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Native: Account management

Péter Szilágyi edited this page Jan 17, 2017 · 4 revisions

To provide Ethereum integration for your native applications, the very first thing you should be interested in doing is account management.

Although all current leading Ethereum implementations provide account management built in, it is ill advised to keep accounts in any location that is shared between multiple applications and/or multiple people. The same way you do not entrust your ISP (who is after all your gateway into the internet) with your login credentials; you should not entrust an Ethereum node (who is your gateway into the Ethereum network) with your credentials either.

The proper way to handle user accounts in your native applications is to do client side account management, everything self-contained within your own application. This way you can ensure as fine grained (or as coarse) access permissions to the sensitive data as deemed necessary, without relying on any third party application's functionality and/or vulnerabilities.

To support this, go-ethereum provides a simple, yet thorough accounts package that gives you all the tools to do properly secured account management via encrypted keystores and passphrase protected accounts. You can leverage all the security of the go-ethereum crypto implementation while at the same time running everything in your own application.

Encrypted keystores

Although handling accounts locally to an application does provide certain security guarantees, access keys to Ethereum accounts should never lay around in clear-text form. As such, we provide an encrypted keystore that provides the proper security guarantees for you without requiring a thorough understanding from your part of the associated cryptographic primitives.

The important thing to know when using the encrypted keystore is that the cryptographic primitives used within can operate either in standard or light mode. The former provides a higher level of security at the cost of increased computational burden and resource consumption:

  • standard needs 256MB memory and 1 second processing on a modern CPU to access a key
  • light needs 4MB memory and 100 millisecond processing on a modern CPU to access a key

As such, standard is more suitable for native applications, but you should be aware of the trade-offs nonetheless in case you you're targeting more resource constrained environments.

For those interested in the cryptographic and/or implementation details, the key-store uses the secp256k1 elliptic curve as defined in the Standards for Efficient Cryptography, implemented by the libsecp256k library and wrapped by github.com/ethereum/go-ethereum/accounts. Accounts are stored on disk in the Web3 Secret Storage format.

Keystores from Go

The encrypted keystore is implemented by the accounts.Manager struct from the github.com/ethereum/go-ethereum/accounts package, which also contains the configuration constants for the standard or light security modes described above. Hence to do client side account management from Go, you'll need to import only the accounts package into your code:

import "github.com/ethereum/go-ethereum/accounts"

Afterwards you can create a new encrypted account manager via:

am := accounts.NewManager("/path/to/keystore", accounts.StandardScryptN, accounts.StandardScryptP);

The path to the keystore folder needs to be a location that is writable by the local user but non-readable for other system users (for security reasons obviously), so we'd recommend placing it either inside your user's home directory or even more locked down for backend applications.

The last two arguments of accounts.NewManager are the crypto parameters defining how resource-intensive the keystore encryption should be. You can choose between accounts.StandardScryptN, accounts.StandardScryptP, accounts.LightScryptN, accounts.LightScryptP or specify your own numbers (please make sure you understand the underlying cryptography for this). We recommend using the standard version.

Account lifecycle

Having created an encrypted keystore for your Ethereum accounts, you can use this account manager for the entire account lifecycle requirements of your native application. This includes the basic functionality of creating new accounts and deleting existing ones; as well as the more advanced functionality of updating access credentials, exporting existing accounts, and importing them on another device.

Although the keystore defines the encryption strength it uses to store your accounts, there is no global master password that can grant access to all of them. Rather each account is maintained individually, and stored on disk in its encrypted format individually, ensuring a much cleaner and stricter separation of credentials.

This individuality however means that any operation requiring access to an account will need to provide the necessary authentication credentials for that particular account in the form of a passphrase:

  • When creating a new account, the caller must supply a passphrase to encrypt the account with. This passphrase will be required for any subsequent access, the lack of which will forever forfeit using the newly created account.
  • When deleting an existing account, the caller must supply a passphrase to verify ownership of the account. This isn't cryptographically necessary, rather a protective measure against accidental loss of accounts.
  • When updating an existing account, the caller must supply both current and new passphrases. After completing the operation, the account will not be accessible via the old passphrase any more.
  • When exporting an existing account, the caller must supply both the current passphrase to decrypt the account, as well as an export passphrase to re-encrypt it with before returning the key-file to the user. This is required to allow moving accounts between machines and applications without sharing original credentials.
  • When importing a new account, the caller must supply both the encryption passphrase of the key-file being imported, as well as a new passhprase with which to store the account. This is required to allow storing account with different credentials than used for moving them around.

