Help me understand wallet backups.

[Kaos116]

Hello all - I'd like to get some simple answers to some, what should be, simple wallet questions. These are all in refrence to Bitcoin-QT.

To start, let's make it easy and not worry about encription for the moment.

1. Making a copy of 'wallet.dat' is sufficient to recover a wallet, assuming the copy was made before any problems started?

2. Once a fresh install of Bitcoin-QT has been made, replacing the new 'wallet.dat' with the copy/backup 'wallet.dat' will load all data that was in the copy wallet?

3. At this point, my assumption is that somehow it's going to sync again and update any transactions in your wallet that you made since the backup?


I have a few more questions, but they will build off the answers/confirmation of these 3 questions.

Thanks,

Kaos

[Martijnvdc]

1. Make a copy of wallet.dat is indeed sufficient, but you have to make a new backup whenever you add a new address to your wallet.
I recommend you print out a paper wallet though.
2. Yes, but you'll have to wait for the whole blockchain to be downloaded again. All your funds will be accessable to you.
3. Yes, that's a correct.

The blockchain contains all the transactions that ever happened.
Your wallet contains the private keys to your bitcoin addresses, which gives you the power to spend those coins. If you lose your private keys (your wallet.dat), you lose the ability to spend those coins, because you have no way of proving to the network that you are the owner of those addresses.

[DannyHamilton]

1. Making a copy of 'wallet.dat' is sufficient to recover a wallet, assuming the copy was made before any problems started?

Yes, as long as the copy is made while Bitcoin-Qt is shut down and not accessing the file, and as long as the copy is made recently.  Bitcoin-Qt creates a new address every time it sends a transaction.  It also creates a new address every time you click the "New Address" button.  When you create a copy of wallet.dat, that copy already contains the next 100 addresses that Bitcoin-Qt will use.  If you made the copy a long time ago and have used more than 100 addresses, then your backup is no longer sufficient.

2. Once a fresh install of Bitcoin-QT has been made, replacing the new 'wallet.dat' with the copy/backup 'wallet.dat' will load all data that was in the copy wallet?

This is generally true.  Occasionally, you may need to start Bitcoin-Qt with a rescan command line option to get it to scan the full blockchain looking for transaction history.  In some extreme circumstances, it may become necessary to delete the blockchain from your computer and wait for it to re-sync completely from the beginning again.

3. At this point, my assumption is that somehow it's going to sync again and update any transactions in your wallet that you made since the backup?

Correct. That is the intended design.

[DannyHamilton]

1. Make a copy of wallet.dat is indeed sufficient, but you have to make a new backup whenever you add a new address to your wallet.

This is unnecessary. A backup already has the next 100 addresses that you will use. It is generally a good idea to have multiple copies of backups just in case one of the backups is lost or damaged.  I generally recommend creating a backup when the sum of sent transaction count and new receiving addresses is approximately 25, and then storing the 3 most recent backups.

I recommend you print out a paper wallet though.

The OP specifically said: "These are all in refrence to Bitcoin-QT"

How do you recommend printing out a paper wallet from Bitcoin-Qt?

2. Yes, but you'll have to wait for the whole blockchain to be downloaded again.

This is not necessarily true.  It depends on whether they've deleted the existing copy of the blockchain.

[Martijnvdc]

How do you recommend printing out a paper wallet from Bitcoin-Qt?

You can just make a QR code from your keys... And then print it... You can get the private key with dumpprivkey.

This is not necessarily true.  It depends on whether they've deleted the existing copy of the blockchain.

A fresh install doesn't come with the blockchain. You have to download it again.

[DannyHamilton]

This is not necessarily true.  It depends on whether they've deleted the existing copy of the blockchain.

A fresh install doesn't come with the blockchain. You have to download it again.

A fresh install of the software does not delete the data directory.  You have to intentionally choose to delete the data directory if you want to download the blockchain again.

[Martijnvdc]

This is not necessarily true.  It depends on whether they've deleted the existing copy of the blockchain.

A fresh install doesn't come with the blockchain. You have to download it again.

A fresh install of the software does not delete the data directory.  You have to intentionally choose to delete the data directory if you want to download the blockchain again.

Then it's not a fresh install. A fresh install means you install it on a system where the previous version was completely removed, including the data.

[Kaos116]

Thanks for the responses, that is simple enough. A question that came up reading the responses is

 - It sounds like the 'wallet.dat' file is really just the private key. It doesn't contain any transaction data. The transaction data is picked up by scanning the Block Chain?

Let's bring in encryption.

 - Do the question/answers in the original post still hold true if I encrypt the wallet in Btcoin-QT?

Thanks again,

Kaos

[Martijnvdc]

- It sounds like the 'wallet.dat' file is really just the private key. It doesn't contain any transaction data. The transaction data is picked up by scanning the Block Chain?

You can read what's inside the wallet.dat here:
https://en.bitcoin.it/wiki/Wallet
If you have the private keys to the addresses, you have enough information to recover all your coins. You can calculate your balance based on all the transactions in the blockchain.

- Do the question/answers in the original post still hold true if I encrypt the wallet in Btcoin-QT?

No, you will only be able to VIEW your wallet, but you won't be able to SEND any coins if you don't know the password.

[Kaos116]

You can read what's inside the wallet.dat here:
https://en.bitcoin.it/wiki/Wallet

Thanks!