Transport Layer Security (TLS)
Transport Layer Security (TLS) is a security protocol that replaces SSL for data privacy and Internet communication security. TLS encrypts communications between web applications and servers such as between a visitor’s browser loading a website.
TLS is used for many Network Protocols, but of those HTTPS is the most publicly visible.
Handshake
TLS starts with a handshake, where the client and the server negotiate how they are going to set up the Encryption. When that handshake is complete, the connection is secure.
Because the handshake by definition must happen unencrypted, and the intent is to have an encrypted connection, part of the connection is encrypted, and another part is not. The easiest way to implement this is by using two port numbers: one for unencrypted communication, and another one for encrypted communication. Many Network Protocols that apply TLS use this approach.
Alternatively client and server could make use of a STARTTLS request to indicate when the rest of the communication will be encrypted.
TLS applies PFS. Every new connection will use a different Symmetric Key.
Versions
At the time of writing there are 4 versions of TLS:
- TLS 1.0
- TLS 1.1
- TLS 1.2
- TLS 1.3 (the current version)
TLS 1.0 and TLS 1.1 saw the light of day in 1995 and 2006 respectively. Both were deprecated in 2021.
TLS 1.0 had a lot in common with SSL 3.0, except that it was no longer vulnerable to POODLE attacks. TLS 1.1 was an improvement on TLS 1.0 in that it had extra protection against some cryptographic attack vectors.
TLS 1.2 is mostly similar to TLS 1.1, with the exception that it moved from MD5 and SHA1 to SHA256. It also included the ability to negotiate during the handshake what Hashes and ciphers each party thought acceptable. TLS 1.2 also supported AES256, where TLS 1.1 did not. TLS 1.2 was also modified to never allow a fallback to SSL 2.0. TLS 1.2 is not deprecated, but one is encouraged to migrate to TLS 1.3.
TLS 1.3 is the latest version of TLS. It dropped support many ciphers and Hashing algorithms that we now consider “weak”, it dropped support for features like “compression” and “renegotiation”, and it added support for more secure ciphers and Hashing algorithms, like ED25519.