You can share your public key, or the address made from it, so people can send you crypto and check your signatures. Keep your private key, and the seed phrase that encodes it, to yourself. The private key signs transactions, and anyone who has it can spend your funds. The two keys are made as a matched pair. You can get the public key from the private key, but you can't practically work backwards from the public key to the private key. That one-way link is why one key can be public and the other has to stay secret.
How the key pair works
The ssh.com Academy puts it this way: "Public keys and private keys come in pairs." A private key "is a key used for digitally signing documents," and a public key "can be used for verifying digital signatures generated using a corresponding private key."
The link only works in one direction. ssh.com says the public key "can be easily derived from the private key" but "the private key cannot be practically derived from the public key." CertiK's explainer says the same: "a public key can be derived from a private key, but not the other way round."
What the keys are in Bitcoin and Ethereum
- Bitcoin. The Bitcoin developer guide says Bitcoin uses ECDSA with the secp256k1 curve. It says "secp256k1 private keys are 256 bits of random data." That data is "deterministically transformed" into a public key, so a wallet can always rebuild the public key and doesn't need to store it.
- Ethereum. ethereum.org says a private key "is made up of 64 hex characters," and the public key "is generated from the private key using the Elliptic Curve Digital Signature Algorithm."
From private key to address: three steps
A wallet address is usually not the public key itself. It comes from hashing the public key, so the chain runs private key → public key → address, and each step only works forward.
- Bitcoin: According to the developer guide, the public key "is then cryptographically hashed," and this "shortens and obfuscates the public key." It also says hashing adds "security against unanticipated problems which might allow reconstruction of private keys from public key data."
- Ethereum: ethereum.org says the address is made by "taking the last 20 bytes of the Keccak-256 hash of the public key and adding 0x to the beginning," which gives a 42-character address.
When the public key becomes visible
In the standard Bitcoin payment the developer guide describes, only the hash of the public key is shared when you receive funds. The full public key appears on-chain when you spend. The spending transaction has to include "his full (unhashed) public key" along with a signature.
How signing a spend works
To spend, the wallet makes a signature from the transaction data and the private key. In the Bitcoin guide's words, the signature is "made by using the ECDSA cryptographic formula to combine certain transaction data... with Bob's private key." The network checks it against the public key. That proves the spender "owns the private key which created the public key," and the private key is never revealed.
CertiK says this is why public and private keys let users "transact without the need for a third party to verify the transaction."
Seed phrases: the master copy of your keys
Most wallets show a list of words instead of a raw private key. CertiK calls a seed phrase "a sequence of words that encodes your private keys."
BIP39, the Bitcoin Improvement Proposal titled "Mnemonic code for generating deterministic keys," explains how this works. The wallet starts with 128 to 256 bits of random data and turns it into 12, 15, 18, 21 or 24 words. It then runs those words through PBKDF2 with HMAC-SHA512 and 2,048 iterations to get the seed. The salt is "mnemonic" plus an optional passphrase. All of the wallet's keys come from that seed. Anyone who has the words can rebuild every key, so the seed phrase needs the same protection as a private key.
Public key vs private key
How do the public key and the private key differ on their job, who may see them, where they come from, what happens if they leak and what each one produces?
Each cell restates one explainer's general description; the table does not show how any particular wallet or platform handles keys.
In public-key cryptosystems, a private key is a key used for digitally signing documents.
Public is not always harmless: extended public keys
Sharing one public key or address doesn't let anyone spend your funds. Wallets built on BIP32 (hierarchical deterministic wallets) also have an extended public key, often shown as an "xpub." It can generate a whole tree of child public keys. BIP32 says this is useful because "one can derive child public keys of a given parent key without knowing any private key."
BIP32 also gives a warning: "knowledge of a parent extended public key plus any non-hardened private key descending from it is equivalent to knowing the parent extended private key." It concludes that "extended public keys must be treated more carefully than regular public keys." BIP32 uses hardened derivation to contain this risk, so that "a leak of account-specific (or below) private keys never risks compromising the master or other accounts."
In practice, an xpub can't move funds on its own. It does let whoever holds it see all the addresses it generates. If it leaks along with a single child private key, it can expose much more.
Who is responsible for the private key
With a non-custodial wallet, CertiK says, "users are responsible for their private keys." Coin Bureau's explainer (last updated November 26, 2024) describes what's at stake: "If a private key is lost or exposed, the funds in the associated wallet could be permanently lost or stolen." It says the reason is that "blockchain transactions are irreversible." Its advice is to store private keys offline in hardware wallets, away from online threats.
The signature system removes the middleman, so there's no one who can reverse a mistake.
How keys actually get stolen
The attacks these sources describe target people, not the math. CertiK says hackers "will try and gain access to your private key, often in the form of phishing attacks." It calls one rule foundational to web3 security: "never share your private key or seed phrase with anyone." It adds that "legitimate institutions or projects will never ask you to share this information."
Coin Bureau says a public key is "safe to share" and "poses no direct security risk." It still advises sharing it only on trusted platforms and avoiding suspicious links, because phishing often starts there.
A short checklist
- Share your address to receive funds. Share an xpub only with software you trust, because it reveals your full address history.
- Never type a private key or seed phrase into a website, chat or form. Treat any request for one as an attack.
- Keep the seed phrase offline. Losing it can mean losing the funds for good, and leaking it can mean having them stolen.
A longer-term question: quantum computing
The one-way link depends on problems that today's computers can't solve in practice. On August 13, 2024, the US National Institute of Standards and Technology (NIST) published its first finalized post-quantum standards. FIPS 204 (ML-DSA) and FIPS 205 (SLH-DSA) cover digital signatures. NIST says a quantum computer could "break the current encryption that provides security and privacy for just about everything we do online." When such a machine might exist, and how blockchains would move to new signature schemes, is still open. Neither question is settled by the sources here.
Next step: check an address
Funds arrive at your address. To see the transactions tied to it, look it up in a blockchain explorer. A separate guide covers how to use a blockchain explorer to check a transfer.
Limitations: this is a general description of standard Bitcoin and Ethereum key handling, and individual wallets may store, derive or display keys differently.
