[openssl-users] Diffie-Hellman Questions

Norm Green norm.green at gemtalksystems.com
Tue May 24 17:33:42 UTC 2016

Thanks Rich.  More newbie questions.

Looking at the available ciphers I see this:

 >./openssl ciphers -v 'ALL:aNULL' |grep ECDH |grep "Au=None"
AECDH-AES256-SHA        SSLv3 Kx=ECDH     Au=None Enc=AES(256)  Mac=SHA1
AECDH-AES128-SHA        SSLv3 Kx=ECDH     Au=None Enc=AES(128)  Mac=SHA1
AECDH-RC4-SHA           SSLv3 Kx=ECDH     Au=None Enc=RC4(128)  Mac=SHA1
AECDH-DES-CBC3-SHA      SSLv3 Kx=ECDH     Au=None Enc=3DES(168) Mac=SHA1
AECDH-NULL-SHA          SSLv3 Kx=ECDH     Au=None Enc=None      Mac=SHA1

1) What arg to SSL_CTX_set_cipher_list() to I need to use to get these?  
I previously tried "kEECDH:kEDH" and that didn't work.

2) These ciphers all report as SSLv3.   Do I have to use SSLv3 
client/server methods to get access to these ciphers?  I was using TLS 
1.2 (TLSv1_2_server_method()) methods.

Norm Green

On 5/24/16 10:08, Salz, Rich wrote:
>> 1) The wiki says don't use ADH, presumably because ADH provides
>> encryption but not authentication and is exposed to man in the middle
>> attacks. Is that the only reason?
> Use ECDH, it's less expensive computationally.
>> 2) Are the same encryption keys used every time with ADH?
> Yes.  That's the other BIG reason :)  You really want ephemeral, and therefore ECDH
>> 3) Is it possible to use ephemeral DH without using certificates?  I was not
>> able to get that to work.
> Yes.  This is "null" auth.
>> 4) What is the best practice for establishing an anonymous encrypted
>> channel using OpenSSL?
> Postfix does this kind of thing, as does other SMTP software.  Look around for 'opportunistic encryption' perhaps.

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