What is SSL?

SSL (Secure Sockets Layer) is the original protocol for encrypting the connection between your browser and a website, so anyone watching the traffic in between sees only scrambled data instead of what you're actually sending or reading. Every version of SSL itself has since been retired for being insecure — what actually runs behind that familiar padlock icon today is its successor, TLS, though "SSL" has stuck around as the everyday name for both.

SSL vs. TLS in 2026 — the version most people mean doesn't exist anymore

SSL 2.0 and 3.0 both shipped with cryptographic weaknesses serious enough that browsers eventually disabled them entirely. SSL 3.0's end came specifically because of the 2014 POODLE attack, which showed that a connection could be deliberately forced to fall back to SSL 3.0 and then broken with comparatively little effort. TLS (Transport Layer Security) is the protocol that replaced it, and every connection you make over HTTPS today uses TLS — version 1.2 or 1.3 in virtually all current traffic — never SSL itself.

How the handshake actually works

Your browser and the server can't just start encrypting with a single shared password neither of them had in advance, so the connection opens with a handshake that solves that problem in two stages. First, asymmetric cryptography (a public/private key pair) lets the two sides agree on a shared secret without ever transmitting that secret itself in a form anyone intercepting the traffic could use. Once that shared secret exists, the connection switches to symmetric encryption using it — much faster computationally, which is why TLS doesn't use asymmetric encryption for the entire session, only for the initial handshake.

The padlock doesn't mean what most people think it means

A valid certificate proves two specific things: the connection is encrypted, and whoever requested the certificate controlled the domain's DNS at the time it was issued. It says nothing about whether the site behind that domain is trustworthy. The cheapest, most common certificate type — Domain Validated (DV) — is exactly what a phishing site needs, and free automated issuance through services like Let's Encrypt means attackers can get a perfectly valid padlock in minutes for a domain built purely to impersonate a bank or retailer. For a deeper look at exactly what that padlock does and doesn't vouch for, see our guide on whether HTTPS means a website is safe.

The chain of trust, and what happens when it breaks

Every certificate is only trusted because a certificate authority (CA) vouches for it, and every browser maintains a list of CAs it trusts by default. That system depends entirely on CAs actually verifying what they claim to verify — and in 2011, the Dutch CA DigiNotar proved what happens when that assumption fails.

Attackers compromised DigiNotar's systems and issued 531 fraudulent certificates, including one for *.google.com, used to run man-in-the-middle attacks against an estimated 300,000 Gmail users inside Iran. The forgery was only caught because Chrome had separately "pinned" Google's real certificates in its own code — a hardcoded check that flagged DigiNotar's fake as wrong the moment a targeted user tried to log in through Chrome specifically. DigiNotar had run outdated, poorly segmented server software with no antivirus protection on its internal systems; once browser vendors stopped trusting its certificates entirely, the company went bankrupt within weeks.

When the protocol's own code has a bug: Heartbleed

A valid certificate and a sound protocol design don't protect against a flaw in the actual software implementing them. In April 2014, researchers disclosed Heartbleed (CVE-2014-0160), a bug in OpenSSL — the library behind a large share of the internet's HTTPS servers — that let an attacker request small chunks of a server's live memory, potentially including private keys, passwords, and session data. Netcraft estimated roughly 17% of the web's SSL-secured servers were vulnerable at disclosure — around half a million sites using certificates from trusted CAs. The bug had existed, unnoticed, for two years before it was found.

How to actually check a certificate yourself

Click the padlock icon in your browser's address bar and view the certificate details directly — check who issued it, what type it is, and its expiration date, rather than trusting the padlock's presence alone. Qualys SSL Labs' free SSL Server Test goes further, showing exactly which protocol version and cipher a site is actually negotiating and grading its overall configuration — detail the padlock icon alone never reveals.