Sometimes, an exchange might ask you to perform a "Satoshi Test" after you've deposited Bitcoin. This means they want you to send a very small amount of Bitcoin from a specific address in your BitBox wallet to an address they provide. This proves you control that particular address.

This request can be puzzling, especially if the address they name is one you've used before but doesn't seem to have a spendable balance right now. Don't worry, there's a straightforward way to handle this!

For a deeper dive into what Satoshi Tests are, check out our blog post: Satoshi Tests hinder self custody, but AOPP can fix it! 

First things first: Do you have Bitcoin for the test?

If your BitBox wallet is currently empty or you don't have enough Bitcoin to perform the test, here are a few ways to get the small amount needed:

• Ask the exchange: Some exchanges provide a tiny amount of Bitcoin specifically for these tests.
• Buy a small amount: Services like Pocket Bitcoin can help you acquire a small quantity.
• Receive from a friend: If you know someone with Bitcoin, they can send the necessary amount to your BitBox wallet.

Once your BitBox wallet has some Bitcoin, you're ready for the next steps.

Why does the specified address seem "empty"?

You might wonder why an address you've previously used to receive Bitcoin appears to have no funds to send. This is because of how Bitcoin transactions work with Unspent Transaction Outputs (UTXOs):

• Think of UTXOs as individual digital "coins" or "notes" of varying amounts, each sitting at a specific address.
• When you send Bitcoin, the BitBoxApp uses one or more of these UTXOs.
• Any leftover amount from that transaction (called "change") is automatically sent to a new change address in your wallet. This is a standard practice for privacy and efficiency.

So, the address the exchange wants you to use for the Satoshi Test likely had its UTXO spent in a previous transaction. The value moved, leaving that specific address without a directly spendable UTXO, even though it has a transaction history.

The solution: Two main steps

To send from the exact address the exchange specified, you'll first send a small amount to that address (an internal transfer within your wallet). Then, you'll send the Satoshi Test amount from that address.

Step 1: Fund the specific address

The exchange gave you a specific Bitcoin address they want you to send from. You need to send a small amount of Bitcoin to this address first.

Identify the target address: Double-check you have the correct Bitcoin address (e.g., starting with bc1q...) from the exchange. This is the address you need to "refill."

Choose your method to send Bitcoin to this target address:

Method A: Simple internal send

This is the most straightforward way if you don't need to select which of your existing funds are used.

• In the BitBoxApp, initiate a normal "Send" transaction.
• In the "Recipient" field, paste the target address (the one the exchange specified for you to send from).
• Send a small amount, a bit more than the Satoshi Test requires, to cover transaction fees. For example, if the test needs 500 satoshis, send 1000-2000 satoshis to this address.
• Complete and send this transaction.

Method B: Using Coin Control to fund (Recommended for more control)

This method allows you to choose exactly which of your existing UTXOs (your "coins") will be used to fund the target address. This can be useful for managing your UTXOs, especially if you want to use funds from specific change addresses.

• Enable Coin Control: In the BitBoxApp, go to Settings > Advanced Settings. Toggle the switch for Enable coin control to the "on" position.
• Start a new "Send" transaction.
• Enter recipient and amount:
  • In the "Recipient" field, paste the target address (the one the exchange wants you to send from).
  • For the amount, enter a small sum (e.g., enough for the Satoshi Test plus a buffer for fees, like 1000-2000 satoshis).
• Select your inputs: On the sending screen, click the "Toggle coin control" option. This will open a separate window, titled "Send from output(s)" or similar.
  • You'll see a list of your UTXOs, showing the amount, address, and other details for each.
  • Select the UTXO(s) you want to use to fund this internal transfer by checking the box next to them. You are choosing which of your existing "coins" will be sent to your target address. It's often good practice to select UTXOs from your change addresses if available.
  • Click "Continue" or "OK" in the Coin Control window.
• Review and send: You'll return to the main send screen. Review the transaction details (recipient, amount, fees) and then proceed to confirm and send the transaction.

Wait for confirmation: Whichever method you used, allow this internal transaction to be confirmed on the Bitcoin network (usually one confirmation is enough, which can take around 10 minutes or more). The target address now holds a fresh, spendable UTXO.

Step 2: Perform the Satoshi Test using Coin Control

Now that the specific address named by the exchange has a confirmed, spendable balance on it:

1. Ensure Coin Control is enabled: If you didn't enable it in Step 1 (Method B), go to Settings > Advanced Settings and toggle Enable coin control to "on."
2. Start a new "Send" transaction.
3. Enter the exchange's details:
   • In the "Recipient" field, paste the Bitcoin address provided by the exchange where they want you to send the small test amount.
   • For the amount, enter the exact small amount (e.g., 500 satoshis) that the exchange requested for the test.
4. Use Coin Control to select the correct sending UTXO:
   • On the sending screen, click the "Toggle coin control" option. This will open a separate window, titled "Send from output(s)" or similar.
   • Look through the list of your UTXOs. Carefully find and select only the UTXO that is on the target address you just funded in Step 1. You can identify it by the address and the amount you sent to it.
   • Click "Continue" to confirm your UTXO selection.
5. Review and send:
   • Back on the main send screen, double-check all details: the recipient address (exchange's), the amount, and the fee.
   • Proceed to confirm and send the transaction from your BitBox.

By first funding the specific address (using your preferred method) and then using Coin Control to meticulously select that exact UTXO for the Satoshi Test, you'll meet the exchange's requirement and successfully demonstrate control over that address.