TLS uses both kinds of crypto, with a clean division of labor:
- Asymmetric (public-key) crypto — used only during the handshake to authenticate the peer and to agree on a shared secret (e.g. RSA, or ephemeral Diffie-Hellman).
- Symmetric crypto — used afterward to encrypt all the application data (e.g. AES-GCM).
Why split it this way? Two facts force it:
- Asymmetric operations are orders of magnitude slower per byte, so you’d never use them for bulk data.
- Symmetric ciphers are fast but need both sides to already share a key — and you can’t just send a key over an open wire.
So asymmetric crypto solves the bootstrapping problem (agree on a key safely, over a visible channel), and symmetric crypto does the heavy lifting once the key exists.
See also
- TLS provides confidentiality, integrity and authentication over a reliable stream — this split is how TLS delivers confidentiality: the handshake sets up the key, symmetric crypto encrypts the stream
- HKDF has two operations, Extract whitens messy entropy and Expand clones one key into many — the shared secret from the handshake is raw and messy; HKDF turns it into the actual symmetric keys
- A certificate authenticates a Diffie-Hellman exchange by signing the handshake transcript — the asymmetric half is where authentication happens, binding the key agreement to a verified identity
- Head-of-line blocking is the price of a single connection-wide byte order — QUIC fuses this key agreement into its transport handshake (handshake collapse) to get 1-RTT, or 0-RTT on resumption
References
Questions
flashcards/software-engineering/tls
What is the division of labor between asymmetric and symmetric crypto in TLS?::Asymmetric crypto handles the handshake (authentication + agreeing on a shared key); symmetric crypto encrypts all the application data
Why isn’t asymmetric crypto used to encrypt the bulk application data?::It is orders of magnitude slower per byte than symmetric crypto
Symmetric crypto is fast but requires both sides to already share a key, which is the bootstrapping problem asymmetric crypto solves.