cswiger at mac.com
Sat Mar 7 00:28:24 UTC 2015
> 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
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.
More information about the freebsd-questions