A digital signature proves that a message came from a specific sender and was not altered in transit. The sender signs the message with their private key. Anyone can verify the signature with the sender's public key. If the message changes by even one character, the signature fails. That combination of authentication and integrity makes digital signatures useful for contracts, software updates, and financial transactions.

The process is straightforward. The sender hashes the message to create a digest. They encrypt the digest with their private key. That encrypted digest is the signature. The receiver decrypts the signature with the sender's public key to recover the digest. They hash the message themselves and compare. If the digests match, the message is authentic and unchanged. Digital signatures do not encrypt the message. They prove who sent it and that it arrived intact. Code signing uses digital signatures to verify that software came from the developer and has not been tampered with. Certificate authorities use their own signatures to vouch for the certificates they issue. The chain of trust relies on signatures all the way down.

Digital signature properties

• Authentication — proves who signed
• Integrity — detects any change to the message
• Non-repudiation — signer cannot deny signing
• Not encryption — the message is not hidden

A digital signature is not a scanned handwritten signature. It is a cryptographic proof that the message is authentic.