If you've used a decentralised application, you've probably clicked "approve" before doing a swap or a deposit. That approval gives a smart contract permission to move a specific token from your wallet. Approvals are a normal part of how tokens work, but old or unlimited approvals are one of the most common ways wallets get drained.

Why approvals exist

Tokens like USDT and USDC follow the ERC-20 standard, which includes two functions that work together:

• approve(spender, amount): you allow a spender (usually a contract) to move up to a set amount of your tokens
• transferFrom(owner, to, amount): the spender moves tokens within that allowance

This lets applications pull tokens when you ask them to, such as when a swap takes your USDC and gives you another token in return.

Limited vs unlimited approvals

Many applications ask for an unlimited approval by default, so you don't have to approve again each time. It's convenient, but it means that if the contract is malicious or later compromised, it can take your entire balance of that token at any time, without asking again.

Many wallets let you edit the approval amount before confirming. If you only plan to use an application once, approve only what you need.

How drains happen

• Phishing sites imitate a real application and ask you to approve a malicious contract.
• Signature tricks use "permit" style signatures that grant approvals without a normal transaction, so they can look like a harmless login.
• Compromised contracts: an application you once trusted is exploited, and every outstanding approval becomes a risk.

In each case, the thief doesn't need your seed phrase. They only need an approval you already gave.

Checking your approvals

Block explorers and approval-checking tools show which contracts can spend your tokens. Tools such as Revoke.cash let you connect a wallet, list its approvals by network, and revoke them. Some explorers have a similar "token approvals" page. Look for:

• Contracts you don't recognise
• Unlimited approvals to applications you no longer use
• Approvals on networks you rarely use

Revoking

Revoking is itself a transaction: you set the approval to zero, and it costs gas.

1. Connect the wallet on the correct network.
2. Review each approval and the spender address.
3. Revoke the ones you don't need.
4. Confirm each revoke transaction in your wallet.

Prioritise large balances and unfamiliar spenders.

Habits that keep you safe

• Read the wallet prompt. "Approve" or "Permit" means you're granting spending rights, not just logging in.
• Bookmark real sites rather than following links from messages or search ads.
• Use a separate wallet with a small balance for trying new applications.
• Revoke after one-off use.
• Remember that a plain transfer needs no approval. Sending tokens to a deposit address is a normal transfer. If a "deposit" asks you to approve a contract, stop and check.

The short version

Approvals let contracts move your tokens. Unlimited approvals to the wrong contract can empty a wallet without your seed phrase. Approve only what you need, check your approvals now and then, and revoke what you don't use.

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Rollups are the most common kind of layer 2 network on Ethereum. They bundle many transactions together, process them off the main chain, and post the results back to Ethereum. There are two main families, optimistic and zero-knowledge, and they differ in how they prove that the results are correct.

Why rollups exist

Ethereum's block space is limited, so fees rise when demand is high. Rollups move execution elsewhere and use Ethereum mainly for data and settlement. ethereum.org's layer 2 overview explains that this lets them offer lower fees while relying on Ethereum's security.

Optimistic rollups

Optimistic rollups assume transactions are valid by default and post them to Ethereum. There's a challenge window during which anyone can submit a "fraud proof" showing an invalid result. If nobody does, the result stands.

• Example: Arbitrum One.
• Everyday experience: fast confirmations from the sequencer, with low fees.
• Trade-off: withdrawing directly to Ethereum through the official bridge can take days because of the challenge window. Third-party services can move funds faster, with their own risks and fees.

Arbitrum's finality documentation separates soft finality (sequencer feed, under a second) from hard finality (once data is final on Ethereum).

Zero-knowledge (ZK) rollups

ZK rollups generate a cryptographic validity proof for each batch, showing the new state is correct. Ethereum verifies the proof, so no challenge window is needed.

• Everyday experience: low fees; withdrawals to Ethereum can settle once proofs are verified.
• Trade-off: generating proofs is computationally heavy and the technology is more complex.

Side by side

The sequencer

Most rollups today use a sequencer to order transactions. It gives you fast confirmations, but it's often run by a single operator. If it goes down, transactions can be delayed. Many rollups have mechanisms that let users force transactions onto Ethereum if the sequencer misbehaves, but these are slower.

What it means for everyday transfers

• Fees include Ethereum data costs, so they can rise when Ethereum is busy.
• Gas is usually paid in ETH on Ethereum rollups.
• Each rollup is its own network with its own token contracts, so match the network when sending.
• Bridging out to Ethereum or another chain takes extra steps; it's often simpler to use a service that supports the rollup directly.

