# Digital signatures

A digital signature proves you hold a private key without revealing it. Learn how Bitcoin signatures authorise every spend and why they can't be reused.

Canonical page: https://mybitcoinexplorer.com/learn/digital-signatures
Topic: Cryptography
Published: 2026-10-29
Updated: 2026-10-30


## Proof without revealing the secret
A digital signature lets you prove you know a [private key](https://mybitcoinexplorer.com/learn/private-keys) without showing it. Anyone with your matching [public key](https://mybitcoinexplorer.com/learn/public-keys) can check the proof.

> Diagram: Signing uses the private key; verifying needs only the public key — 

## What gets signed
When you spend, your wallet signs a hash of the **transaction itself** (which inputs, which outputs, which amounts). So:
- a signature for one transaction can't be reused for another,
- if anyone changes an output, the signature stops being valid.

Exactly which parts are covered is chosen with a **sighash flag**. The default, `SIGHASH_ALL`, covers everything.

## Two schemes in Bitcoin
| Scheme | Used by | Lesson |
|---|---|---|
| ECDSA | Legacy and SegWit v0 outputs | [ECDSA signatures](https://mybitcoinexplorer.com/learn/ecdsa) |
| Schnorr | Taproot outputs (since 2021) | [Schnorr signatures](https://mybitcoinexplorer.com/learn/schnorr-signatures) |

Both use the same curve, secp256k1. See [Elliptic curve cryptography](https://mybitcoinexplorer.com/learn/elliptic-curve-cryptography).

> A future large quantum computer could in theory derive private keys from public keys. That's an active area of research for Bitcoin developers; no such computer exists today.


## Sighash types
| Flag | Signs | Typical use |
|---|---|---|
| ALL | All inputs and outputs | Normal payments |
| NONE | All inputs, no outputs | Rare, risky |
| SINGLE | All inputs, the matching output | Special protocols |
| ANYONECANPAY (combined) | Only this input | Crowdfunding-style transactions |

Almost every wallet uses ALL: the signature locks the whole transaction.

## Signatures you can see
Open a transaction on MyBitcoinExplorer and expand its raw data: each SegWit input's witness begins with a signature of about 71 bytes, or 64 bytes for Taproot.


## Key takeaways

- A signature proves knowledge of a private key without revealing it.
- Bitcoin signatures commit to the transaction, so they can't be reused.
- Bitcoin uses ECDSA and, since Taproot, Schnorr signatures.

## Common questions

### Can someone fake my signature?

Not without your private key, as far as anyone knows. Forging one would require breaking elliptic curve cryptography.

### Is my signature public?

Yes, once your transaction is broadcast. That's safe: signatures don't reveal the private key, as long as the wallet uses fresh random nonces.

### Can a signature reveal my private key?

Not with correct software. It only becomes possible if a wallet reuses or leaks its secret signing nonce.

### Do all inputs need separate signatures?

Yes, each input proves its own right to spend, unless they share a key and the scheme allows aggregation.

## Related lessons

- [ECDSA signatures](https://mybitcoinexplorer.com/learn/ecdsa)
- [Schnorr signatures](https://mybitcoinexplorer.com/learn/schnorr-signatures)
- [Private keys](https://mybitcoinexplorer.com/learn/private-keys)
- [Public keys](https://mybitcoinexplorer.com/learn/public-keys)