SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) are cryptographic protocols that encrypt communication between a client and a server. While SSL is technically deprecated and replaced by TLS, the term "SSL" is still commonly used to refer to both. TLS ensures that data transmitted over a network cannot be read or tampered with by third parties, providing confidentiality, integrity, and server authentication.
How It Works
A TLS connection begins with a handshake process. The client initiates contact and sends its supported TLS versions and cipher suites. The server responds with its chosen version and cipher suite, along with its digital certificate. The certificate contains the server's public key and is signed by a trusted Certificate Authority (CA), allowing the client to verify the server's identity. The client and server then negotiate a shared session key using asymmetric cryptography, and all subsequent data is encrypted with this symmetric key.
Modern TLS 1.3 simplified the handshake to a single round trip, improving both performance and security. It eliminated support for older, vulnerable cipher suites and made forward secrecy mandatory, meaning that even if the server's private key is later compromised, previously recorded encrypted traffic cannot be decrypted. Older versions like TLS 1.0 and 1.1 are now deprecated due to known vulnerabilities.
TLS misconfigurations remain a common security finding. These include supporting deprecated protocol versions, using weak cipher suites, presenting expired or self-signed certificates, failing to implement HSTS headers, and not redirecting HTTP traffic to HTTPS. Certificate transparency logs help detect unauthorized certificate issuance, and automated certificate management through services like Let's Encrypt has reduced the operational burden of maintaining valid certificates.
Why It Matters
TLS is the foundation of secure communication on the internet. Without it, every piece of data transmitted between a user and a web application, including passwords, session tokens, personal information, and financial data, can be intercepted and read by anyone with network access. Security assessments evaluate TLS configuration to ensure that applications use current protocol versions, strong cipher suites, valid certificates, and proper enforcement through HSTS to prevent downgrade attacks.
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