Thank you. I understand that not all bitcoin clients have to download the blockchain to operate however, something I'm not clear on is whether or not those clients properly sync up a wallet.dat. As I understand it, if I store away a wallet.dat in a vault and then pull it out six months later, it needs to see all the new blockchain transactions to be caught up, and I'm concerned that software that does not download the full blockchain will not provide the necessary transactions to the wallet.dat. I hope that makes sense.
Nothing is changed in the wallet.dat when you receive or send transactions. The wallet.dat only holds your private keys and that's why you can keep it offline because it doesn't need to be synced. You only need to be synced if you want to know your balance.
simple:
wallet.dat <-- priv keys(access key to your bitcoin adresses)
blockchain <-- transaction history of everyone publicly available for anyone with a internet connection (looks same for everyone independent of the wallet.dat)
wallet.dat + blockchain -> balance can be calculated
The wallet.dat only changes if you add additional addresses to your wallet. It doesn't change if you receive btcs. It's pretty much like with bank account numbers. They also don't change if you receive money.
And that's why lightweight clients work because those 2 things are separated. The wallet.dat is very small and the blockchain is very big. But you only need the blockchain if you want to know your balance so why not keep the wallet.dat for yourself and use the blockchain of someone else? Remember the blockchain is the same for everyone. That's what electrum for example does. It keeps your wallet on your PC so your safe as long as your PC is safe and uses the blockchain of a server. If the server crashes you can also use another server or in 100 years if noone uses electrum anymore download your own blockchain.
