A private key is a secret number that lets you spend your bitcoin. An address, made from that key, is what you give people so they can pay you. A digital signature is how your wallet proves to the network that you hold the key, without ever revealing it. A wallet is the app or device that keeps your keys, and a seed phrase of 12 or 24 words is the backup from which all of them can be rebuilt.
Why this matters to you: Bitcoin has no bank that can reset a password or reverse a payment, so whoever holds the keys controls the coins. Every later lesson, from transactions to hardware wallets, builds on these few ideas, and a ten-minute exercise at the end lets you see them work without risking any money. If you are brand new, start with What Is Bitcoin and What Problem Does It Solve?
The Big Picture: A Public Ledger of Locked Coins
Bitcoin keeps one shared record, the blockchain, copied on tens of thousands of computers around the world. It doesn’t record names or account balances. It records coins, and each coin carries a lock: a condition that says, in effect, “this can only be spent with a valid signature from the key behind this address.”
A good picture is a row of glass lockboxes in a public square. Anyone can walk past and see how much sits in each box. Anyone can drop coins in through the slot. Only the person with the right key can take coins out. Each of your addresses is the label on one of your boxes, and only your private keys open them.
Where the picture stops: nothing physically moves. When you “send” bitcoin, the shared record is updated so the coins sit under a different lock.
Five terms carry the whole lesson:
| Term | In one line |
|---|---|
| Private key | The secret number that authorizes spending |
| Public key | A number calculated from the private key, used to check signatures |
| Address | A short code made from the public key; what people pay to |
| Digital signature | Proof, attached to one transaction, that the key holder approved it |
| Seed phrase | 12 or 24 words that can recreate every key in your wallet |
Private Key: The Secret That Spends
A private key is a number picked at random from an enormous range: about 1077 possibilities (a 1 followed by 77 zeros). Your wallet picks one for you, and because the range is so vast, nobody can guess yours or stumble on it by accident.
That protection depends on one thing: the number must be truly random. In July 2026 it came to light that a bug in the built-in software of some Coldcard hardware wallets (small devices made to keep keys away from the internet) had been creating new wallets from numbers far less random than intended. Attackers worked out some of those keys on their own computers and took the coins. The devices weren’t broken into (our Coldcard advisory explains what owners should do). A key is only as safe as its randomness and its secrecy.
Whoever has your private key can spend your coins, and there is no one to call to undo it. In practice you’ll rarely see a raw private key: modern wallets keep keys out of sight and show you a seed phrase instead.
Public Key and Address: The Parts You Can Share
From the private key, your wallet calculates a public key. The calculation is a one-way street: going forward takes a fraction of a second, and going backward, from public key to private key, is practically impossible with today’s computers. That is why the public key can be shown to anyone.
From the public key, the wallet then makes an address, such as one starting with bc1q, that you give people who want to pay you. Most address types are made by running the public key through hash functions (calculations that turn any input into a short fixed-length fingerprint). Every address also carries a built-in checksum, a few extra characters calculated from the rest, so a mistyped address is almost always rejected by the wallet before any money moves.
Private key → public key → address. Each arrow runs one way only: nobody can work backwards from your address or your public key to your private key.
An address is closer to a single-use payment reference than to a bank account number. Every payment to an address is visible to anyone who looks it up, so use a new address for each payment (your wallet can make as many as you need) and your history won’t pile up in one place. Our guide Why does my Bitcoin address keep changing? explains more.
Digital Signatures: Proving You Hold the Key Without Showing It
To spend, you must convince tens of thousands of strangers’ computers that you hold the right key, over a network where everything you send is public forever. Sending the key would hand it to the world. A digital signature solves this.
Think of a combination lock. If you open it in front of a witness, you have proved you know the combination without ever saying what it is. A signature does the same job for a payment. Your wallet runs a calculation that takes two things, your private key and the exact transaction, and produces a signature. It sends the signature along with your public key (a Taproot address, met below, already contains it), so anyone can check the signature in an instant. Only someone holding the private key could have produced it, and the key itself is not inside it.
Compare it with a signature in ink:
| Handwritten signature | Digital signature | |
|---|---|---|
| Changes from one document to the next? | No, it looks much the same every time | Yes, it is different for every transaction |
| Can someone copy it onto another document? | Yes, with practice | No, it only fits the transaction it was made for |
| Does it protect the content? | No, a figure can be altered afterwards | Yes, change one amount or address and it fails |
| Who checks it? | A person, by eye | Computers across the network, by calculation |
In an ordinary payment the signature covers the whole transaction: which coins are spent, the amounts and the destinations. So nobody who relays your transaction can redirect it or change the amount, and a signature published on the blockchain can’t be reused to spend your other coins.
