What is a digital signature?

A digital signature is a value computed from data with a private key, which anyone holding the matching public key can check. If the check passes, two things are proven: the data comes from the holder of that private key, and it has not changed since it was signed. Signatures protect much of what you rely on without noticing: software updates and drivers, the TLSTLS: Transport Layer Security. The encryption that protects most internet traffic, such as HTTPS. certificates that prove a website's identity, signed PDFs and e-invoices, e-mail signed with S/MIME, and the code your developers ship. In the EU, qualified electronic signatures under the eIDAS Regulation also carry legal weight. A signature is only as trustworthy as its private key: if the key is stolen, an attacker can sign malware that looks genuine, which is why signing keys belong in hardware security modules or similarly protected stores, with few people able to use them. Today's signatures use RSA or elliptic curves, which a large quantum computer would break; their post-quantum replacements are ML-DSA and SLH-DSA.

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