Certificate Public Key Algorithm Explained

The public key algorithm field in an SSL certificate specifies the cryptographic algorithm used for the certificate's public key — most commonly RSA or EC (Elliptic Curve). The public key is what clients use to verify the CA's signature on the certificate and to establish the TLS session.

RSA Keys

RSA (Rivest–Shamir–Adleman) is the traditional public key algorithm for TLS certificates. Key sizes:

• 1024-bit: Deprecated. No longer accepted by modern CAs or browsers.
• 2048-bit: Current minimum. Still widely deployed. Considered safe until approximately 2030.
• 4096-bit: Higher security margin, but significantly slower TLS handshake. Rarely necessary for web certificates.

EC (Elliptic Curve) Keys

EC keys provide equivalent security to RSA with much smaller key sizes, resulting in faster TLS handshakes and lower CPU overhead:

• P-256 (prime256v1 / secp256r1): 256-bit key, equivalent security to ~3072-bit RSA. The most common EC curve in TLS certificates. Universally supported.
• P-384 (secp384r1): 384-bit key, equivalent to ~7680-bit RSA. Used where higher assurance is required (government, financial).
• P-521: Rarely used in TLS.

Which to Choose

For most web servers, P-256 ECDSA is the best choice: smaller, faster, and just as secure as 2048-bit RSA. RSA 2048-bit is a solid fallback for compatibility with very old clients (ancient mobile browsers, embedded systems). Modern servers often serve ECDSA certificates and fall back to RSA for older clients.

Reading Key Info in the Decoder

The Public Key section in the decoder shows the algorithm (RSA or EC), key size in bits, and — for EC keys — the curve name. The Signature Algorithm field shows what algorithm the CA used to sign the certificate itself, which may be different from the leaf cert's key type (e.g., a CA with an RSA root can sign an ECDSA leaf certificate).