Skip to content
Knowledge Hub/Article
Article

What Is an SSL Certificate? SSL vs TLS vs HTTPS

We explain how a certificate is validated, TLS versions, the difference between DV, OV and EV, lifetimes shrinking to 47 days and renewal automation.

Oğuzhan Gerçek··10 min read
What Is an SSL Certificate? SSL vs TLS vs HTTPS

Short answer: An SSL certificate is a digital document, signed by a certificate authority (CA), that binds a domain name to a public key; the browser uses it to confirm it has reached the right server and then sets up an encrypted connection. SSL is the name of the older protocol that TLS was built on; what is actually in use today is TLS 1.2 and 1.3. HTTPS is HTTP protected by TLS. Since March 15, 2026, certificates can be valid for at most 200 days, falling to 47 days in March 2029, so renewal has to be automated.

What are SSL and TLS?

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) protect traffic between a client and a server against eavesdropping, tampering and message forgery. According to RFC 6101, SSL 3.0, whose last published version dates from November 18, 1996, was the basis for TLS. Old versions have been retired one by one:

  • SSL 2.0 and 3.0: Prohibited by RFC 6176 (March 2011) and RFC 7568 (June 2015).
  • TLS 1.0 and 1.1: Deprecated by RFC 8996 (March 2021) because they lack support for current algorithms.
  • TLS 1.2 and 1.3: The current specification of TLS 1.3 is RFC 9846, published in July 2026. It obsoletes RFC 8446, the original TLS 1.3 specification, and RFC 5246, which specified TLS 1.2, and it sets new requirements for TLS 1.2 implementations.

RFC 9852, published the same month, says that new protocols using TLS must require TLS 1.3. The SSL name lives on in certificates and in products such as SSL VPNs (VPN article); there is a short definition in our glossary.

How does TLS work?

A connection starts with a handshake. According to RFC 9846, during the handshake the two sides agree on a protocol version, select cryptographic algorithms, authenticate each other and establish shared secret keying material; the server is always authenticated, the client only optionally. Traffic is then protected with those keys: only the two endpoints can read it, and tampering in transit is detected. The main differences between TLS 1.3 and TLS 1.2 are:

  • Legacy algorithms, along with static RSA and Diffie-Hellman cipher suites, have been removed.
  • Every public-key-based key exchange now provides forward secrecy: in NIST's definition, compromising a long-term private key later does not compromise the session keys it was used to establish.
  • All handshake messages after the ServerHello are encrypted.

What is HTTPS?

HTTPS is HTTP running over TLS. RFC 9110 (June 2022), which replaced the older RFC 2818, makes TCP port 443 the default for https URIs (port article). Under the RFC, the client must verify that the identity in the certificate matches the domain name in the address; if it doesn't, the browser either asks the user for confirmation or terminates the connection. A site can use the HSTS header (RFC 6797) to declare that it accepts only secure connections, and RFC 9325 recommends HSTS for web servers.

According to Google, the share of Chrome navigations using HTTPS reached the 95-99% range around 2020. Google has announced that from October 2026, with Chrome 154, Chrome will ask for the user's permission before the first visit to any public site without HTTPS. There is a short definition in our glossary.

What is an SSL certificate? X.509, CAs and the chain

Web certificates use the X.509 format, which RFC 5280 (May 2008) profiles for the internet. According to the RFC, a certificate is a data structure that binds a public key to a subject, such as a domain name, and the CA that signs the certificate vouches for that binding. A certificate has a limited lifetime, recorded in its notBefore and notAfter fields.

Browsers and operating systems keep the root CAs they trust in a trust store. As AWS explains, the client tries to build a chain from the server's certificate through subordinate CAs up to a root in that store; RFC 5280 calls this a certification path. If no chain can be built, the connection fails with an error. You can restrict which CAs may issue certificates for your domain with a CAA record in DNS (RFC 8659); we cover record types in our DNS article.

Certificate types: DV, OV, EV, wildcard and SAN

The CA/Browser Forum's object registry distinguishes certificates by what the CA has verified:

  • DV (domain validated): No identity is asserted; the CA only verifies control of the domain, for example through a DNS record the applicant adds (Baseline Requirements).
  • OV and IV: The certificate also asserts the identity of an organization or an individual.
  • EV (extended validation): Identity is verified under a separate document, the EV Guidelines.

