Why is using ECDHE_ECDSA stronger than using RSA?

The primary reason why using ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) with ECDSA (Elliptic Curve Digital Signature Algorithm) is considered stronger than using RSA, particularly in the context of TLS (Transport Layer Security), is:

• If the server’s private key is later compromised, all the prior TLS handshakes that are done using the cipher suite cannot be compromised.

This is due to the property known as “forward secrecy” provided by ECDHE. With ECDHE, each session has its own unique set of keys, which are not derived from the server’s long-term private key. Therefore, even if the server’s private key is compromised at a later date, previous encrypted communications remain secure because the session keys cannot be retroactively calculated.

The other options provided are not accurate because:

• ECDHE_ECDSA does provide data authenticity (through ECDSA) and confidentiality (through the ephemeral keys generated by ECDHE), but this is not a point of comparison with RSA, which can also be used to provide these security properties.
• ECDHE_ECDSA does not necessarily use a much larger key size. In fact, one of the advantages of elliptic curve cryptography is that it can provide the same level of security as RSA with a much smaller key size.
• The use of a pseudorandom function to generate keying material is not unique to ECDHE_ECDSA and is a common practice in various cryptographic protocols, including those that use RSA.