Wrong Address Woes – What Happens if You Send BCH to a BTC address (and vice versa)


In the early days of the Bitcoin Cash hard fork, several users encountered a very serious problem with their transactions. In what might be a simple misreading of an address or confusion over the distinction between these two currencies, users were sending their funds to addresses on the wrong blockchain.

Users found out that this was a simple mistake to make. Bitcoin and Bitcoin Cash share the same address space – so an address that’s valid on one is also valid on the other. And at the time, there was no formatting distinction between the two either. So all it took was accidentally copying an address that someone intended to be user for BTC, and suddenly a user was sending their BCH into the void (or so it seemed). The reverse could be true as well, with BTC (Bitcoin) being sent to an address intended to be used for Bitcoin Cash.

Let’s take a look a what actually happens when this mistake is made, and what some solutions to the problem are.

A Deeper Look at BTC/BCH Addresses

The Address Space and Private Keys

When looking at this problem, it is important to understand why Bitcoin and Bitcoin Cash addresses were easily confused, and why this problem would occur in the first place.

Bitcoin addresses are derived from 256 bit private keys using elliptic curve cryptography, coupled with some extra steps that make addresses easy to verify and encoding that makes them easier to deal with. When a Bitcoin address is generated, a random 256 bit number is generated. The public key is then derived using an elliptic curve algorithm (specifically secp256k1). The public key is hashed twice, once with SHA256 and then once with RIPEMD160. Finally, a version prefix and checksum are added and the whole address is encoded in base58.

Both Bitcoin and Bitcoin Cash used the exact same mechanism for generating addresses from the same exact pool of private keys. This offers up some good news with regards to dealing with this problem!

Cross-Chain Address Ownership

So if the mechanism for generating addresses is the same in BCH and BTC, then that means that the owner of a BTC address is also the owner of the exact same address on the BCH chain! The private key that the address was generated from proves ownership of the coins, and so the owner of the address will own coins sent to that address on either chain.

“Cross-Chain” Transaction Scenarios and Solutions

User sends BCH funds to their own BTC address

Let’s take a closer look at what happens when a user accidentally sends Bitcoin Cash to their own Bitcoin address. It is common for users to send their own funds between different wallets, like purchasing on an exchange with a custodial system and sending funds to a wallet where they control the private keys.

When the user broadcasts this transaction, it is processed entirely by the Bitcoin Cash network. Even though this user’s address came from their Bitcoin (BTC) wallet, nothing happens on the Bitcoin chain because these are two entirely different systems.

However, the BCH had to go somewhere! It may seem that it has been lost, but in reality it is now owned by the user’s address on the Bitcoin Cash blockchain.

It is likely that our friend is panicking because they don’t see the funds showing up in their Bitcoin Cash (BCH) wallet like they intended. However, they do own the money and just need to claim it!

Since the address spaces are the same between chains, our user has the private key that controls this Bitcoin Cash. All they need to do is export the private key from their Bitcoin wallet and import it into their Bitcoin Cash wallet, and the wallet will now recognize the funds as part of their BCH balance.

Further Complications

Thankfully in this situation, the mistake is a simple fix. The user controls the private key for the address they mistakenly sent their funds to, and so they have retained control of the funds the entire time. However, if a user sends funds to someone else’s BTC address, they will have to explain this process to the (hopefully known) recipient and hope they will be generous enough to send the BCH funds back to our friend. If the owner of the address cannot be contacted in the real world, our friend is entirely out of luck.

The activation of segwit (segregated witness) on the Bitcoin (BTC) network further complicates things. The above process only holds true for simple P2PKH transactions. If Bitcoin Cash is sent to a Segwit BTC address, the funds cannot be recovered on the BCH blockchain because these address types are not supported on the network

The CashAddr Format

Thankfully, the developers in the Bitcoin Cash ecosystem have created a new address format unique to BCH that is easily distinguishable from legacy Bitcoin addresses. A base58check encoded BTC address (non-segwit) looks like this:


Whereas a base32 CashAddr BCH address looks like this:


It is now much harder to make the same mistakes when dealing with addresses thanks to this format

Wrong Address? Not All Hope Is Lost!

Once we understand that Bitcoin and Bitcoin Cash share the same address space (and therefore private key space), it is a bit less scary to deal with this problem of “cross-chain” transactions. No transaction is really cross chain, it is simply sent to the mistaken address on the chain you sent it from. So, if you control the private keys for an address on one chain, you can easily claim funds you mistakenly sent to that address on the other. If you send BCH to your BTC address, you simply need to import that private key in your BCH wallet to claim the funds.

There are complications due to segwit and further problems for addresses our user doesn’t own. These are a reminder to always exercise caution when creating cryptocurrency transactions, as mistakes are often costly and irreversible. However, new formats like CashAddr have made these mistakes harder to make, so hopefully you never have a need to recover funds in the first place!

Full Node Friends – Understanding and Running A Fully Validating Wallet


In previous tutorials, we’ve discussed the basic differences between a full node wallet and an SPV or Simplified Payment Verification wallet. Now let’s take a further look into why full nodes are important to cryptocurrency networks, and how you can get started running one!

Why Full Nodes?

Full nodes, along with mining nodes form the backbone of cryptocurrency and smart contract networks like Bitcoin and Ethereum. These wallets have several important characteristics that help secure these networks and ensure they remain decentralized. This is not to detract from the importance of SPV wallets for adoption and convenient day-to-day use! Simplified wallets allow users to quickly and efficiently onboard to the world of crypto, but those aren’t the focus of this discussion.

