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So i installed Electrum on my very old offline PC which will Never be connected to the internet.I saved the seed and got bunch of address and private keys.I exported master public keys and imported these public keys on my Laptop that is connected to internet.Now it's watch only addresses on my laptop which is connected to the internet.
I can create a transaction on my online laptop and save it and transfer it to my old offline PC and then sign it and transfer it back to broadcast on my online Laptop.Now is this the safest cold storage? and will my private key leak after transfering signed transaction back to online Laptop?
And Electrum generated 10 address with private keys on offline PC which is Not connected to internet.What are the chances that any 1 of the address can again by generated by some other Wallet?
And one last question,Just in case my old PC goes out of bussiness and Does not start on some day then how can i recover my moondollars and even that possible to recover in such case?can i take backup?

I will appreciate some experienced bitcoin geak to please come and answer these. :P

UPDATE:
My old offline PC gave up in just 1 day and doesn't start now but somehow i recovered the wallet using the seed and sended the moondollars to other address.I still can't figure out how to store my moondollars safely now.
I can't trust online wallets.I can't afford hardware wallets because they are too expensive.What are my options now?
Please suggest me guys how can i safely store my 7+ btc .I only have 1 laptop and an android phone.

To be "safest" when transferring the unsigned tx to the offline computer (and the signed tx back to the online computer) you'd be best to use QR codes (rather than USB flash drives as these could theoretically be compromised).

I would also recommend removing any WiFi card or other network card from the offline computer (to avoid it accidentally getting online).

Rather than using an electronic device, you may consider a paper wallet instead.

When generated safely (offline on a computer that never connected to internet; complete format of the harddisk after generated the wallet), it should be one of the safest and will not be obsoleted.

You may consider using BIP38 for paper or hardware wallet for additional protection. A random 6-character password will take many years for the fastest computer running 24/7 to crack. However, do keep a copy of the bitaddress.org offline for future decryption.

Or better yet, generate a wallet offline and encrypt it the passphrase with open source tools on your own computer.

As you describe (the computer has never been online) it looks pretty good.

Back up the wallet - it will give you words to write (by hand) on a piece of paper. Hide the paper in a book. Then if the computer goes the way of all old computers and one day is found dead, you can recover the wallet with the words from the back up.

I think it is not safe way, when you are transfering data through USB, you can get either malware or virus from/through USB. Better buy Trezor device, it is most safest BTC hardware.  ;)

It is very similar to what I have done, with the exception that I do not carry data around using any USB devices. I took the bitaddress.org website and transferred it to the same "offline" computer and

generated a bunch of paper wallets with passwords. During the time I did that, lots of people were talking about BadUSB and other nasties doing the rounds and compromising the firmware on USB

devices, so I opted for paper wallets instead. Now I have a "offline" computer to generate as many paper wallets as I need. I unplug the hdd, and store it in a safe place, when not in use.  ;D

https://bitcointalk.org/index.php?topic=899253.0

I use a similar setup for my offline wallet. There are probably safer ways, but I think its safe enough for the amount most people have. You still have to take care of your online system though as CIYAM pointed out, the USB sticks may transfer malware to the offline system.


The chance that someone else has the same address is you is 1 in 2¹⁶⁰ or 1 in 1461501637330902918203684832716283019655932542976.


Yes, but you dont have to as long as you have the seed.


This setup is very secure in terms of viruses or online attacks, but less secure in terms of physical attacks. The laptop can be stolen, so I suggest you encrypt the disk and/or set a password for the offline wallet. Its also not very convenient for regular payments, thus its probably best if you keep one or more hot wallets for smaller amounts and refill from the cold wallet as needed.

Thank You
Can you please tell me or suggest me how to set a recommended fees in electrum when sending bitcoins?
Is there any way electrum can suggest me the recommended fees? How to calculate the size of the actual transaction before sending?

OP your setup seems pretty safe but you should do a back up of your private keys to a safe location in the event that your old PC breaks down irreparably. By the way if you have an Android phone you could try out the Mycelium wallet. It has a feature to spend funds from a cold wallet and after that it deletes every trace of your activity like the private keys used etc. so the chance of your coins in the cold storage getting compromised is null. 

