A crypto wallet does not hold your coins. The coins are recorded on the blockchain. The wallet holds the private keys that can move them. The Bitcoin developer documentation puts it simply: "Bitcoin wallets at their core are a collection of private keys." Whoever controls those keys controls the funds. So the real questions are where the keys are stored, who holds them, and what happens if they are lost or stolen.
What the wallet actually does
A wallet app does three jobs:
- Stores or derives private keys. These keys sign the transactions that move funds from your addresses.
- Shows balances and history. It reads them from the blockchain.
- Builds and signs transactions. It then broadcasts them to the network.
ethereum.org describes wallets as "applications that give you control over your account" and notes that wallet providers "don't have custody of your funds". This applies to non-custodial wallets, where only you hold the keys.
The seed phrase is the whole wallet
Most modern wallets are hierarchical deterministic (HD) wallets. The Bitcoin developer guide explains that "the HD protocol uses a single root seed to create a hierarchy of child, grandchild, and other descended keys". That root seed is "the only data the user needs to backup in order to derive every key" a wallet program creates with given settings.
Most wallets show that seed to you as a mnemonic, or seed phrase, using the BIP-39 standard:
- Length. A BIP-39 phrase has 12, 15, 18, 21 or 24 words. They encode 128 to 256 bits of entropy plus a checksum.
- Optional passphrase. BIP-39 allows an extra passphrase. If none is set, an empty string is used. A different passphrase produces a completely different set of keys.
- How the seed is made. The phrase is turned into a 512-bit seed with PBKDF2, using HMAC-SHA512 and 2,048 iterations.
What this means in practice:
- Anyone with your phrase (and passphrase, if you set one) has full control of every address the wallet derives. This holds on any compatible software, anywhere.
- No real service will ask for it. ethereum.org states: "No legitimate service, support agent, or website will ever ask you for these." A request for your seed phrase is a theft attempt.
- Do not photograph it. ethereum.org warns that screenshots of seed phrases or private keys "might sync them to a cloud data provider".
- Note your wallet settings with the backup. The Bitcoin developer guide warns that HD wallet programs are not expected to be fully compatible with each other. Restoring a phrase in a different app may not show all your addresses without the same derivation settings.
Where should your keys live?
Answer a few questions about who holds your keys and how you use your crypto to see which risks from this guide apply to you.
Who holds the private keys that can move your crypto?
- A trading platform or another third-party custodian → next question
- I do, in my own wallet → next question
Do you know what happens if the custodian fails, how it safeguards keys, and whether it can lend or pool your assets without your consent?
- Yes, I have checked those points → You hold a claim on the custodian
- No, or not sure → Ask the custodian first
How do you use this balance?
- A small balance I move often → The common pattern: a hot wallet
- A larger holding I keep for the long term → next question
Is your seed phrase written down offline, kept with a note of your wallet settings, and tested with a restore of a small amount?
- Yes, all three → The common pattern: cold storage
- No, or only some of it → Fix the backup before relying on it
The firm holds the keys, so a hack, shutdown or bankruptcy can cut off your access. Crypto accounts have no government backing, so the government has no obligation to help you get your money back.
The SEC bulletin lists questions to ask: are the assets insured, what happens if the custodian fails, how are keys safeguarded, is your consent needed before assets are lent or pooled, how is your data handled, and what are the full fees.
Keys on a connected device give fast access but are exposed to malware, phishing and compromised devices. Turn on two-factor authentication where offered, never type your seed phrase into a website, and never screenshot it.
With a hardware wallet the private key stays on the device. The main risk moves to loss, damage or theft of the device or backup. Check every address and amount on the device's own screen before you approve.
The seed phrase is the whole wallet: whoever has it controls every address it derives, and if you lose it no one can recover it. Write it on paper, not in a photo. Note your wallet settings, because another app may not show all addresses without them. Test a restore with a small amount.
This sorts the trade-offs described in the article; it does not judge any wallet product, custodian or investment, and details differ by wallet software and blockchain.
Hot vs cold wallets
The difference is whether the keys sit on a device connected to the internet.
| Hot wallet | Cold wallet | |
|---|---|---|
| Where keys live | Phone app, desktop app, browser extension, web app | Hardware device or an offline computer |
| Main strength | Fast access for frequent transactions | Keys never touch the internet |
| Main risk | Malware, phishing, compromised device | Loss, damage or theft of the device or backup |
The SEC's December 2025 investor bulletin on crypto custody says hot wallets are convenient but expose assets to cyberthreats. It says cold wallets are generally more secure against those threats, but a lost, damaged or stolen device can mean permanent loss.
How cold signing works
The Bitcoin developer guide describes an offline wallet as a signing-only setup "intended to be run on a device which does not connect to any network". The process:
- An online wallet prepares an unsigned transaction.
- The offline device signs it.
- The online wallet broadcasts the signed transaction.
The documented benefit: if the offline device is not compromised and you check each outgoing transaction, your funds stay safe even if the online wallet is compromised.
A hardware wallet automates this. It is a dedicated signing-only device. You confirm the transaction on its own screen, and it returns the signature to the connected wallet. ethereum.org says that with hardware wallets "your private key never touches the internet and stays completely local on your device."
Paper backups
A seed phrase written on paper is a form of cold storage. It cannot be hacked remotely. It can be destroyed, lost, read by someone else, or copied wrong. A single wrong word makes the backup useless, although the BIP-39 checksum catches many copying errors.
Custodial wallets: someone else holds the keys
When you keep crypto on a trading platform or with another third-party custodian, that firm holds the keys. You hold a claim on the firm.
The SEC bulletin's main points on third-party custody:
- Failure risk. If the custodian is hacked, shuts down or goes bankrupt, you may lose access to your crypto assets.
- Lending and pooling. Some custodians may lend out or rehypothecate deposited assets, or commingle customer assets.
- Questions to ask a custodian:
- Are the assets insured? - What happens if the custodian fails? - How are keys safeguarded? - Is your consent needed before your assets are lent or pooled? - How is your personal data handled? - What are the full fees?
The FTC warns that cryptocurrency accounts are not backed by a government. If a storage provider fails or is hacked, "the government has no obligation to step in and help get your money back."
With self-custody, the SEC bulletin notes, you have full control and full responsibility. No custodian can freeze or lose your funds. If you lose your keys and seed phrase, no one can recover them for you.
Choosing a setup
There is no single right answer, but common patterns follow from the trade-offs above:
- Small, active balances. A hot wallet or a custodial account, with two-factor authentication turned on.
- Larger, long-term holdings. A hardware wallet, with the seed phrase backed up offline in a secure place.
- Any setup.
- Never type your seed phrase into a website or share it with "support". - Check every address and amount on the signing device's screen before you approve. - Test a restore with a small amount before relying on a backup.
Limitations: this is a general explainer, not advice on any product or investment. Details differ by wallet software and blockchain.