So what makes full nodes so special and important for Bitcoin and other decentralized forms of money? The first critical characteristic of full nodes is that these wallets store the entire blockchain. Unlike SPV wallets which only store data relevant to the wallet user’s addresses, full node wallets download and process the whole blockchain database. There are some special methods the software uses to prune out unnecessary data, but at a high level we can say that these wallets store every transaction ever recorded in the history of Bitcoin, Litecoin, etc.

Now why is this data-intensive act of storing the whole blockchain necessary and valuable? It’s important because full nodes validate every single transaction in the blockchain. And in doing so, they ensure everyone is following the rules! By processing all the transactions up to and including the current block, wallets ensure that transactions are cryptographically sound, and therefore no one is trying to falsify ownership of any currency. If a single transaction in the history is off, the cryptography won’t be correct and the node will not recognize this fake version of the blockchain.

By rejecting bad blocks, full nodes help tell other parties on the network that something is up…and other nodes in turn will not allow shenanigans to occur. Bitcoin and other digital currencies are decentralized because anyone on the network can run a fully validating node and help make sure everyone is following the rules. If someone tries to fake a transaction (give themselves more money, re-spend money they’ve already spent, etc.), nodes on the network will refuse to accept this false version of history. And if nodes reject the malicious transaction, the bad actor effectively can’t spend that money!

Full nodes are just like cashiers who are trained to recognize counterfeit $100 bills. The know what a fake bill looks like, and they won’t let you spend it. But taking the analogy a bit further, a full node is a cashier that won’t take your fake money and also tells everyone else around them that your money is no good.

How to Run A Full Node

General Setup

Fortunately, running a full node is extremely easy. All you need is a computer with a decent internet connection, plenty of hard drive space, and the right software.

Almost all cryptocurrencies list their full node implementations as top recommended wallets. For example, bitcoin.org recommends running the Bitcoin Core client for Bitcoin (BTC).

Once you download and install the full node, you’ll need to wait for the initial download of the blockchain to complete. This does require some patience, as the most popular cryptocurrency blockchains exceed 100 GB in size at the time of this writing. On a 250Mbps connection, my node was able to sync the Bitcoin Cash blockchain overnight.

Once this initial sync is complete, your node will continually download and broadcast blockchain data and validate that all of the consensus rules of the currency are being followed!

Bitcoin Cash Nodes

For Bitcoin Cash (BCH), I’m trying out the Bitcoin ABC implementation. I installed this via the Ubuntu package manager aptitutde:

sudo add-apt-repository ppa:bitcoin-abc/ppa
sudo apt-get update
sudo apt-get install bitcoind bitcoin-qt

Once installed, simply open up the Bitcoin ABC software and follow some initial setup prompts. None are technical or complicated. Again, the blockchain will take some time to sync but your full node is up and doing its thing!

Ethereum Full Nodes

For the smart contract network Ethereum, geth is the client of choice. geth is just as simple to install, but is a bit less user friendly in terms of interface. The quickest way to start it is to run via the command line. To install:

sudo apt-get install software-properties-common
sudo add-apt-repository -y ppa:ethereum/ethereum
sudo add-apt-repository -y ppa:ethereum/ethereum-dev
sudo apt-get update
sudo apt-get install ethereum

And to start the client:

geth account new

Be a Full Node Friend

Fully validating nodes are an important part of cryptocurrency networks. They help keep these digital currencies secure and decentralized by ensuring that all transactions on the network follow the rules. No one can falsify transactions with thousands of full nodes watching, and because anyone can help ensure the rules are being followed, there is no need for central authorities to maintain trust.

And thankfully, running a full node is pretty easy to do. If you have the resources and desire, consider being a “full node friend” on the currency network of your choice – you’ll be keeping digital money secure for everyone!

Don’t Just Hodl, Spedn! – Cool Ways to Use Your Cryptocurrency


The “hodl” meme in the cryptocurrency world has gotten out of hand. With dreams of lambos abound, it seems everyone is just sitting around with fat crypto wallets waiting for the next big jump in price.

Now, there is absolutely nothing wrong with savings. I wouldn’t tell you to spend all your fiat either, and it’s always great to have money set aside for the future. But when it comes to cryptocurrencies, we’re doing the community a disservice by focusing too much on the price.

Bitcoin and all of its descendents are meant to be digital cash! We’re in the era of the most fascinating way to exchange value ever created, so let’s not sit on our crypto-assets like they’re a couple of boring old gold bars.

Why Spend Cryptocurrencies?

So why should you spend some of your digital assets? I always go back to the unique properties of cryptocurrencies that make them so interesting in the first place. Currencies like Bitcoin Cash, Litecoin, Ethereum, and (to some extent) the original Bitcoin Core chain are:

  • Secure
    • Crypto transactions are push transactions, so you never have to reveal personal information to a merchant like you do with a credit card.
  • Global and decentralized
    • These networks run worldwide without borders. Purchase goods, donate, and share with anyone anywhere without asking anyone’s permission
  • Low barrier
    • There is no KYC requirements, no paperwork, no approvals – download the wallet software and you now have a bank in your hands
  • Low fee
    • With the exception of the Bitcoin Core chain, you can send anyone any amount of money for a penny or less. And your fee isn’t going to a middleman, it’s going to support the network!

