TLS
acronymcryptographynetworkingsecurity
Stands for: Transport Layer Security
The protocol that encrypts and authenticates most internet traffic, the S in HTTPS.
Transport Layer Security (the modern successor to SSL, current version 1.3 in RFC 8446) protects data in transit and verifies you are talking to the right server. It negotiates a cipher suite and, by default now, forward secrecy for each session.
Every HTTPS connection begins with a negotiation that most people never see: the client proposes what it supports, the server picks, and both sides prove they agree before any application data moves. When that negotiation fails, the browser shows a generic error and the useful detail is one layer down, in the handshake itself.
The parts worth knowing separately are the certificate, the cipher suite, and the version. The certificate answers who the server claims to be and who vouches for it. The cipher suite is the set of algorithms chosen for key exchange, bulk encryption, and integrity. The version determines which suites are even on the table, which is why TLS 1.3 removed so many options rather than adding them: the fastest way to eliminate a class of downgrade attacks was to delete the weak choices entirely.
The details are also a fingerprint. The exact ordering of what a client offers is characteristic enough to identify software without decrypting anything, which is the basis of JA3 and its successors. Something intended purely as capability negotiation turned out to leak identity, and both defenders and evaders now build on that fact.
Also known as: tls, ssl, https, transport layer security