Guide2026-04-08•9 min read

RSA vs ECDSA

· CSR.plus Team

•RSA 2048 is the compatibility baseline; ECDSA P-256 offers smaller keys and faster handshakes.
•ECDSA keys are about 10× smaller than equivalent RSA keys, reducing certificate and handshake size.
•Modern browsers, CDNs and clients support ECDSA — keep RSA 2048 only for legacy devices if needed.

2048 bit

RSA key size

recommended minimum

256 bit

ECDSA key size

P-256 curve, equivalent strength

~35%

Handshake saving

fewer bytes, faster negotiation

What the key does

The certificate key is used for the TLS handshake: the server signs an exchange to prove it holds the private key. Its strength bounds the security of that signature, and its size affects handshake performance. RSA and ECDSA are the two algorithms you will actually encounter in certificates.

RSA: the universal standard

RSA 2048 is supported by virtually everything that speaks TLS, including 20-year-old devices. Its math is simple to audit and every CA, client and middlebox handles it. The cost is larger keys: an RSA signature and certificate are bigger, and handshakes send a few hundred extra bytes.

ECDSA: smaller and faster

ECDSA with the P-256 curve gives roughly the same security as RSA 3072 while using a 256-bit key. The certificate is smaller, the handshake is lighter and CPU cost is lower — a real win for high-traffic servers and mobile clients. The only catch: old clients and some middleboxes still misbehave with ECDSA certificates.

Compatibility in practice

All current browsers, operating systems, CDNs and IoT stacks support ECDSA certificates. Legacy devices — old Android, Windows 7 without updates, aging embedded systems — may not. If your analytics show any such traffic, keep an RSA 2048 certificate as a fallback via TLS certificate switching.

How to choose

Default to ECDSA P-256 for new certificates: better performance, smaller handshakes, broad support. If you must serve legacy clients, issue an RSA 2048 certificate as well and let the server pick the right one. Verify the result with our SSL Checker, which reports the negotiated key type.