Questions to ask about any rollup

1. Is it optimistic or ZK?
2. Who runs the sequencer, and what happens if it stops?
3. How long does a withdrawal to Ethereum take through the official bridge?
4. Do the services you use support this network for both deposits and withdrawals?

Key terms

• Batch: a group of rollup transactions posted together.
• Fraud proof: evidence that an optimistic rollup batch is invalid.
• Validity proof: a cryptographic proof that a ZK rollup batch is correct.
• Challenge window: the period in which fraud proofs can be submitted.
• Sequencer: the operator that orders transactions on the rollup.

The short version

Rollups make Ethereum transactions cheaper by processing them off-chain. Optimistic rollups assume correctness and allow challenges; ZK rollups prove correctness upfront. For everyday use, both are fast and cheap. The main practical issues are matching networks and knowing how long it takes to move funds back to Ethereum.

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When you press "send" in a crypto wallet, your transaction doesn't go straight into a block. First it waits in a holding area called the mempool (memory pool). Understanding the mempool explains why transactions sometimes sit as "pending" and how fees influence how quickly they confirm.

What happens after you press send

1. Your wallet signs the transaction with your private key.
2. It broadcasts the transaction to one or more nodes on the network.
3. Nodes check it (valid signature, enough balance and fee) and pass it along to other nodes. ethereum.org's networking layer docs describe how nodes gossip transactions to one another.
4. It sits in each node's mempool, waiting to be included in a block.
5. A block producer selects transactions from its mempool, usually favouring higher fees, and includes them in a block.

While your transaction is in step 4, your wallet shows it as pending.

Why some transactions wait longer

• Fee too low. When the network is busy, block producers prioritise higher-paying transactions.
• Congestion. More transactions than block space means a queue.
• Nonce gap. On Ethereum-style networks, transactions from one address must be processed in order. If an earlier one is stuck, later ones wait.
• Dropped transactions. If a transaction waits too long, some nodes may drop it from their mempool, and it may need to be re-sent.

Fees and priority

ethereum.org's gas documentation explains that Ethereum fees include a base fee set by the network and a priority fee (tip) for the validator. Raising the tip can help a transaction get picked up sooner during busy periods. Many wallets set this automatically.

Speeding up or cancelling

Most wallets let you:

• Speed up: re-send the same transaction (same nonce) with a higher fee. The faster one wins.
• Cancel: send a zero-value transaction to yourself with the same nonce and a higher fee, replacing the original.

Both only work while the original is still pending. Once a transaction is in a block, it can't be cancelled.

Privacy and front-running

The public mempool is visible to anyone running a node. On some networks, automated bots watch it for profitable trades and try to get ahead of them ("front-running"). This mostly affects trades on decentralised exchanges, not simple transfers between addresses.

Practical tips

1. Use your wallet's suggested fee unless you understand the trade-off.
2. Don't send the same payment again just because it's pending; you may end up paying twice.
3. Look up the transaction ID on a block explorer to see its real status.
4. Resolve stuck transactions first before sending new ones from the same address.

The short version

The mempool is the waiting room for transactions before they're added to a block. Fees decide priority when the network is busy. If something's pending, check the explorer, use speed-up or cancel in the same wallet, and avoid resending payments blindly.

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Every crypto wallet rests on a pair of related numbers: a private key and a public key. From the public key comes the address you share to receive funds. Understanding how the three fit together explains why crypto addresses can be shared safely, why private keys can't, and why nobody can "reset" a lost key.

The private key

A private key is a very large random number. It's the secret that proves you control an address. When you send crypto, your wallet uses the private key to produce a digital signature on the transaction. The network can check that signature without ever seeing the key.

Key facts:

• If someone learns your private key, they can sign transactions and move your funds.
• If you lose it (and any seed phrase that can regenerate it), no one can recover it.
• Wallets usually hide the raw key and show you a recovery phrase instead.

The public key

The public key is derived from the private key using one-way mathematics (elliptic-curve cryptography). It's easy to go from private to public, but practically impossible to go back. That's why the public key can be revealed without risk.

The address

An address is a shorter identifier derived from the public key. On Ethereum and compatible networks, an address is the last 20 bytes of a hash of the public key, written as 0x followed by 40 hexadecimal characters. ethereum.org's transactions guide explains how a transaction includes a signature from the sender's key and a recipient address.

