How a hardware wallet works
The device creates your private keys and keeps them inside. A wallet app prepares the transaction, the device shows its details, and you approve it with a button.[1]
MetaMask's help center describes the device as a firewall between attackers and the wallet: even if someone gains access to your MetaMask, they cannot move assets without physical access to the device to approve each transaction.[2]
ethereum.org's security guide makes the same point about the keys: they stay local on the device and never touch the internet, which reduces the risk of theft even if a hacker controls your computer.[3]
Using it with a wallet app
Hardware wallets connect to wallet apps such as MetaMask, which act as the interface while signing happens on the device.
MetaMask supports two connection types. Ledger, Trezor and Lattice connect directly by USB. Air-gapped devices such as Keystone, NGRAVE ZERO, AirGap Vault and OneKey pass transactions back and forth as QR codes, so the device is never plugged in.[2]
Mobile support is narrower. MetaMask lists Keystone, Ledger and NGRAVE ZERO for MetaMask Mobile. It also warns that Firefox 112 and later changed the connection method Ledger relied on, so Ledger users may need a Chromium-based browser.[2]
BIP-44 paths take the form m/44'/60'/account'/change/address_index for Ethereum, where 44 is the purpose field and 60 is the coin type SLIP-44 assigns to Ether. With account 0 and change 0, the first address is m/44'/60'/0'/0/0 and the next is .../0/1.[5]
Risks
A hardware wallet cannot protect you from approving a malicious transaction. Keep the seed phrase backup offline.
How the seed phrase backs up the device
The recovery words follow BIP-39. The device generates 128 to 256 bits of random entropy, adds a checksum of entropy ÷ 32 bits, and splits the result into 11-bit groups. Each group is a number from 0 to 2047 that points to one word in a 2,048-word list.[4]
Worked example with the BIP-39 table: 128 bits of entropy + 4 checksum bits = 132 bits, and 132 ÷ 11 = 12 words. 256 bits + 8 checksum bits = 264 bits, and 264 ÷ 11 = 24 words.[4]
Many devices offer an optional passphrase on top of the words. BIP-39 feeds the words and the string "mnemonic" plus the passphrase into PBKDF2 with 2,048 rounds of HMAC-SHA512 to produce a 512-bit seed. Every passphrase produces a valid but different wallet, so a forgotten or mistyped passphrase opens an empty wallet, not an error.[4]
ethereum.org warns against taking screenshots of the phrase, because photos can sync to a cloud provider where attackers can reach them.[3]
What to check on the device screen
For a transaction, compare the recipient address, amount and network shown on the device with what you meant to do. ethereum.org's security guide lists sending to the wrong address as a common mistake.[3]
For signed messages, EIP-712 typed data includes a domain with the app's name, a version, a chainId and the verifying contract address. The wallet should refuse to sign if the chainId does not match the active chain, so a mismatch there is a warning sign.[7]
Token approvals deserve extra care. ethereum.org advises against unlimited spend limits, because an unlimited approval can let a contract drain that token from your wallet. A hardware wallet signs an unlimited approval as readily as a limited one.[3]
Common mistakes
Restoring the phrase in another app and seeing an empty account can mean a different derivation path or passphrase, not lost funds. Check that the app uses m/44'/60'/0'/0/x and the same passphrase.[5]
Storing the recovery words next to the device defeats the purpose: anyone who finds both has everything they need.
The Bottom Line
A hardware wallet is the safest everyday way to hold larger amounts in self-custody, as long as you read what you sign.
Educational content, not financial advice.