Every full node (a computer running Bitcoin software that checks all the rules for itself) checks the signatures in each new transaction and block it receives, and rejects anything that doesn’t match. That is what stops theft on the network: you can’t spend coins you can’t sign for.
One common confusion: signing is not encryption. Bitcoin transactions aren’t hidden. Amounts and addresses are public for anyone to read; the signature only proves who approved the payment.
What a Wallet Actually Stores
A wallet is the app or device that manages your keys. It stores:
- your private keys, or the seed they are calculated from;
- the public keys and addresses made from them;
- conveniences such as your transaction history and labels.
It doesn’t store bitcoin. Your balance lives on the blockchain; the wallet reads the blockchain and adds up the coins your keys can spend. Two consequences follow:
- Losing the device isn’t losing the bitcoin. If you still have your seed phrase, you can rebuild the same wallet on a new device and the same coins appear.
- A copy of the keys is as good as the original. Anyone who copies your seed phrase has your wallet too. There is no “original” that outranks the copy.
A wallet can even work with no private keys at all. A watch-only wallet holds only public information, often a single extended public key (xpub) from which all your addresses can be calculated: it can show your balance and make new receiving addresses, but it cannot spend. That is usually how a hardware wallet pairs with a computer: the app on the computer is watch-only, while the device keeps the private keys, signs inside, and shows each address and payment on its own screen for you to check. Keep the xpub private too: it can’t spend, but it reveals your addresses and balance.
The Seed Phrase: One Backup for Every Key
When you create a wallet, most apps and devices show you a list of ordinary words, usually 12 or 24 taken from a standard list of 2,048. This is your seed phrase, also called recovery words or a recovery phrase. The words are a human-friendly way of writing down one big random number, and from that number the wallet calculates every key it will ever use, in the same order every time: the keys you have now and the ones it hasn’t made yet.
Think of it as the master pattern from which a locksmith can cut every key on your keyring, including keys for locks you haven’t installed yet. There are about 3.4 × 1038 different 12-word phrases a wallet will accept, far too many to guess.
Three things follow:
- The words are the wallet. Type them into compatible wallet software anywhere in the world and the same keys, addresses and coins appear.
- The words need no password. Hardware wallets are locked with a PIN, and many erase themselves after a set number of wrong guesses. A thief who reads your words skips the PIN entirely, because the words alone rebuild the wallet on any device (unless you add the optional passphrase covered in Course 2).
- The words belong on paper (or metal), offline. Write them by hand, check them twice, and never photograph them or store them in a phone, email or cloud account.
How the words are generated, how to store them so they survive fire, flood and curious visitors, and how the optional passphrase works are the subject of Course 2: seed phrases explained and seed phrase storage.
What You Can Share, and What You Never Share
| Item | Safe to share? | Why |
|---|---|---|
| A receiving address | Yes | It only lets people pay you and see what was paid to that address. |
| A transaction ID (a payment’s reference code) or signature | Already public | Anyone can read them on the blockchain; they can’t be reused to spend. |
| Extended public key (xpub) | Only with your own software | No one can spend with it, but it reveals every address and your balance. |
| Private key | Never | It spends your coins. |
| Seed phrase | Never, to anyone, for any reason | It recreates every private key you have. |
Sharing an address is safe because the one-way calculation can’t be run backwards: the address lets people pay you and see what was paid to it, and nothing more. (Whether a future quantum computer could change that is a separate, long-term question, covered in Is Bitcoin Safe?)
Sharing your recovery words is never safe, because the words are your keys. That is why so many scams aimed at wallet owners end with a request for them: a “support agent” who needs them to fix a problem, a website that asks you to “verify” or “sync” your wallet, a fake app that asks you to restore from your words. As one hardware wallet maker puts it, no legitimate company will ever ask for your wallet backup.
One simple rule covers every case: the only time you type your words is when you yourself are restoring your own wallet, in wallet software or a device you chose and installed from the maker’s official source. Our lesson on the scams that empty self-custody wallets walks through the common ones.
Self-Custody, Hot Wallets and Cold Wallets
Self-custody means you hold your own keys. The alternative is a custodial account: when you buy bitcoin on an exchange and leave it there, the exchange holds the keys, and what you see is the company’s promise to pay you. If it fails, freezes your account or is hacked, that promise can break: “not your keys, not your coins.” The trade-off is responsibility: nobody can lose your coins for you, and nobody can recover your keys for you either. Course 2 covers this in custodial vs non-custodial wallets.
