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RSALabs recommend using at least 4096 bit long RSA keys to encrypt emails but we have some performance difficulty with 4096 RSA. my noob question is what is ECDH (i didn't understood from wikipedia formulas how it works)? ECDH is the RSA but also we add a symmetric secret to our private key ?

can ECDH work like RSA to upload a public key on a key server for people to encrypt their outgoing emails to me? ECDH provide good security like very long RSA keys ? how its performance compared to plain RSA ?

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    Don't use a different nick for each question please.
    – CodesInChaos
    Dec 7 '12 at 13:19
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    There are multiple questions here. (1) what is ECDH? (2) What is the user experience with ECDH, (3) what is the performance of ECDH? (4) What is the security of ECDH. The last two depend more on implementation than on theory. Please clarify which question you want answered.
    – MCW
    Dec 7 '12 at 13:36
  • Pier - can you please clarify your question, as per Mark's comment. I'll reopen once this becomes a clear security question. Also, if you register and login, you can use the same username each time.
    – Rory Alsop ♦
    Dec 7 '12 at 15:09
  • @MarkC.Wallace The performance characteristics of ECDH are inherently different from RSA. In particular key generation is much fast, and an ECDH key-exchange is cheaper than an RSA private key operation. I believe all the questions are answerable.
    – CodesInChaos
    Dec 7 '12 at 15:23
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I'm quite confused by the question, but I'm going to take my best shot. Aside: Normally I strongly dislike this approach, but intuition tells me that nagging the OP to clarify questions that s/he admits s/he doesn't fully understand is a worse violation of SEC:SE principles.

RSA, and all public key encryption, is very processor intensive. I'm not surprised that you have performance problems with a strong RSA key. Symmetric key encryption is less processor intensive, but involves a fairly complex key management overhead, which is usually at least partially manual. Manual key management is painful.

ECDH is a hybrid solution. Perform encryption/decryption with a symmetric key (Good performance), and use the public keys to generate and manage session keys (solid key management with high automation, low overhead and low pain threshold). Best of both worlds; you get the performance of symmetric keys without the key management overhead.

That is the 50,000 meter theory; if SEC:SE were a technical discussion site we could delve into the details.

The second set of questions is much more problemmatic. Bruce Schneier has a pithy quote that I can't call to mind right now to the effect that 90% of the problems with crypto are not in the design, but in the implementation. How much performance improvement will you encounter? Depends on the implementation? How strong is the encryption? Depends on the implementation.

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  • In practice both RSA and ECDH are used in a hybrid scheme. The difference is that a private key operation with ECDH is cheaper at a high security level than for RSA.
    – CodesInChaos
    Dec 7 '12 at 13:47
  • Mark your answer confuse me. its always a session key (like pgp) in RSA only or ECDH because we never can use RSA for whole message , it was always session key as i know. my question is if we use RSA 4096 it is secure but very slow. question is does ECDH solve this problem ? it can make smaller keys for better performance and provide more security than larger keys ? i hope you understand what i want ..
    – pier
    Dec 7 '12 at 13:58

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