Bank transfers rely on account credentials and the bank’s verification. On a blockchain, no institution checks your identity for you—the network recognizes only one thing: a digital signature. Understand how it works, and you’ll understand why private keys are so important.

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[A pair of keys]

Digital signatures use a pair of keys: the private key is used to create the signature, and the public key is used to verify it. The private key is a large randomly generated number. The public key is calculated from the private key using elliptic-curve operations. Bitcoin and Ethereum both use the secp256k1 curve.

This operation is one-way: deriving a public key from a private key is easy, but working backward from a public key to a private key is computationally infeasible. So the public key and the address derived from it can be shared publicly, while the private key must be kept to yourself.

【What a signature proves】

When you send a transaction, your wallet signs its contents with your private key, and nodes across the network verify the signature using your public key. If verification succeeds, it proves two things at once: the transaction really came from someone who controls the private key, and its contents have not been changed since it was signed. Change even a single byte in the amount or recipient address, and the original signature will no longer verify.

Keep in mind that a signature does not encrypt the transaction contents. The amounts and addresses in on-chain transactions are still visible to everyone. A signature answers “who sent this, and has it been altered?”—not “can other people see it?”

【Preventing a transaction from being used twice】

Every Ethereum transaction includes an incrementing nonce, and each nonce can be included in a block only once. Transactions also include a chain ID (EIP-155), so a signed transaction cannot be replayed on another chain. Bitcoin, meanwhile, prevents the same coins from being spent more than once with the rule that each unspent output can only be spent once.

【Signatures aren’t just for transfers】

A signing request that pops up in your wallet isn’t necessarily a transfer. It could also be a token approval or an off-chain signed message. Some off-chain signatures, once signed, have the same effect as granting approval directly, allowing the other party to use them to transfer your tokens.

So before signing, check the type of action, the address you’re interacting with, and the amount. If you don’t understand it, don’t sign yet. Anyone who asks you to provide your private key or seed phrase to “verify a signature” is running a scam.

【In a nutshell】

On-chain, a signature is your “handwritten signature,” and your private key is the only pen that can write it. If the pen is lost or stolen, no institution can help you revoke it. Understanding this won’t make me instantly better, but it can help me avoid one basic mistake. There’s no harm in taking it slow; staying in the game matters more than how much you make on any one trade.

In what situation did this finally click for you?