Authentication, Certificates and PKI

Context: Information Security and Cryptography · “how do I know whom I am talking to?” · closes the authentication gap left by Key Establishment and Diffie-Hellman

Quick Revision

• 🎯 Objective: bind a public key to an identity so a verified partner is known ➔ a certification authority (CA) signs (public key + identity) to make a certificate.
• ⚡ Key Constraint: certificates only move trust, not remove it — you must trust the CA, handle revocation of compromised keys, and remember a certificate proves identity, not that the server is safe.

📝 The authentication gap

• DH’s flaw ➔ the original protocol gives a shared secret but no proof of partner identity (man-in-the-middle).
• Signatures don’t fully fix it ➔ signing the exchanged values $A,B$ just shifts the problem to authenticating the signature verification keys.
• Solution ➔ certify public keys: a trusted CA signs the public key together with identity information → a certificate.

📜 Certificate contents

• A certificate = extra trusted info for a public key ➔ owner of the matching private key · validity (expiry date/time) · subject name · issuer name (the CA) · other parameters.

♻️ Revocation & limits

• Compromise ➔ private keys can leak → the certificate must be revoked before expiry.
• Mechanisms ➔ Certificate Revocation Lists (CRLs); Online Certificate Status Protocol (OCSP) and OCSP stapling (server attaches a fresh signed status).
• Weaknesses ➔ name misinterpretation (special/look-alike characters); anyone can buy a certificate — its existence means the identity was checked, not that the server is secure/benign.

⚠️ Common Mistakes

• 💡 A valid certificate ≠ a safe site ➔ malicious operators can obtain legitimate certificates; the lock icon proves an authenticated identity + encrypted channel, not trustworthiness of the operator.
• 💡 Trust is transitive to the CA ➔ security rests on the CA’s honesty and key security; a compromised or careless CA undermines every certificate it issued.

🧠 Active Recall

Signing the Diffie–Hellman values with digital signatures seems to authenticate them — why do we still need certificates?

Answer

• Short answer: a signature only helps if you already trust the verification key; without that, an attacker can sign with their own key, so the problem is merely displaced to authenticating the signing key.
• Why: Anchoring trust ➔ a CA-issued certificate binds a public key to an identity via a signature you can trace to a trusted root, terminating the regress — provided revocation (CRL/OCSP) keeps compromised keys out.