Wallets, Private Keys and Seed Phrases

What you will learn

• What a wallet, public address, private key and seed phrase are
• The difference between custodial storage and self-custody
• How hardware wallets work and how to back up a seed phrase
• Why the main risk is human error, not code breaking

Explanation

Your bitcoin are not stored "in" a wallet. They are recorded on the blockchain. A wallet is software or a device that stores the keys you need to spend them.

• A private key is a secret number that lets you sign transactions. Whoever has it controls the coins.
• A public address is derived from your keys. You can share it to receive bitcoin, like an account number.
• A seed phrase (also called recovery phrase) is a list of usually 12 or 24 words from a standard list of 2,048 words. It can recreate all private keys of a wallet. If someone sees it, they can take everything.

There are two main ways to hold bitcoin:

• Custodial: a provider such as an exchange holds the keys for you. It is simple, and you can often reset your password. But you depend on the provider's security, solvency and rules. If the provider fails or freezes withdrawals, you may lose access. In the EU, check that the provider is authorised under MiCA (lesson 5).
• Self-custody: you hold the keys yourself. No company can freeze your coins, but no one can help you if you lose the seed phrase or are tricked into revealing it.

A hot wallet is an app on an internet-connected phone or computer. A hardware wallet is a small device that keeps private keys offline and signs transactions inside the device, so malware on your computer cannot easily read the keys. Buy hardware wallets only new and directly from the manufacturer or an authorised seller, and check that the device generates a new seed phrase itself. Never use a seed phrase that came printed in the box.

Backup rules: write the seed phrase on paper or metal, never in a photo, cloud note, email or chat. Store it in a safe place protected from fire and water. Some people keep a second copy at another secure location. Test recovery with a small amount before storing large sums.

Worked example

Hypothetical example: why guessing a seed is hopeless, and where the real risk is.

Assumptions: a 12-word seed phrase contains 128 bits of randomness. An attacker can test 1 trillion (1,000,000,000,000) seeds per second.

1. Number of possible seeds: 2^128 ≈ 3.4 × 10^38.
2. Seconds to try all: 3.4 × 10^38 ÷ 10^12 = 3.4 × 10^26 seconds.
3. Seconds per year: about 31,560,000 (3.156 × 10^7).
4. Years needed: 3.4 × 10^26 ÷ 3.156 × 10^7 ≈ 1.1 × 10^19 years.

Conclusion: attackers do not guess seeds. They steal them: through fake wallet apps, fake support agents, phishing websites or photos of the seed in a cloud account. A seed generated with true randomness is safe from guessing; a seed you share is lost at once.

Common mistake

Practice activity

Write a one-page storage plan for a hypothetical holding. Answer: (1) custodial or self-custody, and why; (2) where the seed phrase is stored; (3) how you will test recovery; (4) who could access the coins if something happens to you.

Limitations: inheritance and very large holdings may need more advanced setups and legal advice.

Key takeaways

• Whoever controls the private keys or seed phrase controls the bitcoin.
• Custodial storage depends on a provider; self-custody depends entirely on you.
• Hardware wallets keep keys offline; buy them new from trusted sources.
• Never share or digitise your seed phrase, and test your recovery with a small amount.

Next step

In the next lesson, you will learn how buying works on an exchange and how to calculate the full cost including spreads and fees.