Transport Layer Security (TLS) [TLS, SSL]

Transport Layer Security (TLS) is a cryptographic protocol that encrypts and authenticates data sent between applications over a network, most visibly between browsers and websites using HTTPS.

How TLS works

TLS succeeded Netscape’s SSL protocol; TLS 1.0 was published in 1999, TLS 1.2 in 2008 (RFC 5246) and TLS 1.3 in 2018 (RFC 8446). All SSL versions and TLS 1.0/1.1 are deprecated (RFC 8996). A connection starts with a handshake:

• the client and server agree on the protocol version and cipher suite;
• the server proves its identity with a digital certificate, which the client validates;
• both sides derive session keys, in TLS 1.3 always via ephemeral Diffie–Hellman, which provides forward secrecy;
• application data is then encrypted and integrity-protected with authenticated ciphers such as AES-GCM or ChaCha20-Poly1305.

TLS 1.3 removed obsolete algorithms and reduced the handshake to one round trip.

Why TLS matters

TLS secures HTTPS, email transport (SMTP, IMAP), VPNs, APIs and countless applications, protecting against eavesdropping and tampering. Its history shows the main risks:

• Implementation bugs – Heartbleed (CVE-2014-0160) in OpenSSL leaked server memory, including private keys, in 2014.
• Protocol weaknesses – BEAST (2011) and POODLE (2014) attacked older versions; FREAK and Logjam (2015) were downgrade attacks to weak export-grade cryptography.
• Abuse by attackers – most malware and phishing sites now use TLS too, so defenders rely on TLS inspection and fingerprinting methods such as JA3/JA4 to spot malicious connections.

Best practices

• Allow only TLS 1.2 and 1.3 and disable legacy cipher suites.
• Automate certificate renewal and protect private keys.
• Validate certificates properly in applications – never disable verification in code.
• Test configurations regularly with tools such as SSL Labs or testssl.sh.