The Fun Part – How to Spend Cryptocurrencies

Purchasing Goods and Services

There are tons of merchants that will accept the most popular cryptocurrencies, especially online. I’ve bought several interesting items with various digital currencies – A JavaScript reference book for my shelf, T-shirts that share my love of Bitcoin, and even special apparel for Brazilian Jiu Jitsu.

Check out websites like Accept Bitcoin Cash or SpendBitcoins for ideas on where you can trade digital money for real-world goods.

Donate to your Favorite Organizations

Cryptocurrencies are great for donations as they make it so easy to send money quickly. Just snap a picture of a QR code address and your charitable contribution is on its way.

I’m a big fan of free and open source software, so I’ve sent tips to other developers and software projects I find useful.

Share it with Friends

Once again, the barrier to entry for digital currencies is low. Do you have crypto-curious friends? Have them download your favorite mobile wallets and send them a dollar or too. It’s simple and you may make a new crypto enthusiast for life!

After my recent lecture at Saint Vincent College, I was able to send a dollar a piece to several students by just instructing them to download a BCH wallet and snapping a picture of their QR code addresses. Crypto sharing is caring.

Spedning is Fun!

Sure, it’s easy to acquire cryptocurrencies and forget about them, stashing funds for a rainy day or big price spike. But the beauty of Bitcoin and its peers is the ability to exchange money in a way we’ve never done before. Adoption will be key for the future of digital money, and it’s quite easy and fun to participate in the economy.

Hodl some, spedn some. The crypto community thanks you.

Understanding Address Balances for UTXO Blockchains


When you open your Bitcoin, Bitcoin Cash, or Litecoin wallet, you’ll see a balance just like you do when you open your bank app. At the end of the day, you just want to know how much currency you own, right?

You may be curious, however, how your total balance is calculated in the world of cryptocurrencies. With your local bank, a centralized authority (the bank itself) keeps track of the state of your account as one unit. The bank tracks deposits and withdrawals, and keeps a running tally of your available balance for you.

The Bitcoin blockchain, however, does things a little differently. This blockchain (and the BCH and LTC blockchains, to name a few others) use a concept called the UTXO to deal with available balances. If that sounds completely foreign, don’t fret. It turns out UTXO-based chains function quite like the physical cash in your wallet!

UTXOs explained

What is a UTXO?

An unspent transaction output, commonly referred to as a UTXO is a chunk of cryptocurrency that is owned by a user’s wallet and available for the user to spend. More specifically, a UTXO is owned by a particular address in the user’s wallet, and therefore the associated private key.

A raw UTXO looks something like this when pulled from a block explorer API:

"txid": "2e2a921b819c261822dfa0931523a54b0c8900182c20d4be25ff333982a8f76a",
"amount": 0.10401187,
"confirmations": 306

This UTXO is pulled from the bitcoin.com REST API, with some bits of data removed for simplification. If you want to try querying this yourself, you can opening this API call in your browser.

Deciphering UTXO data

Now let’s look a little closer at this UTXO. The first data field that we see is the txid, which is a long string of data that looks meaningless. This data is the hash of the transaction that created this UTXO. In other words, this particular transaction sent money to this address.

The second item is fairly self explanatory: this is the Bitcoin amount sent to the address in this UTXO.

Finally, the number of confirmations indicates how many times a new block has been added on top of the block containing this transaction. The more confirmations, the more “sure” we can be that this transaction is a permanent part of blockchain history and owned by the address.

How do UTXO’s function?

UTXOs function in a way that is remarkably similar to physical cash. Think of a UTXO like a five dollar bill in your wallet.

A UTXO is a bill available for you to spend in a future Bitcoin transaction. Let’s say your grandma sent you $5 in a card for Christmas. You now have $5 in your wallet ready to use when you go to the store.

Much like dollar bills, UTXOs must be spent entirely in a new transaction. If you go to the store to buy a bag of chips and a drink for $2.50, you cannot tear the $5 bill in half and give it to the cashier, can you? You give the person the entire bill, and get $2.50 back in change.

Bitcoin UTXOs function in the exact same way in a transaction. If you have a UTXO your address owns for 0.1 Bitcoin and you want to send your friend 0.05 Bitcoin, your wallet will create a transaction that sends their address 0.05 BTC in a new UTXO, and sends 0.05 back to your address in change!

UTXO’s and Your Wallet Balance

Now that we understand how UTXOs work, understanding how your wallet tracks your balance is pretty straightforward! Your wallet contains a bunch of private keys and a bunch of addresses derived from those keys. Each address can have a bunch of UTXOs associated with that address, and your wallet balance is the sum total of all those UTXOs. It’s that simple. Just like you may have some 1’s, 5’s, and 20’s in your physical wallet, your Bitcoin wallet can have a bunch of UTXOs in any denomination of Bitcoin.

When you go to send Bitcoin to another user, your wallet bundles up as many UTXOs as it needs to create a transaction in that amount and uses them as “inputs” for that transaction. Unlike physical cash, however, your wallet can turn your $5 and $10 UTXOs in to a fresh $20 bill.

What’s in your (Bitcoin) wallet? UTXOs of course!

Again, UTXOs are the dollar bills of the Bitcoin world. Blockchains based on this model include popular digital currencies such as Bitcoin Core, Bitcoin Cash, and Litecoin. Other popular currencies such as Ethereum use an account based model that functions more like a traditional bank account, tracking inputs, outputs, and balances as state changes over time. The good news is, understanding the slightly more complex UTXO model is fairly trivial with a good analogy, and this model functions like the cash we use every day.

