What are private keys and public keys?
Bitcoin uses a system called public-key cryptography to secure ownership of funds. Understanding the difference between your private key and your public key is fundamental, it determines who controls your Bitcoin and how you protect it.
The public key: your address
A Bitcoin public key is a cryptographically derived identifier that others can use to send you Bitcoin. It's the equivalent of your bank account number, safe to share, and mathematically impossible to reverse-engineer into something that grants access to your funds.
When someone says "send Bitcoin to this address," they're giving you a form of their public key.
The private key: your master password
Your private key is a secret number, 256 bits of randomness, that proves you own the Bitcoin associated with your public key. Anyone who has your private key has complete, unconditional control of your Bitcoin. They can send it anywhere instantly, and transactions are irreversible.
There is no "forgot my password" link. There is no customer service to call. This is why the Bitcoin community says:
"Not your keys, not your coins."
The seed phrase
Most wallets represent your private key as a 12- or 24-word recovery phrase (also called a seed phrase or mnemonic). This word list is generated from a standardized word list and can recreate your private key on any compatible wallet.
If your phone breaks, your hardware wallet is lost, or the app stops working, your seed phrase is the backup that recovers everything. Without it, the Bitcoin in that wallet is permanently inaccessible.
Best practices for key security
- Never store your seed phrase digitally, no photos, no cloud notes, no email drafts
- Write it on paper, in multiple copies, stored in separate physical locations
- Never share it with anyone, including anyone claiming to be from Astra support
- Consider a hardware wallet for larger amounts (keeps keys offline)
How Astra fits in
When you first accumulate Bitcoin through Astra, it lives in your Astra account, which uses Coinbase custody on your behalf. You don't manage keys yet. As your holdings grow and you feel ready, you can move your Bitcoin into self-custody using your own wallet and private key. Astra fully supports that transition.