Paper Wallets and Cold Storage for Bitcoin and Ethereum

An Ethereum Paper Wallet

You made a fortune trading crypto currencies, but don’t like the general idea of actual money being stored on the hard disk of your worm-infested PC? Then cold storage and paper wallets are for you.

In this post, I am going to explain what cold storage and paper wallets are, and how you can create one, the really safe way.

Offline Accounts

Due to the nature of how blockchain-based currencies such as Bitcoin, Litecoin and Ethereum work, it is possible to generate a wallet or an account and send money to it, without the account ever having seen the network – you can generate and store it entirely offline, so no one could possibly steal it over the network.

When coins are being sent to an address, the blockchain merely stores the information “address X owns N coins”. Only in order to spend those coins again, one needs the private key associated with the address; the private key is used to sign a transaction which moves coins from the address to another one.

So all we need to do is use a software to generate our address and private key pairs.

Generating Wallets

We are going to use the tools provided by MyEtherWallet, and for the respective currencies. You can just go there and do the generating on those websites – all generation happens client side, in the browser, and not on their servers.

However, we want to be really safe and sure that our keys never touch the net, so we go one step further. We are going to create our wallets on an air-gapped computer. An air-gapped computer is a computer which is not connected to the network.


You need mainly two things – a computer which is not connected to the internet, and a means of safely and securely storing your generated keys. If you don’t have an extra computer around (which you totally should have, you can never have enough), you can use the one you are using right now, you just have to go offline for a while.

I will set up the key generation software on an extra RaspberryPi 3 I have lying around, and store my new wallet on a USB drive, and print it as a paper wallet. The USB drive will also be used to transfer the required software to the air-gapped computer.

Download the Software

For Ethereum, go to and download the distribution zip file (

For Bitcoin, go to and download the latest release source code archive (zip or tar.gz). (It is the same software for Bitcoin and Litecoin).

Store the archives you downloaded on a USB stick if you are going to transfer it to your air-gapped computer.

Working Offline

Setup an offline computer. All you need is a working browser, such as Chrome or Firefox. Transfer the software from the USB stick to your computer and unpack it.

For Ethereum, go to the unpacked archive and open index.html in your browser (by double-clicking it). You now have the same environment as on the websites mentioned before. Create your wallet as instructed.

Safe the encrypted JSON “UTC” key file. Then print your paper wallet – either generate a PDF or print it directly (obviously that requires a printer).

For Bitcoin, open in your browser. You need to generate some randomness by moving your mouse around. When done (counters go to 100%) you now have multiple options – generate a single wallet, a number of fancy looking paper walled, or just bulk wallets as a text file. Download the result and store or print it as needed.

You can now start to send money to your wallets.

Checking your wallet’s balance

You can check your wallet balance online. For Ethereum, you can enter your public address at For Bitcoin, enter it at No need to have your wallet online, as all the information needed is on the chain.

Secure Storage

You now have PDFs or pieces of paper with your *unencrypted* private key. No password is needed to spend money from your wallet with this key, so you have to keep it safe. And be sure to make a backup and store it in multiple locations (what happens if your house burns down?).

I hope you found this post helpful. If so, you can throw me a couple Satoshis via my donation address. Thank you 🙂







Bitcoin Transaction Fees Done Right

It happens ever so often – someone tries to send Bitcoins and the transaction is just stuck with zero confirmations forever. The reason is that the transaction is never mined, because the transaction fee was set too low.

In this article, I am going to explain how you set your transaction fee right, and then I’m going to explain what you can do to get your stuck transaction going again, or how to cancel a transaction.

Why do transactions get stuck?

Each transaction has a certain size, depending on some factors, usually it is between about 200 and 1000 byte. When a miner creates a block, they incorporate transactions into the block and get rewarded the transaction fee. The transaction fee is set by the sender of the transaction (i.e. you).

Now, the block size of Bitcoin is limited to 1MB. And because Bitcoin is so popular and there are many transactions, every block is full. The miners will therefore only incorporate transactions with the highest transaction fees. If you set your fee too low, your transaction might stick around for hours, or days, or even weeks without being mined.

What is the right transaction fee?

So, how is the best transaction fee determined? There are websites which can calculate the fee for you. I like this one: – have a look – it lists the predicted block- and time delay by transaction fee.

The transaction fee is usually shown in Satoshi per Byte. One Satoshi, the smallest unit in Bitcoin, equals 0.00000001 BTC. At the time of writing, the page suggests, if you want your transaction to clear in under one hour, you should set a transaction fee of 331-360 Satoshi per byte.

