What is a cipher suite?

A cipher suite is the named bundle of cryptographic algorithms that a client and server agree to use for one TLS connection. A single suite names the algorithm for exchanging keys, the algorithm for encrypting the traffic, and the way message integrity is checked. During the handshake the client offers a list of suites it supports, and the server picks one they share.

The names look dense but are readable once you know the parts. In a TLS 1.2 name like ECDHE-RSA-AES128-GCM-SHA256, ECDHE is the key exchange, RSA is how the server’s identity is authenticated, AES128-GCM is the authenticated encryption (the GCM part carries the integrity tag), and SHA256 is the hash used for key derivation and handshake integrity. TLS 1.3 simplified this: it defines a short fixed list of suites, drops the older key-exchange and authentication parts from the name, and removes every legacy option, so a TLS 1.3 name is just the encryption and hash, such as TLS_AES_128_GCM_SHA256.

Cipher suites are where a lot of "your SSL is weak" findings come from. Older suites carry algorithms that are now broken or too small: RC4, 3DES, CBC-mode constructions with known attacks, and key exchanges without forward secrecy. A scanner grades a server down when it still offers these, even if the certificate is perfect, because a client and server can be pushed onto the weak suite.

The practical guidance is short. Prefer forward-secret key exchange, which means ECDHE. Prefer authenticated encryption, which means the GCM or ChaCha20-Poly1305 suites. Enable TLS 1.3, whose suites are all safe by design, and drop the legacy suites from TLS 1.2 rather than leaving them available as a fallback. Leaving a weak suite enabled "just in case" is what a downgrade attack relies on, so the safe list is the whole list.