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duraconf - A collection of hardened configuration files for SSL/TLS services http://www.appelbaum.net/
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duraconf - A collection of hardened configuration files for SSL/TLS services Hopefully this will help you make a more informed choice about what cipher list should be used for different applications. What you find here are recommended configurations, you should seriously consider using these, but you have to make some choices. When you pick a cipher list, you have a couple different options of how you go about it: 1. make a very specific declaration of what is acceptable. This has the advantage of being able to define very closely of what you want, but the disadvantage of having to stay on top of the latest crypto advancements, with every crypto library upgrade. 2. make a general declaration of which cipher list to use. this has the advantage of allowing you to rely on your crypto libraries to make (hopefully) informed choices for you (and to deactivate known bad/weak/recently broken) ciphers while you don't have the burden of ensuring that they are always resulting in a good cipher suite. The disadvantage is that you cannot fine tune what exactly you get in return. 3. A mixture of being specific and letting your crypto library decide from general statements. This can be useful if, for example, you find out that some particular crypto has become too weak, for example you might use a generic list but then exclude MD5, because your crypto libraries haven't removed that yet. 4. Decide on a threat model for possible attacks that may expose an important private key. Ciphers are often offered in a mode that provides Perfect Forward Secrecy. While there are performance considerations, if you run a high security operation where traffic disclosure would be a serious problem, it is an important property to consider. Generally it seems safer to have the crypto library take the bulk of the decision since it should be for the most part fire-and-forget, while the other options require that you always stay up to date on things and tweak as needed. For practical use, and for people who can afford to follow crypto news, a mixture of both is surely a good idea. So start with the general cipher list and when you become aware that something is bad then just add this specific part to your otherwise general cipher list until the crypto library defaults get updated to fix that. Unfortunately, its not possible to come up with one cipher configuration that is going to work for all configurations. There are many different programs that implement different versions of libraries that have different ciphers available. In fact, a different versions of the same program may be linked against different libraries which have different ciphers available. An important configuration issue for service operators and users is understanding Perfect Forward Secrecy. Generally, PFS sessions are computationally more expensive than connections without PFS properties. It is extremely important to remember that using SSL and/or TLS does not ensure that your traffic is encrypted for all time. Generally, SSL/TLS services offer two general modes of operation - one mode is ephemerally keyed and the other is not. A TLS server that only offers AES256-SHA is strong against an attacker who will never recover the secret key used by the server and cannot break AES256. However, if an attacker is able to recover the server's key, the attacker will be able to retroactively decrypt all traffic that has been recorded where the AES256-SHA cipher was used. If that same server uses an ephemeral cipher such as DHE-RSA-AES256-SHA, the attacker cannot recover previous encrypted sesssions without breaking RSA and/or AES256 for *each* session. In both cases, when the attacker has the private key, all future communications with the server are unsafe. Clients generally deal with this by looking up a revokation list or by using something like the OCSP. Realistically, they're in a lot of trouble and that kind of trouble is out of scope. If you're in doubt it's probably a reasonable thing to use DHE or EDH modes unless you have load issues. The cipher lists you will find here actually vary depending on which version of the crypto library that you have. For example, if you were to find this list recommended: HIGH:MEDIUM:!aNULL:!SSLv2:!MD5:@STRENGTH In one version of openssl this will mean the following list of ciphers: $ openssl ciphers -v 'HIGH:MEDIUM:!aNULL:!SSLv2:!MD5:@STRENGTH' 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 EDH-RSA-DES-CBC3-SHA SSLv3 Kx=DH Au=RSA Enc=3DES(168) Mac=SHA1 EDH-DSS-DES-CBC3-SHA SSLv3 Kx=DH Au=DSS Enc=3DES(168) Mac=SHA1 DES-CBC3-SHA SSLv3 Kx=RSA Au=RSA Enc=3DES(168) 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 In a newer openssl, this list of ciphers will be different: $ openssl ciphers -v 'HIGH:MEDIUM:!aNULL:!SSLv2:!MD5:@STRENGTH' ECDH-RSA-AES128-SHA SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(128) Mac=SHA1 ECDH-ECDSA-AES128-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(128) Mac=SHA1 AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1 CAMELLIA128-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(128) Mac=SHA1 PSK-AES128-CBC-SHA SSLv3 Kx=PSK Au=PSK Enc=AES(128) Mac=SHA1 It is also worth noting that this is setting a policy, and your site may have different policies, depending on your intended audience. There are many questions to consider in determining a policy. For example, in the worst case, when a client doesn't support the higher strength ciphers my server supports, do I want to keep up the image that medium strength ciphers are secure enough in my specific use case, environment and opponents? Or should I not allow anything but the highest strength ciphers, and those clients that do not support them are just denied? Its likely that in many cases there is no possibility of making it clear to the user that their setup does not allow for secure use of your services, and what their options are. I _think_, at least with apache, it should be possible to redirect users whose setup doesn't provide a compatible cipher suite, to an informational web page which explains further steps they can and should take (i have no idea how).. Unfortunately, in most cases, users will not get any message at all and they will have no clue why they are shut out. This could result in unhappy users with no idea of where to turn, and potentially a higher support burden. Notes on format of cipher designations -------------------------------------- Format of cipher designations differ, but in general they follow the format described in ciphers(5). A few notes: The order specified is the preference order, and the list is separated by colons. The list can be specific ciphers (eg. RC4-SHA), a list of suites containing a certain algorith (SHA1), or a cipher suite of a certain type (TLSv1). There are also cipher strings which are a grouping of different ciphers into a specific category (eg. HIGH). When removing ciphers that you do not want, you have a choice between indicating ! or -, the difference is subtle but important. It's good practice to use ! if you really do not want this class to ever get used, and to use - when you want to allow them to be still used if you later added something to your existing cipher list. Finally, there is also the @STRENGTH parameter, which sorts the cipher list in order of encryption algorithm key length. Suggested reading ----------------- https://www.eff.org/pages/how-deploy-https-correctly