How a private key works
On Ethereum and other EVM networks, a private key is a 256-bit number. The public key and the address are calculated from it, but you cannot go back from the address to the key. Your wallet uses the key to sign each transaction.[1]
Written out, an Ethereum private key is 64 hexadecimal characters. The address is the last 20 bytes of the Keccak-256 hash of the public key, shown as 0x plus 40 hex characters, 42 characters in total. The public key comes from the private key through elliptic curve cryptography (ECDSA).[1]
Only externally owned accounts have private keys. A contract account is controlled by its code and has no private key, so there is no key to export for a smart contract address.[1]
How a private key is used
You rarely see it: the wallet keeps it encrypted and signs for you. Some wallets let you export it to move a single address to another app.
Each signed transaction includes the account nonce, a counter that goes up by one with every transaction. Because of the nonce, a signature for one transaction cannot be replayed to send the same payment twice.[1]
The signature also commits to a chain ID under EIP-155, where Ethereum mainnet is chain ID 1. A transaction signed for mainnet is not valid on a testnet or another chain, even though the same private key controls the same address on all EVM networks.[4]
Risks and scams
ethereum.org says no legitimate service, support agent or website will ever ask for your private key. Once someone has the key, they can move the funds, and a transaction sent on Ethereum is irreversible.[7]
Private key vs seed phrase
A seed phrase can regenerate every private key in a wallet; a private key controls only its own address.
Example: where a wallet's keys come from
Illustrative example: an HD wallet does not store one random key per address. It keeps a master key and derives child keys along a path. For Ethereum the common path is m/44'/60'/0'/0/0, where 44' marks the BIP-44 layout and the apostrophe means hardened derivation.[5]
The 60' is the coin type registered for Ether in SLIP-0044; Bitcoin uses 0' and all testnets share 1'. Changing the last number gives the next address: m/44'/60'/0'/0/1 is the second address on that path. Each of those addresses has its own private key.[6]
BIP-32 warns that a parent extended public key plus any non-hardened child private key is enough to rebuild the parent extended private key. Exporting one non-hardened child key can therefore expose every key on that branch if the extended public key is also known.[2]
How to check it yourself
To confirm which address a key controls, import it into a trusted wallet app and compare the address it shows with the one on a block explorer. Never paste a real key into a website to check it; ethereum.org lists sites and messages that ask for keys among the common phishing patterns.[7]
Do not screenshot a key or save it in a notes app. Screenshots can sync to cloud storage, where an attacker who gets into the account can read them. A hardware wallet keeps the key on the device, so it never touches the internet.[7]
Common mistakes
Sharing a key with someone who says they are from Ethereum support. ethereum.org warns against replying to messages from "official Ethereum support" and describes support scammers who find targets in public Discord channels.[7]
Sending new funds to an address after its key leaked. A leaked key cannot be rotated, so new deposits are exposed too. The FTC notes crypto payments are typically not reversible and crypto held in accounts is not insured by the government.[3]
The Bottom Line
Your private key is your ownership of an address. Keep it inside your wallet, and if you ever export it, treat it like cash.
Educational content, not financial advice.