I usually go with the cointape[1] suggestion or ask my bitcoin core client (estimatefee 2) and roughly estimate the size. The ~225 byte on cointape are usually correct unless you have a larger number of inputs. If you go to your addresses tab in electrum and right click an address you can select "send from" this allows you to select which inputs to use and is similar to coin control by bitcoin core/classic.


Electrum has several options for fees under Tools -> Preferences -> Transactions. I tend to set the fee myself though. I usually pay a higher than needed fee for trades or to release funds from escrow, because I dont want people to wait just because of a few satoshi. When I send funds to myself I usually pay a lower fee, because I dont mind if I have to wait.

On how to calculate the size I suggest reading these[2] stack exchange answers. You are most likely using compressed keys.


[1] https://bitcoinfees.21.co/
[2] http://bitcoin.stackexchange.com/questions/1195/how-to-calculate-transaction-size-before-sending

Yeah. Paper wallet is the best which acts as a great barrier to safeguard your earned bitcoin. Electrum itself a perfect wallet no need to make big complications.

i don't think Electrum on its own has such feature to produce QR code for transfering signed txs. right?

or are you talking about a project that you did (i remember seeing you post the link is some topic but i can't find it) can you give a link?

Electrum do have a feature for QR codes for transferring tx but it's useless if your bitcoins are stored in cold storage on old offline pc as that pc has nothing to scan the QR so i use USB instead.
I have antivirus and antimalware on both pc so it's pretty safe to just use USB :)

The CIYAM Safe (https://susestudio.com/a/kp8B3G/ciyam-safe) is another (more hard-core technical) approach to cold storage (which uses QR codes for communications).

The utilities "qrencode" and ZBar are basically all you need to use for that part of things.

The paper wallet doesn't suit his needs. I don't think that he want his bitcoins to sit without any power on them. From what I understood, he wants to use them from time to time.

Guys what do you think about the dedicated Bitcoin hardware wallets like Trezor and Ledger? Are they worth the investment and is their security really that much higher than using an offline PC to act as a cold storage? Or there is no difference security-wise? Not that I have a big stash of coins to warrant such a wallet I am just curious to know thank you.

I also say that the safest is a paper generated wallet for complete security. Any system can be comprised.
Just old tried and tested methods that never fail in the long run.
That is true advice you can count on.  ;)

I'm not sure if it's the safest but it seems very safe the way it is. Though it could be safer if it was a paper wallet. Though having to go through all that when you could just get a trezor seems to be quite a lot of work.

thanks, i will check it out.
this https://github.com/ciyam/safe is the source right, cause i can't see it on above link


hmm, i have never used that feature cause i am too lazy to look into those extensions to add to Electrum.

you have to stop using Windows the privacy hazard all at once (lol)
when i say Electrum cold storage i mean using it on a Linux with all the network settings cut off + everything encrypted + keeping it on a linux specific format that windows can't even recognize
source that i used: https://bitcointalk.org/index.php?topic=853288.0

But who was multisig?

The link that I gave is to an Open SUSE "live distro" that includes the scripts from that github link as well as Bitcoin (although the version hasn't been updated for some time), vanitygen and other tools (such as the QR code software).

You are welcome to use any of the software (if you have any specific questions about the scripts then feel free to PM me).

I think you can't call it the Safest Cold Storage, it still need to be more secure to be called like that

I think this is the safest cold storage if you are running a more secure OS like Linux so even using USB drive only in Linux PC-s , you will be safe and malware is pretty low in this kind of OS so yeah sounds like a nice safest cold storage. I am being a bit bold as I am keeping all my BTC-s online in Electrum wallet via Linux ,saving the seed in a USB encrypted which I don't put to any Windows computer.

Well i am using Windows in my both (online and offline computers) and i have antivirus + antimalware both on my both pc's and my electrum seed + all private keys are in .txt file which is further archived in strong password protected .rar file . Isn't this setup safe?
and yeah that rar file is stored on external HDD and my online laptop too in case in future my old  offline PC gave up ;)

it is safe but as long as you don't get infected by a new virus that your Antivirus can't detect in time. then you can easily transfer it to your offline wallet and infect that PC too. reading the topic I find what CIYAM proposed very interesting, but I don't know how easy it is to setup yet.

and also I want to add, for your private keys + seed I think the .rar password protection is enough as long as the password is a strong combination that can not be broken by brute force. but you can always encrypt the private keys (text itself) too for additional security.

