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Unless you are using a hardware wallet it is strongly recommended to read this page. In the case that your current wallet hasn't been protected adequately e. For a brief overview see also: Wallet Security Dos and Don'ts. Paper wallets can be used to store bitcoins offline in non-digital format.
Using securely generated paper wallets significantly decreases the chances of your bitcoins being stolen by hackers or computer viruses. Fundamentally, a paper wallet is merely a physical record of a HD wallet private seed. Hardware wallets are a major step to enhanced security and usability. See the Hardware wallets page for more information on which hardware wallet solutions are currently available. No software is perfect, and from time to time there may be security vulnerabilities found in your Bitcoin client as well.
Be sure you keep your client updated with the latest bug fixes, especially when a new vulnerability is discovered. We maintain a list a known vulnerabilities on this wiki - you can watch that page to get updates. Note that you don't need to be running the latest major client version: Bitcoin transactions send Bitcoins to a specific public key. A Bitcoin address is an encoded hash of a public key. In order to use received Bitcoins, you need to have the private key matching the public key you received with.
This is sort of like a super long password associated with an account the account is the public key. Your Bitcoin wallet contains all of the private keys necessary for spending your received transactions. If you delete your wallet without a backup, then you no longer have the authorization information necessary to claim your coins, and the coins associated with those keys are lost forever. The wallet contains a pool of queued keys. By default there are keys in the key pool.
The size of the pool is configurable using the "-keypool" command line argument. A brand new address is generated to fill the pool back to So when a backup is first created, it has all of your old keys plus unused keys. After sending a transaction, it has 99 unused keys. After a total of new-key actions, you will start using keys that are not in your backup. Since the backup does not have the private keys necessary for authorizing spends of these coins, restoring from the old backup will cause you to lose Bitcoins.
Creating a new address generates a new pair of public and private keys, which are added to your wallet. Each keypair is mostly random numbers, so they cannot be known prior to generation. If you backup your wallet and then create more than new addresses, the keypair associated with the newest addresses will not be in the old wallet because the new keypairs are only known after creating them. Any coins received at these addresses will be lost if you restore from the backup.
The situation is made somewhat more confusing because the receiving addresses shown in the UI are not the only keys in your wallet. Each Bitcoin generation is given a new public key, and, more importantly, each sent transaction also sends some number of Bitcoins back to yourself at a new key.
When sending Bitcoins to anyone, you generate a new keypair for yourself and simultaneously send Bitcoins to your new public key and the actual recipient's public key. This is an anonymity feature — it makes tracking Bitcoin transactions much more difficult.
So if you create a backup, and then do more than things that cause a new key to be used, and then restore from the backup, some Bitcoins will be lost. Bitcoin has not deleted any keys keys are never deleted — it has created a new key that is not in your old backup and then sent Bitcoins to it. A backup is therefore recommended roughly every 50 transactions or address creations just to be safe.
If a wallet or an encrypted wallet's password has been compromised, it is wise to create a new wallet and transfer the full balance of bitcoins to addresses contained only in the newly created wallet. Examples of ways a wallet may be compromised are through password re-use, minimal strength passwords, computer hack or virus attack.
There are a number of ways to create a new wallet with Bitcoin-Qt or bitcoind but this is a process that has been tested with bitcoind 0. We use the copy command to minimize the chance of any data loss but you are warned to make backups of any wallet.
Unless you are using a hardware wallet , you must take care that the system is free of malware, viruses, keyloggers, remote access tools, and other tools that may be used to make remote copies of your wallet, Bitcoin-related passwords, or Bitcoin private keys. When your computer is compromised, the precautions taken below may provide additional protection.
A hardware wallet typically holds the private keys on its internal storage that is not accessible by any malware. The device signs the transactions internally and only transmits the signed transactions to the computer.
The separation of the private keys from the vulnerable environment allows the user to spend bitcoins on a compromised computer without any risk. Among its features are bind-hooks to set up a tomb's contents in the place where other programs expect them, for example in our case mount -o bind the. First install tomb from https: Now close the tomb and store its keys safely, make sure you memorize the password. Have a look at Tomb's documentation, there is a number of things you can do like steganography or printing out keys on a paper to hide and such.
Every time you like to access your wallet open the tomb and the. One can also store the bitcoin binary inside the tomb and even start the bitcoin client using the exec-hooks. Tomb's manual page "man tomb" explains the possibilities. The advantage of this approach over an encrypted home is that it becomes extremely portable across computers and even online shells: The first step is to make a new user.
In order for that new user to have an encrypted home directory, you'll first need the encryption utility. You'll need to come up with a secure new password for that user.
Then switch user to the new user. Since the home folder of this user is encrypted, if you're not logged in as that user, data that is saved there can't be browsed, even by a root user.
If something goes wrong with your system, and you need to decrypt the new user's files, you'll need its decryption key. It will ask you for your user's password and give you the decryption key.
You can run it again later if you need to, but run it now so that you can get your data if your Linux install gets botched. The encrypted folder data is not encrypted while it's in memory, and so if it's ever sent to the swap partition it can be stolen from there unless that too is encrypted - be aware that this will mean you cannot use Hibernate anymore, as the bootloader won't be able to restore the hibernation data. Then click on a folder in the new user to display the file browser, then keep going up folders until you see the new user home directory, then right click to bring up the Properties dialog, then click on the Permissions tab, then in the Others section, set the folder access to None.
Then click on the Security tab, and in the Passwords section, deselect 'Remember passwords for sites' and deselect 'Use a master password'. Then click on the Advanced tab, then click on the Update tab, and then in the 'Automatically check for updates to' section, deselect 'Add-ons' and 'Search Engines'.
This also applies to the storage of passwords, private keys and other data that can be used to access any of your Bitcoin balances. Assuming that you have installed the Windows Bitcoin client and run it at least once, the process is described below. For help finding this directory, see Locating Bitcoin's Data Directory. For example, if you installed Bitcoin in the default directory, mounted your Bitcoin encrypted drive as E: After doing this, any time you want to use Bitcoin, you must first mount the Bitcoin encrypted disk image using the same drive designation, and then run Bitcoin from the shortcut that you created, so that it can find its data and your wallet.
Anyone who can access an unencrypted wallet can easily steal all of your coins. Use one of these encryption programs if there is any chance someone might gain access to your wallet. There is also a list of open source encryption software.
Decrypting and encrypting the wallet. There is also a method to print out and encrypt your wallet. Brute-force password cracking has come a long way. A password including capitals, numbers, and special characters with a length of 8 characters can be trivially solved now using appropriate hardware. The recommended length is at least 12 characters long. You can also use a multi-word password and there are techniques to increase the strength of your passwords without sacrificing usability.
The Usability of Passwords. However, simply using dictionary words is also insecure as it opens you up to a dictionary attack. If you use dictionary words, be sure to include random symbols and numbers in the mix as well. If you use keyfiles in addition to a password, it is unlikely that your encrypted file can ever be cracked using brute-force methods, even when even a 12 character password might be too short.
Assume that any encrypted files you store online eg. Gmail, Dropbox will be stored somewhere forever and can never be erased. You might want to read What is your way to create good passwords that can actually be remembered? Short complex password, or long dictionary passphrase? Backing up your wallet is not necessary if you use a wallet with implemented BIP hierarchical deterministic wallet.
For advise on the backup process see Backing up your wallet. It is likely that advanced tools can still be used to recover the wallet. The Linux shred command can be used to overwrite the wallet file with random data prior to deleting; this particular copy of the file will then be practically impossible to recover.
Using shred and similar tools on Windows however does not guarantee that still other copies don't exist somewhere hidden on your HD.