SSL/TLS: Meaning, How It Works and Secure Versions

What Does SSL/TLS Mean?

SSL stands for Secure Sockets Layer, TLS for Transport Layer Security. Both are protocols, meaning fixed rules for how two computers set up a secure connection.

TLS is the successor to SSL. Today, TLS is used almost everywhere. Still, people often say "SSL", for example in "SSL certificate". The old name has stuck.

SSL/TLS protects three things:

• Confidentiality: no one along the way can read the data. See encryption.
• Integrity: if data is changed on the way, this is noticed.
• Authenticity: the certificate shows that you are talking to the real server.

From SSL to TLS: the Versions

• SSL 2.0 (1995) and SSL 3.0 (1996): developed by Netscape. Both are considered broken and are prohibited.
• TLS 1.0 (1999) and TLS 1.1 (2006): officially deprecated since 2021. Modern browsers no longer support them.
• TLS 1.2 (2008): still secure, as long as only strong methods are allowed.
• TLS 1.3 (2018): the current standard. Faster and more secure, because weak methods were removed entirely.

The German BSI recommends TLS 1.2 and TLS 1.3 in its technical guideline TR-02102-2.

How the TLS Handshake Works

Before any data flows, the browser and the server agree on the connection. This process is called the handshake. With TLS 1.3, it looks roughly like this:

1. Client hello: the browser lists the methods it supports and sends its part of the key exchange right away.
2. Server hello: the server picks a method and sends its part of the key exchange.
3. Certificate: the server sends its certificate and proves that it holds the matching private key.
4. Check: the browser checks the certificate. Does the domain name match? Is it still valid? Does it come from a trusted certificate authority?
5. Encrypted connection: both sides work out the same session key. From now on, all data is encrypted.

Two types of encryption work together here. The key exchange uses asymmetric methods, usually ECDHE. The actual data is encrypted symmetrically, for example with AES. This is much faster.

TLS 1.3 uses a new key for every connection. This is called perfect forward secrecy. Even someone who later steals the server's private key cannot decrypt old connections after the fact.

Certificates and Certificate Authorities

A TLS certificate is a server's digital ID card. It contains the domain name, a public key and the signature of a certificate authority (CA). Browsers trust a fixed list of such authorities. All of this is part of a public key infrastructure (PKI).

• Domain validation (DV): the CA only checks that the applicant controls the domain. Such certificates are also free, for example from Let's Encrypt.
• Organization validation (OV): the company is checked as well.
• Extended validation (EV): the strictest check. However, browsers no longer highlight EV certificates in any special way.

Lifetimes are getting shorter. Since March 2026, public TLS certificates may be valid for 200 days at most. By 2029, the limit will drop step by step to 47 days. If you still renew certificates by hand, you should switch to automation, for example with the ACME protocol.

Where SSL/TLS Is Used

• Websites: HTTPS is HTTP over TLS. The padlock in the address bar shows an encrypted connection.
• Email: mail servers encrypt the transport via STARTTLS or directly with TLS.
• VPN: many VPN solutions build their tunnel with TLS.
• APIs: apps and services almost always talk to each other over TLS.
• Mutual checks (mTLS): here, both sides prove who they are with certificates, for example between servers in the cloud.

Attacks and Weaknesses

TLS itself is secure when it is up to date and set up correctly. Problems came mainly from old versions, weak methods and bugs in software:

• POODLE (2014): used a weakness in SSL 3.0. Since then, SSL 3.0 has been turned off everywhere.
• BEAST (2011): attacked connections using TLS 1.0.
• Heartbleed (2014): not a flaw in the protocol, but in the OpenSSL library. Attackers could read memory, including private keys.
• Downgrade attacks: an attacker forces both sides onto an old, weak version. The only defense is to turn old versions off completely.
• Man-in-the-middle: without a valid certificate check, an attacker can sit in between. See man-in-the-middle attack.

Best Practices for a Secure Setup

• Only allow TLS 1.2 and 1.3: turn off SSL, TLS 1.0 and TLS 1.1.
• Choose strong methods: key exchange with forward secrecy and modern encryption such as AES-GCM or ChaCha20-Poly1305.
• Automate certificates: renew via ACME and monitor expiry dates.
• Turn on HSTS: this makes the browser enforce HTTPS so it cannot be pushed back to HTTP.
• Protect private keys: limit access tightly and replace them at once if you suspect a leak.
• Check on a regular basis: configurations change. A scan shows whether old versions or weak methods are still active.

SnapScan reports weak TLS settings and maps them to the BSI module CON.1 crypto concept and ISO 27001 A.8.24.

TLS and Quantum Computers

Powerful quantum computers could one day break today's methods for key exchange. Attackers could already collect data now and decrypt it later. That is why TLS is now being extended with post-quantum methods. Major browsers already use a hybrid key exchange: a classic method combined with the new ML-KEM standard. Read more under post-quantum cryptography.

Conclusion

SSL/TLS is the basis for secure connections on the internet. SSL is outdated; TLS is what runs today. TLS 1.2 and TLS 1.3 with strong methods and valid certificates are secure. If you turn off old versions, renew certificates automatically and check the setup on a regular basis, you are well prepared.