Public key infrastructure (PKI)

What does Public key infrastructure mean?

PKI, or public key infrastructure, is the combination of certificate authorities, digital certificates, cryptographic key pairs, policies and software that lets people and systems trust that a given public key really belongs to a particular person, organization or device. It is the plumbing behind HTTPS padlocks, secure email and certificate-based digital signatures.

Public key cryptography gives each participant two mathematically linked keys. The private key is kept secret and is used to sign; the public key is shared and is used to verify. The weak point is identity: anyone can generate a key pair and claim to be anyone. PKI solves this by having a trusted certificate authority (CA) check an applicant's identity and then issue a digital certificate, a signed statement binding the public key to a name.

Trust flows through chains. Your software ships with a set of trusted root certificates; roots sign intermediate CA certificates, which in turn sign end-user certificates. When a PDF reader validates a digital signature, it follows that chain back to a trusted root, checks that no certificate has expired, and checks revocation lists or online status services to make sure the certificate was not withdrawn.

Here is a concrete case. A hospital group issues each department head a signing certificate from its internal CA. When a department head signs a policy document, colleagues' PDF readers, configured to trust the hospital's root, show the signer's name and confirm the file is unaltered. Outside readers that do not trust that internal root will show the signature as unverified, even though the cryptography is sound.

That example points to a common misconception: that PKI makes a signature universally trusted. Trust depends entirely on which roots the verifier accepts. Another misunderstanding is that PKI means encryption. PKI supports both encryption and signing, but a digital signature proves origin and integrity without hiding the content. And a hash such as SHA-256 is not encryption either; it is a one-way fingerprint that signatures build on.

In the EU, PKI underpins advanced and qualified electronic signatures, with qualified trust service providers acting as heavily supervised certificate authorities.

Our take: PKI is powerful but carries real operational weight, from certificate renewals to revocation checks. For many small businesses, a well-kept audit trail and a verifiable document hash, which is what eSignSimple provides alongside its certificate of completion and /verify page, deliver practical assurance without running certificates. Certificate-based signing is on our roadmap.

What is an example of public key infrastructure?

When you visit a bank's website, your browser checks the site's certificate against a root certificate authority it trusts; that is PKI at work. The same system lets a PDF reader show that a contract was signed with a certificate issued to a named person. eSignSimple does not issue certificates or run a PKI; we rely on audit trails and a SHA-256 hash checkable on /verify.

How does a PDF reader validate a certificate-based signature, step by step?

When you open a signed PDF, the reader first recalculates the hash of the signed byte range and compares it with the value inside the signature, which tells it whether the content changed. Next it uses the public key in the signer's certificate to check that the signature over that hash is genuine. Then it builds the chain: signer certificate, intermediate CA, root CA, and looks for that root in its trust store.

After that come time and status. The reader checks whether the certificate was valid at the time of signing, ideally using a trusted timestamp from a timestamping authority rather than the signer's own clock, and whether it had been revoked, using a certificate revocation list or an online status check (OCSP). Only if every step passes does it show a fully valid signature. A failure at any one step produces a warning, and the signature panel tells you which.

Why do certificate-based signatures expire, and what is long-term validation?

Certificates have expiry dates, often one to three years for signing certificates, and CAs can stop publishing revocation information for old certificates. Without extra data, a reader validating a ten-year-old contract may be unable to confirm that the certificate was good when the signature was made, and it shows the signature as unknown rather than valid.

Long-term validation solves this by embedding a trusted timestamp plus the certificate chain and revocation responses inside the PDF at signing time, and in some archives by adding fresh timestamps periodically. The PAdES family of standards used in Europe defines these levels. If you rely on certificate-based signatures for documents you must keep for many years, check that your provider produces long-term validation and not just a basic signature.

When do you actually need PKI-based signatures?

You need them when a law or a counterparty requires a specific type of signature: an advanced or qualified electronic signature in the EU, certain public-sector or regulated filings, or tenders that specify certificate-based signing. In those cases, a simple electronic signature will not satisfy the requirement, however good its audit trail. See our explanation of qualified electronic signatures and the European Union legality page for where the line sits.

For most other contracts, including the bulk of small business agreements, a simple electronic signature with an audit trail and a verifiable hash is legally sufficient and much easier for signers, who need no certificate, card reader or special app. eSignSimple provides simple electronic signatures only; it does not issue certificates, run a CA, or create advanced or qualified signatures. Our security page describes how files and records are protected instead.

Which terms are related?

• Digital signature: A digital signature is a cryptographic mechanism that uses public key infrastructure to prove who signed a document and that it has not changed.
• Qualified trust service provider (QTSP): A QTSP is a provider granted qualified status by an EU supervisory body to issue qualified certificates and other qualified trust services.
• SHA-256: SHA-256 is a cryptographic hash function from the SHA-2 family that turns any file into a unique 256-bit fingerprint used to detect tampering.
• Tamper-evident seal: A tamper-evident seal is a cryptographic seal on a PDF that shows whether the document has been changed in any way after it was signed.