Wallets and keys

A wallet is the most misunderstood object in crypto. People imagine a digital purse holding their coins. The reality is closer to a keychain. Your "wallet" is software that stores keys; the coins themselves live on the blockchain at addresses those keys control.

This sounds like a small distinction, but everything in crypto self-custody depends on it. Lose the keys, lose the coins. Share the keys, share the coins. There is no password reset, no customer support line, no way to prove you "own" something without the key.

Public and private keys

Every wallet is built around a pair of keys. The public key produces an address you can share with anyone. People send funds to it the way they would send money to an email address. The private key is the secret that lets you spend whatever sits at that address.

Think of the public key as your bank account number and the private key as the signature that authorizes withdrawals. The math behind them is one-way: the public key can be derived from the private key, but not the other way around. Knowing someone's address tells you nothing useful for stealing their funds.

The seed phrase

Modern wallets do not actually ask you to write down a long string of random characters. They generate a list of 12 or 24 ordinary English words called a seed phrase (or mnemonic). That phrase is the master key. From it, the wallet can mathematically derive every private key, every address, and every account it controls.

A seed phrase looks innocent. It is a list of common words. It is also literally everything: anyone who reads it can recreate your wallet on any device, anywhere in the world, instantly. There is no second factor. There is no recovery email. The phrase is the wallet.

• Write it on paper. Two copies, two locations, both offline.
• Never type it into a website. Never paste it into a chat. Never photograph it.
• Real wallets never DM you to "verify your seed phrase." Anyone who asks is stealing.
• For larger amounts, use a hardware wallet that signs transactions without exposing the keys.
• Test your backup before relying on it. Restore the wallet from the phrase to make sure the words you wrote down work.

Custodial vs self-custody

There are two ways to hold crypto. With a custodial wallet (a Coinbase or Binance account, for example), the exchange holds the keys for you. You log in with a password, you can reset it, and the user experience feels like a normal app. The trade-off is that the exchange technically owns the coins; you have a claim on them. If the exchange freezes your account, gets hacked, or goes bankrupt, you are at their mercy.

With a self-custody wallet, you hold the keys. The coins are yours in the strongest possible sense — no one can freeze them, no one can take them, no one can stop you from moving them. The trade-off is that you are also fully responsible. There is no help desk if you lose the seed phrase.

Most people end up using both: an exchange to convert between dollars and crypto, a self-custody wallet to hold and use what they want to keep.

How a transaction is "signed"

Notice the third step: your private key never travels. The signature is a separate piece of math that proves you knew the key without revealing it. This is what lets you transact safely with strangers across the internet without anyone ever seeing your secret.

The rule, plainly

Not your keys, not your coins.

It is the most repeated phrase in crypto for a reason. Whoever holds the private keys controls the funds, full stop. Everything else — usernames, balances on a screen, customer service promises — is layered on top of that one fact.

In the next lesson we look at why every transaction costs a small fee, how that fee is calculated, and why those fees can spike during busy periods.