If you have a Bitcoin Cash address, you can try viewing your UTXO “dollar bills” yourself using a project I created for this purpose. This project features an API that digests raw blockchain data and outputs an easy to understand format so you can learn these concepts. On top of the API, there’s a nice and simple React frontend that formats the data in a table. The code is available on Github, and if you visit https://jmcintyre.net/sites/myaddrbal_client/ you can try it for yourself! Here’s an example with one of my BCH addresses used above:

Happy crypto learning!

(Bitcoin) Script Kiddies – Understanding Basic Transaction Scripts


In the Bitcoin world, money is not just digital – money is programmable! When transactions between users on the network are created and broadcast, miners and nodes independently verify that these transactions are valid. But this verification is not just checking some basic data points – it involves the execution of special scripts specified in the transaction parameters.

Script Basics

The Bitcoin Scripting Language

Before we can understand how basic Bitcoin scripts operate, we need to know a little bit about the scripting language itself. Unlike common scripting languages such as Python and Bash, the Bitcoin scripting language is quite limited and fairly simple in its execution. “Script” is stack based, meaning data is stored on an execution stack and script operators “push” and “pop” data from this stack. As well, Script is not Turing complete. There are no functions for looping or jumping around in the order of script execution. Operations are completely linear from the beginning of execution to the end. This keeps scripts secure, as it is not possible to tie up machines executing the scripts with an infinite loop.

Some operators are general, but most are specific to the cryptography of Bitcoin. Operators such as OP_ADD, OP_SUB, OP_DUP are pretty self explanatory – they make it possible to add, subtract, or duplicate data on the stack. Operators such as OP_HASH160 are more specific to the way Bitcoin operates – this operator takes the top item on the stack, hashes it using SHA-256 and then RIPEMD160, and finally pushes the result back on to the stack.

Common Bitcoin Script Mechanics

Pay to Public Key Hash (P2PKH) Script Basics

Finally, we need to understand a bit about how scripts are formed by discussing the basics of transactions. When a user “receives” Bitcoin in a transaction, they don’t just have a “bank account” balance on the blockchain. Rather, the blockchain stores what are called unspent transaction outputs associated with a user’s address. These outputs specify a locking condition that must be satisfied in a script when the user tries to spend that output in a future transaction. When the user creates a new transaction with that UTXO, they specify an unlocking script that satisfies that locking script

The most common form of script on the Bitcoin network is called Pay to Public Key Hash. With this type of script, the locking script requires that the user provide their public key and a digital signature formed with transaction data and their private key. This public key and digital signature will “satisfy” the locking script. When this unlocking script is combined with the UTXO locking script and executed, the final result on the Script stack should be true, meaning that the user can spend the Bitcoin.

P2PKH Formation

For a P2PKH script, the locking script specified by an unspent transaction output looks like this:


The unlocking script provides the user’s signature and public key in order

<Signature> <Public Key>

In order to verify that the user owns the Bitcoin they wish to spend, a node verifying this transaction will append the locking script to the unlocking script and then execute it:

<Signature> <Public Key> >OP_DUP OP_HASH160 <Public Key Hash> OP_EQUALVERIFY OP_CHECKSIG

Script Execution

Now let’s walk through how this P2PKH script executes.

<Signature> <Public Key> OP_DUP OP_HASH160 <Public Key Hash> OP_EQUALVERIFY OP_CHECKSIG

First, the signature and public key specified by the unlocking script are pushed on to the stack:

STACK: <Signature> <Public Key>

Next, OP_DUP pushes a copy of the top item on to the stack:

STACK: <Signature> <Public Key> <Public Key>

OP_HASH160 will pop the top stack item and hash it using SHA-256, and then RIPEMD160. Once the hashing operations are complete, the result is pushed on to the stack:

STACK: <Signature> <Public Key> <Public Key Hash>

The user’s public key hash (a data item) specified by the locking script is pushed on to the stack:

STACK: <Signature> <Public Key> <Public Key Hash> <Public Key Hash>

OP_EQUALVERIFY now pops the top two stack items and checks that they are equal. If they are equal, execution continues. If the comparison fails, script execution exits with a failure.

STACK: <Signature> <Public Key>

OP_CHECKSIG now verifies that the signature is valid against the public key specified. An elliptic curve digital signature is created using a private key and a specific message, and any user with that message and public key can verify that the signature is valid without knowing the private key! Note that the message is not a part of the script, but is garnered from the overall transaction data. If the signature is valid, OP_CHECKSIG pushes true on to the stack.

STACK: true

Any Bitcoin script that ends with just true on the stack indicates a valid transaction. The user that created this transaction to spend some currency is in fact the rightful owner of the unspent output they want to use.

Bitcoin Scripts – Simple But Powerful

For someone with programming experience and some computer science background, Bitcoin scripts are generally straightforward to understand since the language is limited and Turing incomplete. Understanding P2PKH scripts requires just a working knowledge of stack data structures and commonly used cryptographic algorithms, but no higher level programming constructs. Now you know what goes on when you send your friend some money from your Bitcoin wallet!

However, the beauty of programmable money is the power to create transactions beyond the normal flow of “Alice sends Bob some cash”. Script opens up the possibility of things like multi-signature transactions, time locked spending, and more!