According to Cloudflare, certificates at every validation level provide the same degree of TLS encryption; the difference lies in the identity assertion. RFC 9525 (November 2023) requires the server's identity to be taken from the subjectAltName (SAN) field and allows a certificate to carry more than one name. In a wildcard certificate, the asterisk can only stand for the whole left-most label: *.example.com covers www.example.com but not a.b.example.com. The RFC warns that a wildcard can also vouch for rogue or buggy hosts in the domain, and that in a multi-name certificate, every name is only as secure as the weakest server offering one of those names.

Certificate lifetimes are shrinking: 200, 100 and 47 days

The CA/Browser Forum, which sets the rules for publicly trusted TLS certificates through its Baseline Requirements, adopted a schedule cutting the maximum validity from 398 days to 47 in Ballot SC-081v3; voting closed on April 11, 2025, with no votes against. Under version 2.3.1, effective October 4, 2026, the schedule is:

  • March 15, 2026: Maximum validity and the maximum reuse period for domain validation data are both 200 days.
  • March 15, 2027: Both drop to 100 days.
  • March 15, 2029: Validity drops to 47 days, and domain validation data can be reused for only 10 days.

EV certificates were tied to the same schedule by Ballot SC102 in July 2026. The ballot's rationale includes the point that a certificate represents a point-in-time state of reality, and that revocation services such as CRLs and OCSP do not adequately protect relying parties at the current scale of the internet.

Automation with ACME, and expired certificates

At this pace, renewing certificates by hand gets hard to sustain. ACME, standardized in RFC 8555 (March 2019), automates domain validation and certificate issuance between a CA and a client; in the authors' informal usability tests, webmasters often needed 1-3 hours to obtain and install a certificate by hand. Control of the domain is proven through a change made on the web server (http-01) or in DNS (dns-01). RFC 9773 (June 2025) also lets the CA suggest when the client should renew.

According to NIST's SP 1800-16 guide (June 2020), an expired certificate makes the application on that server unavailable, and nearly every enterprise has experienced an outage because of one. The guide notes that many organizations lack a comprehensive inventory of their TLS certificates and recommends automation wherever possible, from enrollment through replacement.

TLS configuration and TLS termination

SP 800-52 Rev. 2 (August 2019), written by NIST for US federal agencies, requires TLS 1.2 on citizen- and business-facing servers and has required TLS 1.3 support since January 1, 2024. It does not allow SSL 2.0 or 3.0 and calls for preferring ephemeral keys. Under RFC 9325, TLS 1.0 and 1.1 must not be negotiated at all, and RFC 10015 (July 2026) deprecates RSA and finite-field Diffie-Hellman key exchange in TLS 1.2. We handle these settings on web servers in our system hardening & baseline enforcement service.

Where TLS is terminated is also a design decision. According to AWS, an Application Load Balancer terminates the client connection with its certificate and decrypts requests before sending them to the targets; with a TCP listener on a Network Load Balancer, encrypted traffic can pass through without being decrypted. The connection to the backend servers can be re-encrypted, but the load balancer does not validate those servers' certificates. The termination point is where a load balancer can route on content and a WAF can read the request; with a CDN, it moves to the provider's edge servers. We handle TLS termination in our network & traffic engineering service and application-layer protection in our application security (WAF) service.

Frequently asked questions

What is SSL? It stands for Secure Sockets Layer: the encryption protocol, now retired, that TLS was built on. Certificates are still commonly called SSL certificates.

What is TLS, and how is it different from SSL? TLS is the encryption protocol that replaced SSL. All versions of SSL, as well as TLS 1.0 and 1.1, have been deprecated; TLS 1.2 and 1.3 are the versions in use today.

What is HTTPS? HTTP encrypted with TLS: the browser verifies the server's certificate, and the traffic cannot be read in transit or modified without detection.

What does an SSL certificate do? It lets the browser verify that it has reached the right site and set up an encrypted connection. OV and EV certificates also assert the identity of the organization behind the site.

What is a wildcard SSL certificate? A certificate that covers every subdomain one level below a domain name: *.example.com is valid for www.example.com but not for a.b.example.com.

What happens when an SSL certificate expires? The browser shows a warning and asks for confirmation, or terminates the connection; automated clients usually terminate it too. That is why certificates should be monitored and renewed automatically.

Sources