Short answer
ECDSA keys are much smaller and faster than RSA at the same security level: a 256-bit ECDSA key roughly equals a 3072-bit RSA key. ECDSA gives faster handshakes and lower CPU load, especially under heavy traffic. RSA wins on only one point: compatibility with very old clients. For most public servers the best answer is to serve both and let each client choose.
Head-to-head comparison
| RSA 2048 | ECDSA P-256 | |
|---|---|---|
| Security level | ~112-bit | ~128-bit |
| Private key size | Large | Small |
| Handshake CPU (server) | Higher | Lower |
| Signature verification (client) | Fast | Slightly slower |
| Compatibility, old clients | Broadest | Very broad, but not total |
| Best for | Maximum backward compatibility | Performance, new infrastructure |
Note that the table compares P-256 with RSA-2048 (the most widely used RSA size) — not with its own equivalent. P-256 is security-equivalent to RSA-3072, so the ECDSA column's ~128-bit sits above RSA-2048's ~112-bit.
Note the asymmetry: RSA is faster to verify but slower to sign, while ECDSA is faster to sign but slightly slower to verify. Since the server signs per handshake, ECDSA wins on the server's CPU load — usually what matters under load.
Generate one of each
# RSA 2048-bit openssl req -new -newkey rsa:2048 -nodes \ -keyout rsa.key -out rsa.csr -subj "/CN=example.com" # ECDSA on the P-256 curve (prime256v1) openssl ecparam -name prime256v1 -genkey -noout -out ec.key openssl req -new -key ec.key -out ec.csr -subj "/CN=example.com"
Note that prime256v1, secp256r1 and P-256 are three names for exactly the same curve. P-256 and P-384 are the only curves with genuinely broad support in TLS.
What does the certificate have?
openssl x509 -in cert.pem -noout -text | grep -A2 "Public Key Algorithm"
For RSA you see rsaEncryption and a key size in bits. For ECDSA you see id-ecPublicKey and a curve such as prime256v1. The key type determines the Au= part of the cipher suites the server can use — see what a cipher suite is.
The right answer: serve both (dual-cert)
Modern servers can have both an ECDSA and an RSA certificate installed at the same time. The server presents ECDSA to clients that support it (the vast majority today) and falls back to RSA for the old ones. You get ECDSA's performance without losing compatibility:
# nginx — two certificates on the same server ssl_certificate /etc/ssl/example.com/ecdsa-fullchain.pem; ssl_certificate_key /etc/ssl/example.com/ecdsa.key; ssl_certificate /etc/ssl/example.com/rsa-fullchain.pem; ssl_certificate_key /etc/ssl/example.com/rsa.key;
Why ECDSA keys are so much smaller
The difference comes from the underlying mathematical problems. RSA's security rests on the difficulty of factoring large numbers, and that problem only grows slowly harder as the numbers grow — which is why RSA keys must be large (2048, 3072, 4096 bit) to resist modern attacks. ECDSA rests on the elliptic-curve discrete logarithm problem, which is much harder per bit. The result: a 256-bit ECDSA key gives the same security as a 3072-bit RSA key. This is not just a file-size detail — smaller keys mean less data in every handshake, faster signing and lower memory use, which matters especially on mobile clients and under a high connection rate.
Practical considerations when migrating
If you migrate from RSA to ECDSA, do it additively, not as a hard cutover. Add an ECDSA certificate alongside the existing RSA one (dual-cert above), verify that clients negotiate ECDSA, and keep RSA as a fallback until you are confident no critical clients drop out. Remember the whole chain must match the key type: an ECDSA leaf must chain to an ECDSA intermediate from the CA. Never mix leaf and intermediate across key types — it produces a chain that cannot be completed.
What about security?
Both are secure when used correctly. The real cryptographic level of RSA-2048 (~112-bit) is lower than ECDSA P-256 (~128-bit), but both sit far above what can practically be broken today. RSA's biggest practical risk is people using 1024-bit keys (obsolete) or poor randomness during key generation; ECDSA's is a weak randomness source during signing. With proper libraries neither is an issue.
How CertControl sees it across your fleet
In practice organisations end up with a mix: new servers on ECDSA, older ones on RSA-2048, and a few forgotten ones on RSA-1024. CertControl records the key type and key size on every certificate it finds, and raises a finding on the weak or obsolete ones — so you can migrate systematically rather than discover an RSA-1024 key during an audit. Read more about what a certificate contains in what is a TLS certificate.
Frequently asked questions
Is ECDSA more secure than RSA?
At comparable key sizes ECDSA gives a higher security level per bit — P-256 equals RSA-3072. But both are secure in practice at recommended sizes. The difference is mostly about performance and key size, not real break risk.
Which is faster?
ECDSA gives lower CPU load on the server per handshake, because signing is cheaper. That matters most under high load. RSA verifies slightly faster on the client, but that is rarely the bottleneck.
Can I use both RSA and ECDSA on the same domain?
Yes, and it is often the best option. With a dual-certificate setup you serve both, and each client negotiates the key type it supports — ECDSA for most, RSA as a fallback.
Which ECDSA curve should I choose?
P-256 (prime256v1) for the vast majority; P-384 if a compliance requirement demands a higher level. Other curves are not supported widely enough in TLS for public use.
Should I still use RSA-2048 or move to 3072/4096?
RSA-2048 remains acceptable for most purposes. 3072 gives a larger margin at the cost of performance; 4096 is rarely necessary and noticeably slower. If you want a higher level, ECDSA P-256 is often a better choice than large RSA.