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If Someone has bitcoin in a Wallet, then suppose that the company of the wallet get closed or finished work or destroyed, how could one restore his bitcoin?
Where exactly do the bitcoins exist? Are they "exist" in the blockchain and can be resored from there?
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your question may not be specific enough... general rule: as long as you have the private key (!), you can get your bitcoins. Why? Wallets store private keys, and from these private keys, you derive the pubkeys. And a little more magic code (well, not really) gives you your bitcoin address. Effectivly that's what is stored in a wallet. There are no Bitcoins in your wallet!
Bitcoins exist in the blockchain (keyword is UTXO), and they can be transferred from one address to another. So effectivly the bitcoins are in the blockchain.
So when the company goes down, and they did not provide you the privkeys, you're lost (this is true for "web wallets"). On wallets, that are located on your local machine, you usually have a seed or can somehow extract the priv key. This means, you will still have access to the bitcoins, even if the company doesn't exist any more. The concept of keys, addresses and wallets are well describerd here: http://chimera.labs.oreilly.com/books/1234000001802/ch04.html
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Bitcoins are stored in the blockchain in the form of unspent transaction outputs, i.e. the product of confirmed transactions. These unspent transaction outputs can be spent if you control the private key corresponding to the address they were signed over to. If a private key is lost irretrievably, the coins will never become spendable again. There is no key recovery service. You're responsible of creating sufficient backups and keeping your private keys safe.
If a company shuts down their service, they should pay out any remaining balances to their users. If they have full custody and go out of business without returning balances, you're probably (depending on the exact circumstances) out ouf luck.
Thanks for your answer but let me ask you again please, does that mean that paper waletsis are the best wallets kind as it show you the privkey? Why do not walet company show any user his privkey? – Gamal Thomas – 2017-08-20T01:08:51.493
this justifies a new question in the forum. Paper wallets are a form of storing your values offline (aka "cold storage"). But generally, in online wallets the private key is not shown directly, or even hidden, to protect against malware, and if someone "sees" it on your screen, he can immedeatly make a photo and later on steal your funds... So more a security related discussion. You have to protect the private key, cause you are your own bank. – pebwindkraft – 2017-08-20T06:37:56.657
So in your opinion the paper wallet is the less secure wallet since it shows the privkey. Its a new and different opinion than most of I have read which consider it the best in security, do I get you right here? – Gamal Thomas – 2017-08-20T07:35:42.510