EZ-Pay – Full Node vs. SPV Wallets


When discussing digital currencies, the question is often asked “where is the ‘money’ actually stored?” In the world of fiat currency (US dollars, Euros, etc.), cash stored in your physical wallet is the money. You give a $20 bill to a cashier, and they now have $20. With cryptocurrencies like Bitcoin, the actual currency is stored on a completely public, open ledger called the blockchain. The blockchain stores a complete record of every transfer between individuals in the history of Bitcoin’s existence, so a Bitcoin wallet can easily verify that you own some amount of currency and can send it to another person.

However, there’s a bit of a problem with this. The Bitcoin blockchain contains a record of every single transaction ever recorded, now over ten years of history. The blockchain is HUGE in terms of storage space – nearly 200 gigabytes these days. What if we don’t have that kind of space on our computer? What if we want to have a digital currency wallet on our phone or another capacity-limited device? Fortunately, there’s a type of wallet called an SPV wallet that fixes this problem. Let’s discuss the difference between full node and SPV technology.

Full Node vs. SPV

The Full (Node) Experience

The most full wallet experience is using what is called a full node. The strategy use by a full node wallet is very simple: the entire blockchain containing all transactions is downloaded to the machine running the wallet software.

Because the full blockchain is available to the wallet, verifying ownership of the user’s funds is simple. The wallet software looks at the blockchain ledger and traces the ownership of the currency back to the very beginning of Bitcoin. The blockchain is well secured by cryptography and proof of work, making it near impossible to forge any of these transfers. So, by storing the full blockchain, the wallet software can verify that all the previous transfers of Bitcoin leading up to the transfer to the current owner are valid and considered indisputable history. If your wallet can independently verify the blockchains transactions, it knows for sure that your Bitcoin is truly yours.

Wallets on a diet – Simplified Payment Verification or SPV

As we discussed, however, it can be problematic to download and store the entire blockchain for a wallet in many cases. What if a user with a small laptop, a mobile phone, or other limited-capacity device wants to participate on the Bitcoin network? A user may have limited storage capacity, or may also have limited bandwith for downloading the very large blockchain. But if we can’t download the blockchain, how can we independently verify that the Bitcoin in our wallet is actually ours?

Satoshi Nakamoto, the inventor of Bitcoin, brilliantly solved this problem by developing a technology called Simplified Payment Verification, or SPV. These wallets use some neat cryptographic tricks to avoid downloading the whole blockchain, at the expense of a minimal amount of trust required to verify currency ownership.

So how do SPV wallets work? When an SPV node needs to verify ownership of a user’s funds (in order to create a new transaction where they send money to someone else), the node makes special requests to full nodes it can find on the network. Instead of asking for the whole blockchain, it only asks for specific bits of information it needs to cryptographically verify that the wallet user owns their money.

SPV wallet only downloads what are called the block headers. These headers store important metadata about the transactions included in that block, including a sort of cryptographic summary of transactions called a Merkle tree. Next, the SPV wallet will ask other nodes on the network for transaction data that is relevant specifically to the user’s wallet, like previous transactions that send money to the user’s Bitcoin addresses.

By getting the basic transaction data from other nodes and the block headers, the SPV wallet can use cryptography that verifies that the transaction does indeed belong in a particular block (by verifying it belongs in the Merkle tree in the block header). The SPV node can then verify that the blockchain is valid by checking that all the block headers are valid and have sufficient “proof of work”. It turns out that if the transaction the wallet needs to verify is several blocks “deep” (that is, behind the latest block proved and added to the chain), the wallet can generally trust that the funds do indeed belong to the user without having to verify the whole blockchain!

It is important to note that in order to prevent being scammed by one rogue node on the network, SPV wallets connect to many full nodes to request transaction data. It is far less likely that all the peers an SPV node connects to a trying to scam that node with falsified transaction data, so it is generally considered secure to use SPV nodes for everyday transfers. If a user wants the most secure wallet experience, a full node is a bit better since it verifies the whole blockchain and doesn’t have to trust other parties on the network.

SPV – Wallets, simplified!

Thanks to the interesting cryptography of Merkle trees, proof of work, and block chaining, SPV wallets do not need to download the entire blockchain to securely check if a user owns their Bitcoin. By asking for specific transaction data, an SPV wallet can check that transactions sent to a user’s address belong to a block using a Merkle tree. And by verifying block chaining and proof of work, the node can trust that said transaction has been accepted as part of the Bitcoin history and is therefore owned by the user. Since SPV nodes communicate with multiple full nodes, it is generally true that SPV wallets are secure despite the fact that they do not download and validate the entire blockchain. So never fear – if you’re using a mobile phone wallet or a wallet on your netbook, you can participate in Bitcoin in a way that is secure!

What’s in Your Wallet? Understanding Private Key Control


Just like your cash, cards, and ID, your cryptocurrency assets live in something called a “wallet”. Most all forms of digital money implement this concept in some form, and understanding wallets is critical to safely storing and using your favorite digital currency.

Much like your physical wallet, your Bitcoin, Monero, or Ethereum wallet gives you direct access to the funds inside. A crypto-wallet isn’t like a credit card – if a stranger gets a hold of it, you can’t cancel it. Much like cash, the money stolen would be theirs!

But how does this work? How can a digital asset act like cash when all other forms of digital monetary transactions (credit cards, bank transfers, PayPal) can be “cancelled” if stolen? We must first understand a bit about what a “private key” is, and why who controls it is so important to the security of your cryptocurrency funds.

