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Mobile: Account management
To provide Ethereum integration for your mobile 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 mobile 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 library 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.
Although handling your users' accounts locally on their own mobile device 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, light is more suitable for mobile applications, but you should be aware of the trade-offs nonetheless.
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.
The encrypted keystore on Android is implemented by the AccountManager class from the org.ethereum.geth package. The configuration constants (for the standard or light security modes described above) are located in the Geth abstract class, similarly from the org.ethereum.geth package. Hence to do client side account management on Android, you'll need to import two classes into your Java code:
import org.ethereum.geth.AccountManager;
import org.ethereum.geth.Geth;Afterwards you can create a new encrypted account manager via:
AccountManager am = new AccountManager("/path/to/keystore", Geth.LightScryptN, Geth.LightScryptP);The path to the keystore folder needs to be a location that is writable by the local mobile application but non-readable for other installed applications (for security reasons obviously), so we'd recommend placing it inside your app's data directory. If you are creating the AccountManager from within a class extending an Android object, you will most probably have access to the Context.getFilesDir() method via this.getFilesDir(), so you could set the keystore path to this.getFilesDir() + "/keystore".
The last two arguments of the AccountManager constructor are the crypto parameters defining how resource-intensive the keystore encryption should be. You can choose between Geth.StandardScryptN, Geth.StandardScryptP, Geth.LightScryptN, Geth.LightScryptP or specify your own numbers (please make sure you understand the underlying cryptography for this). We recommend using the light version.
The encrypted keystore on iOS is implemented by the GethAccountManager class from the Geth framework. The configuration constants (for the standard or light security modes described above) are located in the same namespace as global variables. Hence to do client side account management on iOS, you'll need to import the framework into your Swift code:
import GethAfterwards you can create a new encrypted account manager via:
let am = GethNewAccountManager("/path/to/keystore", GethLightScryptN, GethLightScryptP);The path to the keystore folder needs to be a location that is writable by the local mobile application but non-readable for other installed applications (for security reasons obviously), so we'd recommend placing it inside your app's document directory. You should be able to retrieve the document directory via let datadir = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0], so you could set the keystore path to datadir + "/keystore".
The last two arguments of the GethNewAccountManager factory method are the crypto parameters defining how resource-intensive the keystore encryption should be. You can choose between GethStandardScryptN, GethStandardScryptP, GethLightScryptN, GethLightScryptP or specify your own numbers (please make sure you understand the underlying cryptography for this). We recommend using the light version.
Having created an encrypted keystore for your Ethereum accounts, you can use this account manager for the entire account lifecycle requirements of your mobile 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 agaist 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 devices 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.
An Ethereum account on Android is implemented by the Account class from the org.ethereum.geth package. Assuming we already have an instance of an AccountManager called am from the previous section, we can easily execute all of the described lifecycle operations with a handful of function calls.
// Create a new account with the specified encryption passphrase.
Account 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.
byte[] jsonAcc = am.exportKey(newAcc, "Creation password", "Export password");
// Update the passphrase on the account created above inside the local keystore.
am.updateAccount(newAcc, "Creation password", "Update password");
// Delete the account updated above from the local keystore.
am.deleteAccount(newAcc, "Update password");
// Import back the account we've exported (and then deleted) above with yet
// again a fresh passphrase.
Account impAcc = am.importKey(jsonAcc, "Export password", "Import password");Although instances of 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.
// Create a new account with the specified encryption passphrase.
let newAcc = try! 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.
let jsonKey = try! am?.exportKey(newAcc!, passphrase: "Creation password", newPassphrase: "Export password")
// Update the passphrase on the account created above inside the local keystore.
try! am?.update(newAcc, passphrase: "Creation Password", newPassphrase: "Update password")
// Delete the account updated above from the local keystore.
try! am?.delete(newAcc, passphrase: "Update password")
// Import back the account we've exported (and then deleted) above with yet
// again a fresh passphrase.
let impAcc = try! am?.importKey(jsonKey, passphrase: "Export password", newPassphrase: "Import password")Although instances of GethAccount 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.