Please note, there is no recovery mechanisms for losing the passphrases. The cryptographic properties of the encrypted keystore (if using the provided parameters) guarantee that account credentials cannot be brute forced in any meaningful time.

Accounts from Go

An Ethereum account is implemented by the accounts.Account struct from the github.com/ethereum/go-ethereum/accounts package. Assuming we already have an instance of an accounts.Manager called am from the previous section, we can easily execute all of the described lifecycle operations with a handful of function calls (error handling omitted).

// Create a new account with the specified encryption passphrase.
newAcc, _ := am.NewAccount("Creation password");

// Export the newly created account with a different passphrase. The returned
// data from this method invocation is a JSON encoded, encrypted key-file.
jsonAcc, _ := am.Export(newAcc, "Creation password", "Export password");

// Update the passphrase on the account created above inside the local keystore.
am.Update(newAcc, "Creation password", "Update password");

// Delete the account updated above from the local keystore.
am.Delete(newAcc, "Update password");

// Import back the account we've exported (and then deleted) above with yet
// again a fresh passphrase.
impAcc, _ := am.Import(jsonAcc, "Export password", "Import password");

Although instances of accounts.Account can be used to access various information about specific Ethereum accounts, they do not contain any sensitive data (such as passphrases or private keys), rather act solely as identifiers for client code and the keystore.

Signing authorization

As mentioned above, account objects do not hold the sensitive private keys of the associated Ethereum accounts, but are merely placeholders to identify the cryptographic keys with. All operations that require authorization (e.g. transaction signing) are performed by the account manager after granting it access to the private keys.

There are a few different ways one can authorize the account manager to execute signing operations, each having its advantages and drawbacks. Since the different methods have wildly different security guarantees, it is essential to be clear on how each works:

  • Single authorization: The simplest way to sign a transaction via the account manager is to provide the passphrase of the account every time something needs to be signed, which will ephemerally decrypt the private key, execute the signing operation and immediately throw away the decrypted key. The drawbacks are that the passphrase needs to be queried from the user every time, which can become annoying if done frequently; or the application needs to keep the passphrase in memory, which can have security consequences if not done properly; and depending on the keystore's configured strength, constantly decrypting keys can result in non-negligible resource requirements.
  • Multiple authorizations: A more complex way of signing transactions via the account manager is to unlock the account via its passphrase once, and allow the account manager to cache the decrypted private key, enabling all subsequent signing requests to complete without the passphrase. The lifetime of the cached private key may be managed manually (by explicitly locking the account back up) or automatically (by providing a timeout during unlock). This mechanism is useful for scenarios where the user may need to sign many transactions or the application would need to do so without requiring user input. The crucial aspect to remember is that anyone with access to the account manager can sign transactions while a particular account is unlocked (e.g. application running untrusted code).

Note, creating transactions is out of scope here, so the remainder of this section will assume we already have a transaction hash to sign, and will focus only on creating a cryptographic signature authorizing it. Creating an actual transaction and injecting the authorization signature into it will be covered later.

Signing from Go

Assuming we already have an instance of an accounts.Manager called am from the previous sections, we can create a new account to sign transactions with via it's already demonstrated NewAccount method; and to avoid going into transaction creation for now, we can hard-code a random common.Hash to sign instead.

// Create a new account to sign transactions with
signer, _ := am.NewAccount("Signer password");
txHash := common.HexToHash("0x0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef");

With the boilerplate out of the way, we can now sign transaction using the authorization mechanisms described above:

// Sign a transaction with a single authorization
signature, _ := am.SignWithPassphrase(signer, "Signer password", txHash.Bytes());

// Sign a transaction with multiple manually cancelled authorizations
am.Unlock(signer, "Signer password");
signature, _ = am.Sign(signer.Address, txHash.Bytes());
am.Lock(signer.Address);

// Sign a transaction with multiple automatically cancelled authorizations
am.TimedUnlock(signer, "Signer password", time.Second);
signature, _ = am.Sign(signer.Address, txHash.Bytes());

You may wonder why SignWithPassphrase takes an accounts.Account as the signer, whereas Sign takes only a common.Address. The reason is that an accounts.Account object may also contain a custom key-path, allowing SignWithPassphrase to sign using accounts outside of the keystore; however Sign relies on accounts already unlocked within the keystore, so it cannot specify custom paths.

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