In the official Bitcoin Client, the transaction fee is configured in BTC per Kilobyte, so that’s Satoshi per byte divided by 100’000; so, to set 360 Satoshi per byte, you would set a transaction fee of 0.0036 – on the command line:

$ bitcoin-cli settxfee 0.0036

Ideally you put this as a default into your bitcoin.conf:


So, if your transaction has a size of 250 bytes, your fee would be calculated as 0.0036 * 0.25 = .0009 BTC, which at the time of this writing are about $2.54 – be aware that your transaction size might be bigger, though!

How can I get my stuck transaction going again?

That said, your problem still exists – you have sent a transaction, the money is gone, but it is stuck on the network – what now?

Luckily there is a trick – you can double spend the coins with a higher transaction fee. All you have to do is make your bitcoind “forget” that you already sent your money, and then send it again with a higher fee (calculated and set as described above).

First, you stop your bitcoind. I recommend you make a backup of your wallet.dat. And then start it with:

$ bitcoind -zapwallettxes=2

This removes all transactions from your wallet and re-reads them from the blockchain data. This will take quite some time, as the entire block chain is rescanned.

Now your bitcoind is running again but it “forgot” that you have sent your coins. As your transaction is not on the blockchain yet (because it was never mined), it does not re-appear.

$ bitcoin-cli settxfee 0.0036

Now, don’t forget to set the right transaction fee as described above, and then send the transaction again. Your bitcoind will now broadcast your new transaction. It can take some time until the network picks it up, up to one day in my experience – but when it is mined, the old transaction will be abandoned and forgotten.

Note that this strictly only works when your original transaction was never confirmed.

I hope this helped. If you are happy about the result, feel free to send me some Satoshis – thank you 🙂

Why Nuclear Crypto-Buddha?

Hello dear reader, my name is Daniel, and with this first post I am entering the blogosphere. I am a software engineer based in Zurich, Switzerland. I’m a general science and technology nerd, and in this blog, I am going to talk about and discuss the topics which come up on my way to enlightenment.

My first endeavor was to pick a name, and I came up with this, the Nuclear Crypto-Buddha. The name incorporates three things which concern me quite a bit, but the list is not exhaustive. I’m going to give a quick overview on each of the topics below.

Why Nuclear?

When I was seven years old, the Chernobyl Disaster happened, and the world once again became aware of the dangers of the nuclear age. Being a child, I was not able to understand the problem back then, but I was frightened. I wanted to know everything about it, which turns out to be a good way to handle fear.

So I went and started reading about how nuclear power plants and nuclear bombs work; I read about all the science and all the disasters, the well known and the lesser known ones, some of which I am planning to cover in the blog some day. In 2009, I finally visited Chernobyl myself, which was quite an experience.

Now I can say that I feel safe again, but I have lots of largely useless knowledge about everything nuclear.

Why Crypto?

While cryptology played an important role in many ways along my way of becoming and working as a professional software engineer, my main concern of this topic is based on a relatively new phenomenon, namely cryptocurrencies and the concept of blockchains.

While I heard of the infamous Bitcoin early on and even owned some back when they were of little worth, I only later really grasped the concept and the significance of blockchains. Meanwhile, I have invested some money and also I am running an Ethereum node. The picture on the left shows my original Mt.Gox YubiKey.

I am pretty sure that blockchains will play an important role in the future of information technology, and therefore, some of my hopes rely on crypto.

Why Buddha?

Right after my midlife-crisis it struck me, that in the end, every one of us is striving for happiness. Some may not know it, and many confuse it with wealth, but that is what we all want: we want to be happy.

As it turns out, this is neither an original insight, nor is it an easy to pursue goal to become truly happy. It requires work.

Fortunately, there was one guy who had it all figured out a long time ago. His name was Śhakyamuni Buddha. While this guy is often referred to as”the Buddha”, the original teacher and founder of Buddhism, “Buddha” itself actually is a title for someone who is enlightened, it simply means “the awakened” or “the enlightened”.

There is also the term “Crypto-Buddhist“, which refers to someone who secretly is Buddhist. I’m not going to further comment on this 🙂

The guy in the picture on the right is me, on the second day of a stay in at Wat Tan Dong, a secluded Theravada Buddhist temple in the forest far south west of Chiang Mai, Thailand. I had to use that chair because sitting and sleeping on the floor hurt my bones. By the way, that was before I lost more than ten kilos.


So it has been quite a ride so far, and I want to share my experience and knowledge with you. The journey has just only begun. Hang on tight!