A word on private keys

Without getting to far into the technical details, let’s discuss a bit about what a “private key” is and why it is so important. Remember how I said that your crypto-wallet is like digital cash, and your wallet “stores” your Bitcoin or other currency? Well, that’s not quite how that works…

In reality, the amount of Bitcoin that you own is stored on a worldwide, completely public ledger/database called the “blockchain”. This ledger stores a public record of all of the Bitcoin transfers ever conducted, so anyone can see exactly who owns what. Sound scary and insecure? How can you control your digital cash if everyone has access to this open blockchain??

This is where private keys come in. Bitcoin (and other crypto currencies) use a form of cryptography called “elliptic curve cryptography” to generate the Bitcoin “addresses” people can use to send you money. The address is completely public; you can give it to anyone and they can send you funds. However, behind this address is a special “private key” used to access those funds on the blockchain. Your address is generated from this randomly generated private key by using this form of cryptography.

The cryptography used in address generation makes it so that you can’t figure out the private key by going backwards from the public key, or Bitcoin address. However, the private key is used to prove that you own the address without ever revealing it, thanks to the magic of elliptic curve cryptography. It is critical that the private key is always kept secret, because anyone with the private key can access the Bitcoin at the associated address.

Levels of Private Key control in Wallets

Now that we understand the basics of private keys and their importance, we can talk a bit about how different wallets keep these keys safe from the prying eyes of crypto-thieves. All wallets must work in some way that keeps the private keys, well, private so that control of the funds lies with their rightful owner. There are three general approaches to private key control in wallets: a full control model, a hybrid model, and a custodial model.

Full control wallets

Full control wallets offer the obvious – complete and total control of the private keys. With a full control model, the private keys are generated and stored on the user’s device, be it a desktop computer, mobile phone, or even a hardware wallet like the Trezor. With this model, the private keys never leave the user’s device in any shape or form.

The advantage to this model should be fairly obvious – it is by far the most secure model. There is no trust involved with a third party; the funds are completely controlled by you. Users should still exercise care around other security parameters (ensuring a virus-free machine, for example), but generally these wallets offer the most hardened approach to keeping private keys safe.

The disadvantage here is the lack of convenience and ease of use. This wallets require the most technical savvy of these three models, although most “power users” will have no problem understanding and securing these wallets. It is extremely important that the users of these wallets understand how to back up their private keys. If the device is fried or lost and there is no accessible backup, all funds will be lost! Fortunately again, BIP39 mnemonic backups make this tasks easier than it was with the first few Bitcoin wallets.

Hybrid wallets

Some major players in the crypto space have created an interesting hybrid model for private key storage. Web wallets like those at blockchain.info or btc.com implement this model. With hybrid wallets, private keys are generated and then encrypted on the user’s machine (usually in the web browser) before being stored on the company’s server. With these wallets, private keys are only known and accessible to the user, while the company keeps an encrypted backup safe on their servers.

With this model, security is still pretty strong. Because strong encryption is done on the user’s machine, no one with access to the company’s servers have access to the actual private keys without the decryption passphrase, which lies safely with the user. This model requires that the user trust that the company’s code (which is preferably open source) is soundly implemented and doesn’t contain secret backdoors. However, if the encryption is done right no one but the user can actually access the keys. This model is slightly less secure than a full control model,

Although there is a small amount of security tradeoff here, this model comes with increased convenience to the user. Most web wallets have a more traditional username and password login interface, so the user only needs to create and remember a secure passphrase to access their funds, with the site taking care of backups for them. This may be easier for a beginner crypto-enthusiast, and any good site will still offer mnemonic backups and private key exports for the savvy user.

Custodial wallets

The final model we’ll discuss here is the custodial wallet. Many exchanges like Coinbase offer custodial wallets. Like a hybrid wallet, all you need to do is create a username and password and log into a website to access funds. However, the critical difference is that with a custodial wallet, the user doesn’t know their private keys at all!. With custodial wallets, the website takes care of generating and storing all the private keys without revealing them to the user. No backups to manage, and no need to understand how to do much more than log in to a website to use this kind of wallet.

The security pitfalls of this model are pretty serious, in my opinion. With these wallets, the user has no way to back up their private keys. What’s more, the user must completely trust the company or individual implementing this kind of wallet. These companies must have significant security measures in place to avoid attacks on their servers, and they must be trusted to mitigate the access rogue employees could have to user’s money.

In fact, these types of wallets completely break a fundamental security principle of Bitcoin – the user controls their keys, therefore the user controls their money. Custodial wallets far more closely resemble the centralized model of traditional banks.

Don’t worry though – these wallets aren’t all scary! The cost of security comes with a large benefit – ease of use! These wallets have a much smaller learning curve for complete beginners. Just sign up for a wallet account just like you would a forum, email address, or social media account. For someone with little understanding of the world of cryptocurrencies, this type of wallet offers a gentle introduction

My only advice would be that given the security pitfalls of this model, only use custodial wallets to store small amounts or for buying and selling. Most custodial wallets live in currency exchanges, so use them to buy your crypto of choice and send the funds to a more secure wallet.

Know Your Keys

The most important takeaway from this discussion of private key control models is that it is important for a wallet user to know where their private keys are. Again, in Bitcoin and other cryptos, control over your private keys is control of your money. Anyone with access to the keys has access to your money and can spend it freely, so either keep them to yourself or make sure the holder is a wallet maker you trust.

