A private key is a secret 256-bit number that authorises spending from one crypto address; a seed phrase is a list of 12 to 24 words that encodes the randomness from which a wallet derives all its private keys. Anyone holding the seed phrase controls every address in the wallet, and losing every copy means losing access permanently.
What is a private key?
A private key is a randomly generated number, usually 256 bits long. On Ethereum it is written as 64 hexadecimal characters. From it, the wallet calculates a public key using elliptic-curve cryptography (Bitcoin and Ethereum both use the secp256k1 curve), and from the public key it derives your address. On Ethereum, the address is the last 20 bytes of the Keccak-256 hash of the public key, shown with a 0x prefix.
Each step works in one direction only. Anyone can check that a signature came from the key behind an address, but nobody can work backwards from the address to the key. That is how you prove ownership without revealing the secret. As the US SEC's custody bulletin notes, a private key cannot be changed or replaced: lose it and you lose access; expose it and someone else gains access.
What is a seed phrase?
Writing down a separate private key for every address would be impractical, so most wallets follow the BIP-39 standard. The wallet generates 128 to 256 bits of randomness, adds a short checksum taken from its SHA-256 hash, splits the result into 11-bit chunks, and maps each chunk to a word from a fixed list of 2,048. The checksum lets wallets catch most typos when you restore.
| Randomness (bits) | Checksum (bits) | Words in the phrase |
|---|---|---|
| 128 | 4 | 12 |
| 160 | 5 | 15 |
| 192 | 6 | 18 |
| 224 | 7 | 21 |
| 256 | 8 | 24 |
To use the phrase, the wallet runs it through a key-stretching function (PBKDF2 with HMAC-SHA512, 2,048 rounds) to produce a 512-bit seed. If you add an optional passphrase, it is mixed in at this step, creating an entirely different wallet from the same words.
How does one seed phrase create many private keys?
The 512-bit seed feeds a hierarchical deterministic (HD) wallet, defined in BIP-32. It produces a master key and from it a tree of child keys, always in the same order. BIP-44 standardises the branches as m / purpose' / coin_type' / account' / change / address_index, where coin type 0 is Bitcoin and 60 is Ether. The first Ethereum address in most wallets therefore sits at m/44'/60'/0'/0/0.
Because derivation is deterministic, the same phrase rebuilds the same addresses in any compatible wallet. Wallets do not always use identical paths, though, so if a restored wallet looks empty, check that it is scanning the right network and derivation path before assuming funds are gone. For how wallets use these keys day to day, see how crypto wallets work.
Private key vs seed phrase: what is the difference?
| Question | Private key | Seed phrase |
|---|---|---|
| What it controls | One address | Every address the wallet can derive |
| Format | A 256-bit number, often shown as 64 hexadecimal characters | 12 to 24 words from a fixed list of 2,048 |
| Who sees it | Usually nobody; the wallet uses it internally | You, once, when you write it down |
| Role in backup | Can be exported for a single account | The master backup for the whole wallet |
| If someone else gets it | They can empty that address | They can empty the entire wallet |
How are seed phrases actually stolen?
Guessing is not a realistic threat: a 12-word phrase has 2 to the power of 128 possible combinations. Seed phrases are stolen in more ordinary ways:
- Typing the phrase into a fake wallet, "sync" or airdrop website; see how to recognise crypto phishing.
- Sharing it with a fake support agent who contacted you first.
- Photos, screenshots and notes that sync to cloud accounts.
- Malware on the device where the phrase was typed or displayed.
- Using a phrase you did not generate yourself, such as one printed on a card in a hardware wallet box.
- Physical theft of the written backup.
To limit the damage if a written backup is stolen, some people add a BIP-39 passphrase or use a multi-signature wallet. Practical storage options, from metal plates to splitting backups, are covered in how to store a recovery phrase, and creating a phrase correctly is covered in how to set up a crypto wallet.
Keys and Wealtii: Wealtii accounts are not controlled by a seed phrase. You sign in with your email and password plus an email one-time code, and you can add authenticator-app two-factor authentication. Fund assets are held in a Gnosis Safe multi-signature vault on BNB Smart Chain that needs all three of its three signatures to move funds, so you rely on Wealtii rather than on keys you hold yourself. Wealtii will never ask for a seed phrase or private key. Wealtii is not regulated.
Frequently Asked Questions
Is a seed phrase the same as a private key?
No. A private key controls a single address. A seed phrase is a human-readable backup of the randomness from which a wallet derives all of its private keys, so it controls every address in the wallet, across every network the wallet supports.
Can I change my seed phrase?
Not for an existing wallet. If you think your seed phrase has been exposed, create a new wallet with a new phrase and move all funds to it straight away, because the old phrase will always control the old addresses.
What is a passphrase or 25th word?
It is an optional extra word or sentence, defined in BIP-39, that is combined with the seed phrase to create a completely different wallet. It protects you if someone finds your written seed phrase, but if you forget the passphrase, the funds in that wallet cannot be recovered.
Can a seed phrase be guessed?
Not in practice. A 12-word phrase encodes 128 bits of randomness, about 3.4 × 10^38 possible combinations. Seed phrases are lost through theft, photos, cloud backups, phishing sites and fake support agents, not through guessing.
📚 Sources & References
- BIP-39 — Mnemonic code for generating deterministic keys (accessed 21 Oct 2026)
- BIP-32 — Hierarchical Deterministic Wallets (accessed 21 Oct 2026)
- BIP-44 — Multi-Account Hierarchy for Deterministic Wallets (accessed 21 Oct 2026)
- ethereum.org — Ethereum accounts (private keys, public keys and addresses) (accessed 21 Oct 2026)
- U.S. SEC, Investor.gov — Crypto Asset Custody Basics for Retail Investors (12 Dec 2025) (accessed 21 Oct 2026)
Disclaimer: This content is for educational purposes only and does not constitute financial, tax, or legal advice. Digital assets are volatile and you can lose some or all of the money you invest. Never share your private keys or seed phrase with anyone, including people claiming to be support staff. Wealtii is not registered or licensed with any financial regulator; see our Regulatory Status page.
