SPV library API discussion

[AndrewGuarda]

Dear Z-Cash foundation and community.Guarda is doing Z-Cash grant SPV library and Wallet Android project #ZcashFoundation/GrantProposals-2017Q4#16. We'd like to have the communications in a transparent and community-oriented way. It will be really great to have any valuable feedback regarding SPV function and methods that we are going to implement.
Please leave your comments below and we'll take it into the account.

[AndrewGuarda]

Terms of reference
Z-Cash wallet functions:

• Generate Private+public keys for t-addr
• Generate Private (spending) + public keys for z-addr
• Get balance for own t-addr
• Get balance for own z-addr
• Sign and sent transaction using t-addr private key. Change should send back to the sending address
• Sign and sent transaction using z-addr spending key. Change should send back to the sending address
• Support memo field for the transactions
• Send ZEC between t-addr and z-addr and vice versa
• History for own t-addr
• History for own z-addr
• Keys backup
• Restore the wallet from backup for both t-addr and z-addr

OS requirements: from Android 4.4 and higher

The list of SPV library methods:

• Generate private+public t-addr keys. Executes on the device, no network calls, instant result expecting.

public static String generateNewPrivateKey_taddr();
public static String publicKeyFromPrivateKey_taddr(String privKey);

• Generate spending+public z-addr keys. Executes on the device, no network calls, instant result expecting.

public static String generateNewPrivateKey_zaddr();
public static String publicKeyFromPrivateKey_zaddr(String privKey);

• Get t-addr balance, asynchronous network method, the result is callback function

public void getBalance_taddr(String pubKey, WalletCallback<String, Long> onComplete);

• Get t-addr balance, asynchronous network method, the result is callback function

public void getBalance_zaddr(String pubKey, WalletCallback<String, Long> onComplete);

• Send t-addr transaction, asynchronous network method, the result is callback. Separate method to generate transaction on the device.

public ZCashTransaction_taddr createTransaction_taddr(fromAddr, toAddr, amount, fee, memo, privateKey);
public void pushTransaction_taddr(ZCashTransaction_taddr tx, WalletCallback<String, Void> onComplete);

• Send t-addr transaction, asynchronous network method, the result is the callback. Separate method to generate the transaction on the device.

public ZCashTransaction_zaddr createTransaction_zaddr(fromAddr, toAddr, amount, fee, memo, privateKey);
public void pushTransaction_zaddr(ZCashTransaction_zaddr tx, WalletCallback<String, Void> onComplete);

• History for t-addr. Returns following details: Sum, time, date, hash, origin address, destination address, balance after/before, confirmations, block height

public void getTransactionHistory_taddr(String pubKey, int limit, int offset, WalletCallback<String, List<ZCashTransactionDetails_taddr>> onComplete);

• History for z-addr. Returns following details: Sum, time, date, hash, origin address, destination address, balance after/before, confirmations, block height

public void getTransactionHistory_zaddr(String pubKey, int limit, int offset, WalletCallback<String, List<ZCashTransactionDetails_zaddr>> onComplete);

• Restore t-addr using backup

public void importWallet_taddr(String privateKey, WalletCallback<String, Void> onComplete);

• Restore z-addr using the backup. It will require blockchain scan.

public void importWallet_zaddr(String privateKey, WalletCallback<String, Void> onComplete);

[BlinkyStitt]

Why are you sending change back to the senders address? Address reuse should be avoided if possible. Generating change addresses with an HD wallet is pretty simple and there are already standards for it.

[garethtdavies]

@wysenynja there is a good post here https://forum.z.cash/t/i-dont-want-too-many-own-addresses-in-my-wallet/22497/2?u=garethtdavies discussing this issue.

[garethtdavies]

In the history for z-addr method should the destination be displayed? See this earlier discussion zcash/zcash#1438

[bitcartel]

@guardaco Is there a reason to support Android 4.4 instead of targeting the latest Android 8.0 and taking advantage of the latest frameworks and toolchain?

[chaserileyroberts]

How is the RNG of generateNewPrivateKey implemented or accessed? It might be a good idea to allow dependency injection in case we learn later that the RNG is compromised.

[AndrewGuarda]

@wysenynja thank you for the feedback. For the mobile wallet, it's not so useful to have the list of HD addresses, the number of addresses will grow for each outgoing transaction. Finally, the addresses long list will almost useless in mobile interfaces. That's why we decide to use origin address for receiving unspent.

[AndrewGuarda]

@bitcartel there is a number of restrictions for the wallet functionality on 4.4 and early, I'm afraid it will be impossible to have the full wallet functions in that case. For my own opinion the market share of such devices mapped for ZEC users will not so great. Do you have any ideas why we need to support 4.4 and early devices?