OpenSSL Ciphers

[Doug Hardie]

> On 6 March 2015, at 17:35, dweimer <dweimer at dweimer.net> wrote:
> 
> On 03/06/2015 6:36 pm, Doug Hardie wrote:
>>> On 6 March 2015, at 16:28, Charles Swiger <cswiger at mac.com> wrote:
>>> Hi--
>>>> On Mar 6, 2015, at 3:58 PM, Doug Hardie <bc979 at lafn.org> wrote:
>>>>> On 3 March 2015, at 23:21, Doug Hardie <bc979 at lafn.org> wrote:
>>>>> The default list of ciphers is quite extensive and includes some that are apparently causing some potential security issues.  I have a number of applications that use OpenSSL and many don’t have the code to restrict the list.  Fixing all that would take quite a bit of work.  However, looking into /usr/include/openssl/ssl.h I find a definition for the SSL_DEFAULT_CIPHER_LIST.  The comments indicate that that list is the one used when the application doesn’t specify anything.  I changed its definition to:
>>>>> #define SSL_DEFAULT_CIPHER_LIST "TLSv1+HIGH:!SSLv2:RC4+MEDIUM:!aNULL:!eNULL:!3DES:@STRENGTH:
>>>>> However, s_connect will still create a connection with the export ciphers.  I tried adding !EXPORT to that list and it had no effect.  Is the definition actually used by openssl or is it just there for documentation?
>>>> Not hearing anything on this, I suspect it’s not very well understood.  I have started updating the various servers/clients that use SSL/TLS.  The one that has me completely stumped is sendmail.  There is a web page which provides instructions "http://novosial.org/sendmail/cipherlist/index.html”.  However, when I follow them, I can still establish a connection and deliver mail using the export ciphers.
>>>> Has anyone successfully restricted the sendmail ciphers?
>>> You can see which ciphers openssl will support via a statement like:
>>> % openssl ciphers -v 'TLSv1+HIGH:RC4+MEDIUM:!aNULL:!eNULL:!3DES:@STRENGTH:!EXPORT'
>>> DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA1
>>> DHE-DSS-AES256-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(256)  Mac=SHA1
>>> AES256-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(256)  Mac=SHA1
>>> DHE-RSA-AES128-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA1
>>> DHE-DSS-AES128-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(128)  Mac=SHA1
>>> AES128-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(128)  Mac=SHA1
>>> RC4-SHA                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=SHA1
>>> RC4-MD5                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=MD5
>>> RC4-MD5                 SSLv2 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=MD5
>>> ...and you can experiment with TLS negotiation results via something like:
>>> % openssl s_client -cipher 'AES256-SHA:AES128-SHA' -connect www.google.com:443
>>> [ ... ]
>>> New, TLSv1/SSLv3, Cipher is AES128-SHA
>>> Server public key is 2048 bit
>>> Secure Renegotiation IS supported
>>> Compression: NONE
>>> Expansion: NONE
>>> SSL-Session:
>>>   Protocol  : TLSv1
>>>   Cipher    : AES128-SHA
>>>   Session-ID: [ ... ]
>>> Sendmail normally performs crypto via STARTTLS negotiation rather than via SMTPS; there's a CipherList option which can be defined via sendmail.mc / sendmail.cf.  You might need to recompile sendmail with -D_FFR_TLS_1, which I think that novosial page mentions.
>> sendmail has _FFR_TLS_1 compiled in per th tests in the web page
>> mentioned above.  The CipherList option doesn’t seem to work.  I can
>> connect and send mail with that in place using the EXPORT ciphers.
> 
> Doug,
>  I have this added to my /etc/mail/{HOSTNAME}.mc file.
> 
> LOCAL_CONFIG
> O CipherList=ALL:!aNULL:!eNULL:!LOW:!EXP:RC4+RSA:+HIGH:+MEDIUM
> 
> Of course you can use other options, this has been there for a while in mine, carried over from some time a few versions back. Probably should get around to testing it to make sure it actually is still working. It doesn't take long to add it in and run a quick test.


As I replied earlier, I have done that.  I can still use:

openssl s_client -connect localhost:25 -starttls smtp -cipher EXPORT

It will establish the connection and I can send mail.  The details show that one of the export ciphers is being used.  I have tested this on FreeBSD 9.3 and 8.2.