✍️ Digital Signatures

A digital signature is the electronic equivalent of signing a document with a pen — but far more secure. It proves three things: who sent the data (authentication), that the data has not been changed since it was signed (integrity), and that the sender cannot deny they sent it (non-repudiation).

Digital signatures use the asymmetric encryption you learned in the previous lesson, but in reverse. When encrypting, the sender uses the receiver's public key. When signing, the sender uses their own private key. This means anyone with the sender's public key can verify the signature, but only the sender (who has the private key) could have created it.

How Digital Signatures Work — Step by Step

📰 Where Digital Signatures Are Used

SSL/TLS certificates	The Certificate Authority digitally signs each certificate. Your browser verifies the CA's signature to confirm the certificate is genuine.
Software updates	Apple, Microsoft, and Google digitally sign their updates. Your device verifies the signature before installing — if it doesn't match, the update is rejected (could be malware).
Email (S/MIME)	Digitally signed emails prove the sender is genuine and the message hasn't been altered. Used in business and government.
Legal documents	DocuSign and Adobe Sign use digital signatures for legally binding contracts signed online.