# What is a Digital Signature?

> A digital signature is a cryptographic proof attached to data showing who signed it and that the content was not altered after signing — providing authenticity and integrity, and supporting non-repudiation.

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## Content

Signing uses the signer’s private key; anyone with the corresponding public key can verify it. The signature binds the identity to the exact bytes signed: change one bit and verification fails. In practice, signatures authenticate software releases, documents, email (DKIM), code, certificates, and transactions.

### Key points

- **Signing-key protection:** A signature is only as trustworthy as the secrecy of the private key — use HSMs or managed signing services for high-value keys.
- **Chain verification:** Signature validation depends on the signer’s certificate and the trust chain behind it; check revocation, expiry, and who issued it, not just “signature valid.”
- **Important limitation:** A valid signature proves the key signed the data — not that the signer intended to, that the content is good, or that the key was not stolen before use. Signed malware is still malware.

### Related terms

[Public key infrastructure (PKI)](<https://yellowcube.eu/glossary/public-key-infrastructure/>) · [Digital certificate](<https://yellowcube.eu/glossary/digital-certificate/>) · [Cryptography](<https://yellowcube.eu/glossary/cryptography/>) · [DomainKeys Identified Mail (DKIM)](<https://yellowcube.eu/glossary/domainkeys-identified-mail/>) · [Message authentication code (MAC)](<https://yellowcube.eu/glossary/message-authentication-code/>) · [Pretty Good Privacy (PGP)](<https://yellowcube.eu/glossary/pretty-good-privacy/>) · [Cryptographic hashing](<https://yellowcube.eu/glossary/cryptographic-hashing/>)

### Sources

[NIST FIPS 186-5, Digital Signature Standard](https://csrc.nist.gov/pubs/fips/186-5/final) · [NIST SP 800-57 Part 1 Rev. 5, Recommendation for Key Management](https://csrc.nist.gov/pubs/sp/800/57/pt1/r5/final)

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