Self-custody wallets come in two temperatures:
- A hot wallet keeps its keys on a device connected to the internet, such as a phone app. It’s convenient, like cash in your pocket, and best for small amounts.
- A cold wallet keeps its keys offline, so they are never exposed to the internet. A hardware wallet is the common form: your computer or phone passes it the unsigned transaction, it signs inside the device, and it hands back only the signed transaction. It suits savings, like a safe at home.
Many people use both. Our guide Hot wallet vs cold wallet: which do you need? helps you split the amounts, and Hardware Wallets Explained shows how the devices work.
The Address Formats You Will See
You’ll meet four kinds of address. All are ordinary bitcoin on the same network, and paying from one kind to another is routine (some services still can’t pay to the newest kind, bc1p). Two names in the table are network upgrades: SegWit (2017) and Taproot (2021) each brought a new address type.
| Starts with | Name | Since | Worth knowing |
|---|---|---|---|
1 | Legacy | 2009 | The original format; spending from it usually costs the most in fees |
3 | Script hash (P2SH) | 2012 | Often a multisig wallet (one that needs several keys to spend) or an early, compatible form of SegWit |
bc1q | Native SegWit | 2017 | Widely used; all lowercase, which makes it easier to read aloud and type |
bc1p | Taproot | 2021 | The newest; it contains a public key directly rather than a hash of it |
One look-alike to recognize: a long string starting with lnbc is not an address but a Lightning invoice, a payment request for the Lightning Network, the fast payment layer built on top of Bitcoin. It expires (after an hour, unless the invoice sets another time) and needs a wallet that can pay over Lightning.
Why the formats differ, how they affect fees, and how wallets organize thousands of addresses from one seed are covered in Course 2: Bitcoin Address Types and Derivation Paths.
Try It: A Ten-Minute Exercise With No Money at Risk
- See the public ledger. Open mempool.space and search for
1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa, often called the genesis address: it is made from the public key that received the reward for Bitcoin’s very first block (that 50 BTC reward itself appears on block 0, not on this page). As of November 2026, more than 60,000 transactions have paid this address, and not one coin has ever been spent from it. Anyone can see it and anyone can pay it; only one private key could spend from it. - Make a wallet. Install a well-reviewed self-custody phone wallet (one that gives you your own keys) from the maker’s official site or the official app store. Pick one that shows you 12 or 24 recovery words and gives you ordinary Bitcoin addresses, not only Lightning invoices. When it shows your words, write them by hand on paper.
- Note your first address. Tap “Receive” and write down the address it shows.
- Prove that the words are the wallet. With the wallet still empty, delete it in the app (or uninstall and reinstall the app), choose “Restore” (some apps say “Import” or “Recover”) and type your words back in. Restored in the same app, the same first address reappears: the words rebuilt the same keys. A different app may show a different address for the same words if it uses another address type; Course 2 explains why.
When real money goes in, start small and follow the test-send habit in Your First Withdrawal, which walks you through moving coins from an exchange into your own wallet.
Key Takeaways
- Your private key is a secret random number; whoever has it can spend your coins, and nobody can undo that.
- Public keys and addresses are calculated from it one way only, so an address is safe to share. Use a new one for each payment.
- A digital signature proves you hold the key without revealing it, fits only the transaction it was made for, and is checked by every full node.
- A wallet stores keys, not coins. The coins are on the blockchain.
- Your seed phrase recreates every key. Keep it on paper or metal, offline, and never give it to anyone.
- Self-custody means holding your own keys: hot wallets for small amounts, cold wallets for savings.
Check Your Understanding
1. A friend wants to send you 20,000 sats (0.0002 BTC) and asks for “your Bitcoin details.” What do you give them, and what do you never give?
Answer
A fresh receiving address from your wallet’s “Receive” screen. That is all anyone needs to pay you. Never give your seed phrase or a private key: the seed phrase lets the holder spend everything in the wallet, and a private key lets them spend every coin it controls.
2. Your transaction, including its signature, is visible to everyone on a block explorer. Could someone use that signature to spend your other coins, or to change the amount you sent?
Answer
No. A signature is made for one specific transaction and covers its coins, amounts and destinations. Changing anything makes it invalid, and it doesn’t reveal your private key, so it can’t be used to sign anything else.
3. Your phone falls into a lake. Your 12 words are on paper at home. What has happened to your bitcoin?
Answer
Nothing. The coins were never on the phone; they are recorded on the blockchain. Restore your wallet from the 12 words on a new device, using compatible wallet software, and the same keys, addresses and coins reappear. In anyone else’s hands the words would do the same, which is why they stay private.