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I need some guide to decide in which order I must put the following ciphers if I want prioritize security and in cases of tied, consider performance to decide.

ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
ECDH-RSA-AES256-SHA384  TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256)  Mac=SHA384

And what about this list?

DHE-DSS-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=DSS  Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA256
DHE-DSS-AES256-SHA256   TLSv1.2 Kx=DH       Au=DSS  Enc=AES(256)  Mac=SHA256

different list, I don't want mix them ;)

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You can already scratch half of them as you likely have a RSA certificate for which the ECDSA and DSS ciphers do not work. –  Lekensteyn Oct 4 '13 at 15:43
@Lekensteyn Thanks for your comment. Anyway in my case is possible that we go with a self signed certificate cos our client base allow that. Your comment make me ask another question ;) –  gsc-frank Oct 4 '13 at 17:22
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1 Answer

up vote 4 down vote accepted

Security: if you don't do stupid things like using a 512-bit RSA key, these cipher suites are all equally secure: they are all very far in the "cannot break it" zone. So that's a meh. You cannot say that one is more secure than any other.

With an exception though: the "ECDHE" suites use an ephemeral key pair for actual encryption; since the corresponding private key is never stored in a file, this grants a nifty property known as Perfect Forward Secrecy. Basically it makes communications immune to attackers who steal a copy of the server private key after the fact. PFS looks good in technical audits.

Performance issues exist only after having been duly measured. As Knuth said: Premature optimization is the root of all evil. So you should not ask such questions; you should try it out and measure. In any case, answer will depend a lot on the context: involved machines, bandwidth, usage patterns...

Short answer: won't matter.

Long answer:

The "GCM" cipher suites use GCM; the non-GCM cipher suites use AES in CBC mode and an extra HMAC (here, with SHA-384). Performance issues depend on the involved systems:

  • On small 32-bit systems (embedded ARM...), the MAC part of GCM will be expensive, but so will SHA-384 (because 64-bit computations...); I'd guess a tie.
  • On PC, AES cost will dominate;
  • ... except on very recent PC with AES-NI, where AES is very fast, and so is GCM.

So the GCM cipher suite should, usually, be a better bargain. However, it takes a lot of bandwidth, or a very small CPU, to actually notice the difference. Even without AES-NI, a normal server has enough juice to do SSL at full gigabit bandwidth, with CPU to spare.

For the asymmetric cryptography part:

  • The ECDHE suites imply on the server an (elliptic-curve) Diffie-Hellman and a signature for each full handshake, whereas the ECDH cipher suites require only the elliptic-curve Diffie-Hellman, and half of that one is already done.
  • ... But a normal PC will crunch through thousands of those per second and per code, so this very rarely matters.
  • ... Especially since normal SSL clients reuse SSL sessions, which means that the asymmetric cryptography occurs only for the first ever connection of the day from a given client.
  • ECDSA signatures are faster than RSA signatures.
  • ... Depending on key sizes, of course.
  • ... But for verification, this is the other way round.
  • ... And, again, it takes an awful lot of new clients per second for this to actually matter. A normal PC will do hundreds of 2048-bit RSA signatures per seconds, thousands of 256-bit ECDSA signatures per second.
  • ECDSA signatures are shorter than RSA signatures, so this saves a bit of network (but, again, only a few dozen bytes per full handshake).
  • DHE and ECDHE cipher suites also imply a few dozen extra bytes per full handshake.
  • DHE is like ECDHE but without the elliptic curves. You need it to use bigger mathematics to achieve the same security levels (2048-bit modulus instead of a 256-bit curve point), so it is like RSA vs ECDSA: a bit bigger, a bit slower, won't matter in practice.

So, really, you will not make a useful distinction based on security or even performance. In fact, you are already making a fashion statement, by insisting on AES-256 (instead of AES-128) and SHA-384 (instead of SHA-256). You may as well keep it on and bring it to its logical conclusion: use as much GCM and elliptic curves as possible ! This will grant brownie points from impressionable auditors.

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Thomas you are amazing! –  gsc-frank Oct 4 '13 at 16:52
+1 Very informational and in-depth answer, as always. Thanks Thomas! –  Lekensteyn Oct 4 '13 at 17:57
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