By understanding these different models, users have more control over how they choose to secure their wallets and keep their funds safe. Understanding the pros and cons of full control, hybrid, and custodial wallets allows a cryptocurrency user to choose the best wallet for their needs. Ultimately, an understanding of these models allows for better security and comfort with digital money, because a person knows who truly owns their keys and is responsible for keeping them safe.

Playing With Blocks: The Basics of Blockchain Databases (Part 2 – Blockchain for Techies)


In our non-technical overview of blockchain, we discussed what a blockchain database is – a distributed, cryptographically secured, and immutable data structure used in applications like digital currencies. We discussed how blocks are cryptographically linked together to ensure that old records can not be changed, and why these types of databases are useful for applications where the immutability of data is key.

But how does this work technically? Let’s take a deeper look at how blockchains are secured by the application of hashing and proof-of-work.

It’s all in your the block’s head

The block header

The key to understanding blockchain lies in a data structure included in every block of every blockchain. This data structure is called the block header, and it contains several critical bits of information needed to secure the chain as it grows.

Let’s look at the Bitcoin block header as an example. Each Bitcoin block contains an 80 byte header with the following information:

  • Version – the software version of the Bitcoin protocol
  • Timestamp – expressed in seconds since the Unix Epoch
  • Merkle Root – for our purposes here, we’ll say this is a “fingerprint” of all the transactions in this block
  • Difficulty target – A 256 bit integer used in calculating proof of work
  • Nonce – The value added to the block header to demonstrate proof of work
  • Previous block hash – The SHA-256 hash of the previous block header

All of this data is important and serves a purpose in the Bitcoin protocol. However, I’ve highlighted the previous block hash because it is particularly important when discussing how the blockchain is secured!

A quick review of hashes

Before we discuss the critical role that the previous block hash section of the block header plays in securing the blockchain, let’s step back and recall what a hash function does. When “hashing” data, a special algorithm called a “hash function” takes the data and outputs a unique “fingerprint” of the input data. These functions (at least if they are implemented properly) have two very important properties.

First, a particular chunk of data always produces the same hash (or “fingerprint”) every time it is run through the function. If you run Hello through the SHA-256 hashing algorithm, the result will always be 185f8db32271fe25f561a6fc938b2e264306ec304eda518007d1764826381969.

Second, good hash functions avoid collisions, where different data results in the same hash output. Using a proven algorithm such as SHA-256 means that for every different input, a different hash is produced. Even if a single bit of input changes, the hash is radically different. “Hello” will produce a very different output than “hello”, even though only a few bits of the input are changed.

Hashes inside hashes inside hashes

Understanding these properties of hashes, we can better understand the interesting approach that blockchains take to securing the integrity of data in previous blocks when combined with proof of work.

Each time a block is generated, proof of work is generated in the form of the nonce included in the block header. This is computationally intensive and essentially proves that a miner spent a good bit of CPU power to find the answer to this cryptographic puzzle.

Now, when the nonce is included in the block header with the other data including the previous block hash, we can hash the entire block header to generate the unique fingerprint. This “block hash” is unique, and changing any data whatsoever in the block header would create a radically different block hash.

Okay, so each block has an associated hash. What’s the big deal? How does that help secure the blockchain? The magic of blockchain lies in that critical piece of data known as the previous block hash. Recall that changing any bit of data in the input radically changes the output of a hash function. So what would happen if we tried to change a transaction 2 blocks back in our blockchain?

If a node tries to broadcast a fake blockchain to the network with a fake transaction 2 blocks back, the block hashes for each subsequent block would be radically different.

Let’s look at an example. Let’s say we have a really simple blockchain with some transaction data like so (Note – these hashes are made up for demonstration purposes):

Block 2:
Time - 3000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 5345245
Prev block hash - 33a0b89fcce723e9f41f5d756ab1c20584afbe6dfa9ea18838ff3caf0915b5f5
Transaction: Bob pays Alice 6 units

Block 1:
Time - 2000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 2356343
Prev block hash - f4ebb8b56f590188f5824276af552cd51a48ba774e3ad1350c2800b116d8f6f5
Transaction: Alice pays Bob 5 units

Block 0:
Time - 1000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 1232341234
Prev block hash - 0000000000000000000000000000000000000000000000000000000000000000
Transaction: Alice pays Bob 1 unit

Now let’s say Bob gets greedy and tries to say that Alice paid him 10 units in the first transaction:

Block 2:
Time - 3000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 9987983
Prev block hash - 9ae343e333cbb96427eb333bb8c443359e3cf926c9de9845ceb583577b945afb
Transaction: Bob pays Alice 6 units

Block 1:
Time - 2000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 390970
Prev block hash - 3f82f4cfe059b5a69a0fd5b4d34774af5ecdc672d988320d5fd186998969a645
Transaction: Alice pays Bob 5 units

Block 0:
Time - 1000
Difficulty - 1000000000000000000000000000000000000000000000000000000000
Nonce - 235235
Prev block hash - 0000000000000000000000000000000000000000000000000000000000000000
Transaction: Alice pays Bob 10 units

Notice how different the hashes are for blocks 1 and 2 in Bob’s fake blockchain. If these hashes are to be considered valid by a node in this currency’s network, then each block must also demonstrate proof of work. Since the data has changed in a block, a new nonce must be found to show that work was done.

