they can:

1. Share capabilities freely, with a few built-in controls over the types of capabilities that can be retrieved by parent accounts via helper contracts (the CapabilityFactory, and CapabilityFilter)
2. Share additional capabilities (public or private) with a parent account via a CapabilityDelegator resource

Learn more about it in the Hybrid Custody documentation.

Guides

• Building Walletless Applications Using Child Accounts covers how apps can leverage Account Linking to create a seamless user experience and enable future self-custody.
• Working With Parent Accounts covers features enabled by the core HybridCustody contract to access child account assets from parent accounts. This is useful for apps like marketplaces or wallets that are working with accounts that have potential child accounts.

Resources

• Forum Post where core concepts were introduced and discussed.
• GitHub repository where HybridCustody core contracts and scripts are maintained. Check out the repository for more advanced script or transaction examples.
• Example Account Linking project with Magic.
• Starter template for Niftory Account Linking API.