Quest 3

Transactions, nonces, gas — what you're actually paying for

A transaction isn't magic. It's a signed message saying 'I, owner of this private key, want to send X to Y and burn this much gas.'

Lesson

Anatomy of an Ethereum transaction

{
  from:      your address (derived from signature, not in the payload itself)
  to:        recipient address
  value:     amount of ETH in wei (1 ETH = 10^18 wei)
  nonce:     an integer, must equal your address's current transaction count
  gasLimit:  max gas units this tx is allowed to consume
  maxFeePerGas: max gwei per gas you'll pay
  maxPriorityFeePerGas: tip to the validator on top of the base fee
  data:      optional bytes (empty for plain transfer, function call data for contract)
  signature: your private key signing all of the above
}

Gas is a metering unit, not a currency

Every EVM opcode costs a fixed (or formula-based) number of gas units.
ADD: 3 gas
SSTORE (write to storage): 20,000 gas for a new slot, 5,000 for an update
Plain ETH transfer: 21,000 gas
You pay in ETH: total_fee_eth = gas_used × gas_price (where gas_price is in wei).
The base fee goes to the protocol (burned as of EIP-1559). The priority fee tips the validator.

Nonces = replay protection

Your address has a nonce (tx count). Each transaction must use the next nonce in sequence. This prevents replay attacks — if an attacker sees your signed tx and broadcasts it again, the nonce is already used, so the second broadcast reverts.

Worked example

A Uniswap swap might consume 150,000 gas. At base fee 30 gwei and priority 2 gwei: (30 + 2) × 150,000 = 4,800,000 gwei = 0.0048 ETH per swap.

Your task

Compute the ETH cost of a swap given the inputs below.

References

pythonEdit below, click Run tests

{ from: your address (derived from signature, not in the payload itself) to: recipient address value: amount of ETH in wei (1 ETH = 10^18 wei) nonce: an integer, must equal your address's current transaction count gasLimit: max gas units this tx is allowed to consume maxFeePerGas: max gwei per gas you'll pay maxPriorityFeePerGas: tip to the validator on top of the base fee data: optional bytes (empty for plain transfer, function call data for contract) signature: your private key signing all of the above }