Here is the most critical part – since the hash for block 0 has changed and is included in block 1, proof of work has to be re-done for block 1. And since block 1’s hash is included in block 2, proof of work has to be re-done for block 2. In other words, to try and fake a transaction 2 blocks back, Bob has to re-do proof of work for 3 whole blocks!! In the meantime, legitimate nodes only have to try and find a solution for the current block. It is clearly impractical, if not impossible, to “fake” a blockchain more than one or two block old, because proof of work has to be redone for many blocks in the time the legitimate network only has to prove one.

Faking a blockchain take too much work!

Due to the interesting combination of hashing and proof-of-work algorithms, it is incredibly difficult if not impossible to fake history in a blockchain database. Because each block contains the hash of the previous block, changing history blocks back means that every bit of the chain on forward must be forged. While legitimate nodes only have to prove work for one block in that span of time, an attacker would have to calculate for many. Unless a malicious party has some amount of computing power the rest of us don’t know about, it’s nearly impossible to do so.

For the extra curious, Satoshi covers the math behind this concept extensively in section 11 of the Bitcoin whitepaper. While I don’t claim to understand this math very well myself, the paper does a good job of explaining its conclusions that forging blockchain history is a fool’s errand.

Playing With Blocks: The Basics of Blockchain Databases (Part 1 – Blockchain for Everyone)


Blockchain is the latest and greatest buzzword in the information technology world. From open source, decentralized cryptocurrencies like Bitcoin to traditional financial institutions, it seems as though everyone is dying to create and release their own blockchain based applications. But what is blockchain? Why is it such a popular concept, and what is it actually good for? Let’s discuss.

What and why: Blockchain simplified

What is blockchain?

So you’ve heard that blockchain is going to revolutionize everything, but what is it exactly? Let’s cut through the hype and discuss the technical foundations of Blockchain.

A blockchain is a distributed, cryptographically secured database that focuses on making historical data immutable.

In a traditional database, information is often stored on one or a few machines, controlled by a central authority. Access is controlled by this authority (think IT administrator) and the data is kept secure by granting credentials to modify that data to a select few trusted parties. By contrast, a blockchain database is governed by what is called distributed consensus, using mechanisms such as proof-of-work. For more information on proof-of-work, you can read my series of articles on it.. The important thing to note is that (in general), no one central person or authority decides what data is “verified” in a blockchain, a community of network nodes and software does.

If anyone can modify the data in a blockchain rather than a trusted party, then how is this consensus on what is correct achieved? Again, the secret lies in the science of cryptography. Through a mechanism like proof-of-work, a cryptographic puzzle is solved by software with some incentive to do so. In Bitcoin, the node that solves this puzzle is granted new currency. The real magic, however, is the fact that any other node in the network can verify that this answer is correct in a split second, so anyone can independently verify that a block meets the cryptographic standards set by that blockchain’s protocol.

You may be wondering how the cryptography in each block keeps the overall blockchain secure. This is the question of immutability, or how easy it is to modify the history stored in the blockchain. Blockchains solve this by cryptographically “linking” each block to the previous block, thereby making each individual block a critical part of the history stored by that “chain”. Each block has a header full of useful metadata about that block – a timestamp, a “summary” of the included data or transactions, a difficulty target and nonce for mining (part of proof-of-work), and the hash of the previous block’s header. Each block header is run through a one-way, cryptographically secure function called a “hash function” that creates a unique digital fingerprint for the data.

Immutability is achieved when combining the proof-of-work consensus mechanism with this system of chaining each block together. In order to create each block, the cryptographic puzzle solved by the proof-of-work algorithm allows a unique block header hash to be generated. It is computationally difficult to get this value, but very easy to verify it is correct. Now, it’s not that hard to re-solve that hard problem in a matter of minutes…it would be easy to create a fake block at the top of the chain. But what about 10 blocks back? Well, since each block contains a hash of the previous block header that is generated by solving this hard problem, you would have to now fake history for ten whole blocks! It is exponentially more difficult to do so the further back in the chain that you go. Unless you can truly do the work required to fake history in a blockchain, any independent network node could easily see that the rest of your history on forward is invalid. The immense difficulty of “faking” history in a blockchain gives it the most important property it has, its immutability.

Cool, so why is it useful then?

By far the most important aspect of blockchain, in my opinion, is its ability to decentralize applications. With a traditional database, a central authority has to be trusted, which can be a disadvantage in applications that are controversial or have high incentives for fraud. For example, previous attempts at digital money like DigiCash had central services for issuing currency and validating transactions. These were promptly shut down by governments that didn’t like independent currencies very much.

With blockchain, it is possible to have things like completely peer-to-peer money as with Bitcoin, Litecoin, and countless others because no central government or individual has to be trusted! The network is secured by math (cryptography) rather than trust thanks to the blockchain. You don’t have to trust anyone to not defraud you of your money, because the math cannot lie about who owns what.

The other critical function of blockchains beyond decentralization are the preservation of history. Because blockchains are immutable, they can be useful for keeping things like medical records, property transactions, court histories, and more secure from malicious tampering like a traditional database. This does rely on some degree of decentralization, but even within a single company a blockchain is far harder to tamper with than a traditional database.

Cool, now I want a blockchain!

Blockchains are a fascinating and novel way to handle problems with traditional databases in certain applications. Thanks to the decentralized and cryptographically secure nature of these databases, it’s possible to create peer-to-peer applications that don’t require trusting a third party – a key problem to solve for concepts like digital money. As well, their immutability makes them useful even beyond the first few money-centric applications that existed – they may be coming do a real-estate authority, doctor’s office, or justice system near you!