What is a digital signature, and how is it verified?

You sign a transaction with your private key to produce a signature that only you could create; anyone can then use your public key to verify the signature is genuine — without ever learning your private key.

Signing takes your private key plus the message (the transaction) and produces a signature unique to both. Its two magic properties:

• Unforgeable — nobody can derive your private key from the signature, so nobody else can produce a valid signature for your account.
• Publicly verifiable — anyone holding your public key can check that the signature matches the message and was made by the matching private key. If someone tampers with the message or fakes a signature, verification fails.

This is how the network knows a transaction spending your funds really came from you: the sender attaches a signature, and every node verifies it against the sender's public key. Signing creates the authorized transaction; it is a separate step from broadcasting it to the network.

Go deeper:

• Digital signature — Wikipedia — how signing and public-key verification prove authenticity and prevent forgery.