RSA 2048 or ECDSA P-256 for your leaf certificate: where does the difference actually land?

RSA 2048 or ECDSA P-256 for your leaf certificate: where does the difference actually land?

Which key type should sign your TLS handshakes? The honest answer depends on which operation dominates your load.

ECDSA (Elliptic Curve Digital Signature Algorithm) with P-256 gives roughly 128-bit security in a 256-bit key. RSA needs 3072 bits to match that, though 2048 (~112-bit) remains the common floor. The asymmetry in cost is the interesting part.

On the server, signing is what you do per handshake. ECDSA P-256 signing is dramatically cheaper than RSA-2048 signing — often an order of magnitude in raw ops — so a busy terminator favors ECDSA. But RSA verification is cheaper than RSA signing, and clients verify, so the cost lands differently on each side.

The practical constraint is compatibility. Ancient clients lacking ECDSA support are now rare, but if you serve them, dual-certificate deployment lets you present ECDSA to modern clients and RSA to stragglers, selected via the signature_algorithms extension.

— Default to ECDSA P-256 for new deployments: smaller, faster handshakes.
— Keep RSA only as a fallback leaf for legacy reach.
— Avoid P-384 unless a policy demands it; the cost rarely buys you anything.

Further reading: RFC 8446 §4.2.3; NIST SP 800-57 Part 1 for key-strength equivalence.

Bottom line: ECDSA is the modern default; RSA survives as a compatibility shim, not a security upgrade.