01. ETH.BUILD: Key Pair
Overview
Section titled “Overview”An Ethereum account is not a row in a database that some company controls. It is a key pair — a secret number and a public identifier derived from it — that you, and only you, hold. This is the single most important idea in the whole course: on Ethereum, control of the key is ownership of the account. There is no support line that can reset it and no server that stores it. Everything about wallets, signing, and self-custody in later lessons follows from this one fact.
Key concepts
Section titled “Key concepts”- Private key — a 256-bit random number (effectively an unguessably large integer). Whoever knows it can sign transactions for the account and move its funds. It must stay secret, forever.
- Public key — derived from the private key by elliptic-curve multiplication on the
secp256k1curve. The math runs one way only: you can compute the public key from the private key, but you cannot reverse it to recover the private key. - Address — what you share and what shows up in a block explorer. It is the last 20
bytes of the
keccak256hash of the public key, written as0x…(40 hex characters). - Signing and verifying — to send a transaction, your private key produces a signature over it. Anyone on the network can check that the signature matches your address, without ever seeing the private key. This replaces the username/password model entirely: there is no password because you prove identity by signing.
- Seed phrase (mnemonic) — a human-writable list of 12 or 24 words (the BIP-39 standard) that a wallet turns into your private key(s). It is a backup of the key, so it carries exactly the same secrecy requirement as the key itself.
The derivation only flows in one direction: private key → public key → address. You can always go left-to-right, never right-to-left.
Common pitfalls
Section titled “Common pitfalls”- Treating an address and a private key as interchangeable. The address is public and safe to share; the private key and seed phrase never are.
- Pasting a private key or seed phrase into a website, a chat, a screenshot, or a git commit. Any of these is equivalent to handing over the account.
- Expecting a “forgot password” flow. Lose the key with no backup and the funds are gone; leak the key and they can be taken. There is no in-between.
Check yourself
Section titled “Check yourself”- Which of the three — private key, public key, address — can you safely post in public?
- Can someone compute your private key if they know your address? Why not?
- If you lose your only copy of a seed phrase, what happens to the account, and why can no one restore it for you?
Watch the Video
Section titled “Watch the Video”
ETH.BUILD: Key Pair
Video credit: Austin Griffith · Duration: 6 minutes