What Signing Proves
A digital signature uses a private key to bind a message to its author. Anyone with the matching public key can verify that the message is unaltered and came from the key holder, and the signer cannot later deny it, a property called non repudiation.
How It Works
The signer first hashes the message, then transforms that digest with the private key to produce a signature. The verifier hashes the message themselves and checks the signature against that digest using the public key.
- Signing uses the private key, verifying uses the public key.
- Common schemes include RSA signatures and elliptic curve signatures.
- A signature covers the whole message through its hash.
Why Hash First
Hashing lets a signature cover a message of any length with a fixed sized digest, and it keeps the costly key operation small. A change of even one bit produces a different hash and an invalid signature.
Key idea
A digital signature transforms a message hash with a private key so anyone can verify authorship and integrity with the public key, delivering non repudiation that the signer cannot later deny.