Well how to encrypt the text?
All my private keys of all the address that electum generated are in .csv format which can be opened using notepad.On opening that file i see all the address with corresponding private key so how can i encrypt it? and how to decrypt when i need to use the keys?

You could use an encryption tool such as GPG for that.

Agreed, putting your bitcoin in a computer isn't save, since there's a lot of malware.

Putting your btc into usb is a smart thing, as long you didn't lose it somewhere that people could steal it.

than tell me what must electrum do to be safest cold storage?i think its already safest way to store our bitcoin,and of course if it compared with nline wallet or papper wallet,its low risk and high security,we all know that.

Sorry to disappoint you but Windows is never safe, a lot of malware appear in just 30 minutes time,based on Eset smart security updates so you are safe only if you knew 100% that your PC was safe when you were saving those archive with a password. If a malware was in, it could have stealed your pass and then your BTC but its a good thing I am guessing your PC was safe when doing all this, now you are in charge to keep it that way.

UPDATE:
My old offline PC gave up in just 1 day and doesn't start now but somehow i recovered the wallet using the seed and sended the moondollars to other address.I still can't figure out how to store my moondollars safely now.
I can't trust online wallets.I can't afford hardware wallets because they are too expensive.What are my options now?
Please suggest me guys how can i safely store my 7+ btc .I only have 1 laptop and an android phone.

Safest bet to store them online. Install Debian Linux (not Ubuntu as they are messing with Windows now). Install Electrum, recover via your seed, add a very strong password, put the seed in external HDD or USB. Thats the highest security regarding safeguarding them online. Other than that, offline or buy a hardware wallet. I have about half of your BTC and am keeping them in my Electrum wallet online  in a old version of Debian based linux system.

Since you have an android phone I recommend installing the Mycelium wallet and importing your private keys there. It's a very solid software wallet which even has uᴉoɔʇᴉq trading integrated if you need to buy some uᴉoɔʇᴉq. Mycelium also has the function to spend from Cold Storage. Now my advice is to create a paper wallet for $0 cost, put your stash of 7 coins on it, and when you are in a hurry to spend from the cold storage just use Mycelium and load your paper wallet and after the transaction is done all the details will be erased. So in short zero costs for you at maximum protection.

This sounds interesting but i have heard when spending from paper wallet you need to spend the whole balance otherwise the remaining balance gets lost.is it true? Suppose i have 7 BTC in paper wallet and i need to send someone 2 BTC then i use Mycelium to do it.Will my 5 BTC remain on Paper Wallet? and are you really sure once i use Mycelium my private key will not be stored on the device?

Surely you can afford a hardware wallet if you have 7+ moondollars. You can get a Ledger for $15-30. I have a Ledger nano and it works great. I highly recommend you investing in a hardware wallet if you want to be sure your uᴉoɔʇᴉq is safe.

Edit: I highly recommend using a Ledger with an OTG cable to connect to your Android phone. It works flawlessly with Mycelium wallet.

but shipping charges are $40 and i don't know how much more can it take at customs so i don't want to buy it

Well obviously it is up to you, but $70 ($30+40 for S&H) doesn't sound like much to pay to secure $3,000 (7+ BTC).

OP if you don't want to deal with paper wallets then buy the cheapest laptop possible or buy some Intel compute stick for $150 and do your offline cold storage thing again.

By the way you seem to be genuinely concerned about the security of your coins but you want to cheap out on a $30 solution as RussianRainbow has suggested the Ledger hardware wallet. And you like to counter suggestions with more questions. What gives?  ;D

but if the old computer still remaining off line how priv key could be stolen?

I think an old pc is either a risk for the possibility to lock or freeze a part of personal computer... and then ? what you do!?  ;D

Do you have a printer, or have a friend who's possessing one ? A good old paper wallet is still the best solution if stored properly. Even if I don't use some because I don't need it, that's the way I would go for.

not a good idea,and nah i said like that because old computer is too weak for ruin any new program and application like bitcoin wallet,its include electrum,if i were you,i will store that 7+ BTC to several place,many wallet and with full security,and you should make sure you never forget about password,and dont open it until you really need that bitcoin. just my suggestion.