[openssl-users] Security of DH in TLS

[Paul Yang]

> On 26 Jul 2017, at 16:21, SaAtomic <saatomic at keemail.me> wrote:
> 
> The subject is much clearer to me now, thank you.
> 
> The EC key you mentioned is not created manually, correct?
> This key is a result of ECC, which is done by OpenSSL.
> 
> So if I set up a server offering TLS connections and only offer ECDH/ECDHE, no additional data has to be generated manually, correct?

Ahh, that depends on how you use ECC in TLS. If you use something like ‘ECDHE-RSA-XXX-YYY’, you don’t need to manipulate the ECC stuff ‘manually’, since the authentication part of that cipher suite is RSA, just preparing an RSA key/certificate is enough. But if you want to use stuffs like ‘ECDHE-ECDSA-XXX-YYY’, you need to prepare ECC private key (and also the corresponding certificate that encodes the ECC public key). For ECDH ciphers (without the ‘E’ part), that’s more confused, IIRC, in such case the authentication part of that kind ciphers indicates what signature algorithm is used to sign the certificate, but not the ‘public-key-type’ in the certificate, so anyway you probably need to get a ECC key/certificate pair. But the without-‘E’ version of such usage seems rare, I have never meet one in production environment...

And also, don’t mix up the ECC keys used in a `key exchange’ phase during TLS handshake with the keys used in ‘auth’ phase. I suggest you to read the following RFC documentation to get clear understandings on this: https://www.ietf.org/rfc/rfc4492.txt <https://www.ietf.org/rfc/rfc4492.txt>

Hope it helps : )

> 
> Kind regards,
> SaAtomic
> 
> 
> 26. Jul 2017 10:14 by paulyang.inf at gmail.com <mailto:paulyang.inf at gmail.com>:
> 
> On 26 Jul 2017, at 15:56, SaAtomic <saatomic at keemail.me <mailto:saatomic at keemail.me>> wrote:
> 
> Thanks for the reply.
> I'm still not sure I understand this correctly. 
> 
> So the length of modulus is the essential part, determining the security of the DH, right?
> 
> Mostly.
> With ECC, this is defined by the used curves.
> Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
> 
> If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed.
> 
> Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
> 
> Is this correct?
> 
> Thank you for your help,
> kind regards,
> SaAtomic
> 
> ---------
> > Paul Yang paulyang.inf at gmail.com
> > Wed Jul 26 07:19:31 UTC 2017
> > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area)
> > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations...
> > 
> > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’:
> > 
> > openssl pkey -in xyz.key -noout -text
> > 
> > check the Private-Key: (xxxx bit) in the output.

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