Certificates and PKI

The problem certificates solve

Asymmetric encryption needs you to know someone's public key. But if a website just sends you "here is my public key", how do you know it is really your bank and not an attacker in the middle? You need someone you already trust to vouch for it. That is a certificate.

What is inside a certificate

Web certificates follow a standard called X.509. Important fields:

• Subject and Subject Alternative Names (SAN): the names the certificate is valid for, like shop.example and www.shop.example. A wildcard like *.shop.example covers one level of sub-domain.
• Issuer: the CA that signed it.
• Validity: Not Before and Not After dates.
• Public key: the key that belongs to the subject.
• Signature: the issuer's digital signature over all of the above.

$ openssl s_client -connect shop.example:443 -servername shop.example </dev/null 2>/dev/null | openssl x509 -noout -subject -issuer -dates
subject=CN = shop.example
issuer=C = US, O = Let's Encrypt, CN = R11
notBefore=Feb 10 00:00:00 2026 GMT
notAfter=May 11 23:59:59 2026 GMT

The chain of trust

Your operating system and browser ship with a list of trusted root CAs. Roots rarely sign website certificates directly. Instead: the root signs an intermediate CA, and the intermediate signs the website (leaf) certificate. The browser checks each signature up the chain until it reaches a root it trusts.

Root CA            (in your browser's trust store)
  └─ Intermediate CA (signed by the root)
       └─ shop.example (signed by the intermediate)

What the browser checks

1. The name in the address bar matches a name in the certificate.
2. Today is between Not Before and Not After.
3. Every signature in the chain is valid up to a trusted root.
4. The certificate has not been revoked (cancelled early, for example after a private key leak).

If any check fails, the browser shows a full-page warning. Training people to click through those warnings is dangerous.

Certificates are getting shorter

Let's Encrypt, a free CA, has issued 90-day certificates for years, is moving to even shorter ones, and expects automatic renewal. The industry body for browsers and CAs has agreed to cut the maximum lifetime of public website certificates in steps, starting in 2026, so automation is now essential. Expired certificates are one of the most common causes of outages.

PKI in India

Under the IT Act, the Controller of Certifying Authorities (CCA) licenses Certifying Authorities that issue Digital Signature Certificates (DSC) to people and companies. DSCs are used for company filings, income tax and GST, and tenders. Aadhaar-based eSign also uses PKI behind the scenes.

Common mistakes

• Letting certificates expire. Track expiry dates and automate renewal.
• Self-signed certificates on public sites, which teach users to ignore warnings.
• Leaving a private key readable on a server (chmod 600 it) or committing it to Git. If a key leaks, revoke and replace the certificate.
• Thinking a valid certificate means the site is trustworthy. It proves the name, not the intentions.