Because the address is derived from the public key, you can share it freely, like an account number.

How a payment is authorised

1. You create a transaction: recipient, amount, fee and a counter called a nonce.
2. Your wallet signs it with your private key.
3. The signed transaction is broadcast to the network.
4. Validators check that the signature matches the sending address's key.
5. If it does, and you have the balance and fee, the transaction is included in a block.

No password is ever sent. The signature is the proof.

Checksummed addresses

Ethereum addresses can be written with a mix of upper- and lower-case letters. That pattern is a checksum defined in EIP-55: if a single character is mistyped, wallets that check the capitalisation can catch the error. It doesn't protect against copying the wrong address entirely, so you should still compare addresses by eye.

The same address on many networks

On EVM-compatible networks such as Ethereum, Polygon, Arbitrum and BNB Smart Chain, one private key produces the same address on every network. That's convenient, but it also means the address alone doesn't tell you which network to use. The network is a separate choice.

Privacy note

Addresses are public, and so are their transaction histories. Anyone who links your address to your identity can see your balance and activity. Using separate addresses for separate purposes limits how much one address reveals.

Common misunderstandings

• "My address is my password." No. The address is public; the private key is the secret.
• "The exchange can recover my self-custody wallet." It can't. Only the key or seed phrase can.
• "A new address means a new wallet." Many wallets create several addresses from one seed phrase.

The short version

The private key signs, the public key verifies, and the address receives. Share the address freely, guard the key and seed phrase completely, and remember that the network is a separate decision from the address.

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Every transaction on a public blockchain costs a small fee, usually called gas. Gas pays the validators who process transactions and protects the network from being flooded with spam. Understanding how gas works helps you avoid failed transactions, stuck transfers and nasty surprises when sending small amounts.

What gas measures

On Ethereum and compatible networks, gas measures the amount of computation a transaction needs. A simple transfer of the network's native coin uses a fixed, small amount of gas. A token transfer, such as sending USDT or USDC, uses more because it runs a smart-contract function. Complex actions such as swaps use more again.

ethereum.org's gas documentation explains that the total fee depends on the gas used multiplied by the price per unit of gas. Since Ethereum's London upgrade, that price has two parts:

• Base fee: set by the network according to demand, and burned
• Priority fee (tip): an extra amount paid to the validator to include your transaction

Why you need the network's own token

Fees are paid in the network's native token, not in whatever you're sending:

So a wallet holding only USDT on Polygon can't send it until you add a little POL. This is the single most common reason a first transfer fails.

Why fees change

Gas prices rise when many people want to transact at once, and fall when the network is quiet. The fee in your local currency also moves with the price of the gas token. On rollups like Arbitrum, part of the fee covers posting data to Ethereum, so busy periods on Ethereum can push Arbitrum fees up too. Arbitrum's gas and fees page describes this two-part structure.

Failed transactions still cost gas

If a transaction runs out of gas or a contract rejects it, the network still did work trying to run it. The transaction appears as failed on a block explorer, your tokens stay where they were, and the fee is spent anyway.

Common causes:

• Setting a gas limit too low
• Trying to send more tokens than you hold
• Interacting with a contract that rejects the action

Getting fees right

1. Use your wallet's estimate. Modern wallets suggest a fee based on current conditions.
2. Don't lowball in busy periods. A transaction with too low a fee may sit pending.
3. Keep a buffer of gas tokens so you're never stuck.
4. Check service fees too. Exchanges often charge their own flat withdrawal fee, which can be much larger than the network fee.

Small transfers: the fee maths

Fees matter most when amounts are small. If a combined network-plus-service fee is 1 unit:

• On a 10-unit transfer, that's 10%.
• On a 100-unit transfer, it's 1%.
• On a 1,000-unit transfer, it's 0.1%.

It often makes sense to move funds less frequently, in larger amounts, on a network both sides support.

Stuck transactions

On EVM networks, each address's transactions are processed in order by nonce. If an earlier transaction is stuck with a low fee, later ones wait behind it. Many wallets offer a "speed up" or "cancel" option that replaces the stuck transaction with the same nonce and a higher fee. Use these tools carefully, and only from the same wallet.

The short version

Gas is the network's processing fee, paid in its native token. It varies with demand, is charged even on failed transactions, and matters most for small transfers. Keep some gas token in every wallet you use, trust your wallet's estimate, and count service fees as well as network fees.

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