What Is a Digital Signature?

A digital signature is a piece of cryptographic data, generated from a private key and the exact details of one transaction, that proves the key's holder approved that specific transaction without ever exposing the key itself.

What actually goes into it

Signing isn't a stamp applied to a transaction after the fact — the transaction's own details are part of the calculation that produces the signature. Change any detail, such as the amount or the destination address, and the resulting signature would be completely different and invalid for the original transaction. That's why a signature can't be lifted from one transaction and reused on another.

How a network confirms it's genuine

A network never sees the private key. It only sees the transaction, the signature, and the public key tied to the sending address. Using those three pieces, any node can run a verification calculation that gives a simple yes-or-no answer: does this signature actually correspond to this transaction and this public key. No node needs to trust the sender's claim — the math either confirms the match or it doesn't.

Why it can't be forged or reused

Producing a valid signature requires the private key, and nothing about the verification process reveals or reconstructs that key from the signature alone. Because the transaction details are baked into the calculation, a signature that's valid for one transaction won't pass verification for a different one, even a nearly identical one. Someone without the private key can't produce a new valid signature no matter how many old ones they've seen.

For where this fits into checking a transaction before it's sent, see Transaction Verification, and for how the private key behind a signature is protected, see Key Security.