1 \section{Cipher suites}
4 \subsection{Recommended cipher suites}
6 In principle, system administrators who want to improve their servers need to
7 make a hard decision between locking out some users while keeping very high
8 cipher suite security levels or supporting as many users as possible while
9 lowering some settings. \url{https://www.ssllabs.com/} gives administrators a
10 tool to test out different settings. The authors used ssllabs.com to arrive at
11 a set of cipher suites which we will recommend throught this document.
12 \textbf{Caution: these settings can only represent a subjective choice of the
13 authors at the time of this writing. It might be a wise choice to select your
14 own ciper suites based on the instructions in section
15 \ref{section:ChosingYourOwnCipherSuites}}.
18 \subsubsection{Configuration A: strong ciphers, fewer clients}
20 At the time of this writing, we recommend the following set of strong cipher
21 suites which may be useful in an environment where you do not depend on many,
22 diverse external clients and where compatibility is not an issue. An example
23 of such an environment might be machine 2 machine communications or corporate
24 environments where you can define the software which must be used.
27 We arrived at this set of cipher suites by selecting
31 \item Perfect forward secrecy / ephemeral Diffie Hellman
32 \item strong Hashes (SHA-2)
33 \item GCM as chaining mode if possible
36 This results in the string:
38 \begin{lstlisting}[breaklines]
39 'EECDH+aRSA+AES256:EDH+aRSA+AES256:!SSLv3'
42 %$\implies$ resolves to
45 %openssl ciphers -V $string
51 \begin{tabular}{| l | l | l | l | l| l | l |}
53 ID & OpenSSL name & Version & KeyEx & Auth & Cipher & Hash \\ \hline
54 0xC030 & ECDHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & ECDH & RSA &AESGCM(256) & AEAD \\ \hline
55 0xC028 & ECDHE-RSA-AES256-SHA384 & TLSv1.2 & ECDH & RSA &AES(256) & SHA384 \\ \hline
56 0x009F & DHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & DH & RSA &AESGCM(256) & AEAD \\ \hline
57 0x006B & DHE-RSA-AES256-SHA256 & TLSv1.2 & DH & RSA &AES(256) & SHA256 \\ \hline
62 \textbf{Compatibility}
64 Only clients which support TLS1.2 are covered by this cipher suites (Chrome 30,
65 Win 7 and Win 8.1 crypto stack, Opera 17, OpenSSL $\ge$ 1.0.1e, Safari 6 / iOS
66 6.0.1, Safari 7 / OS X 10.9).
70 \subsubsection{Configuration B: weaker ciphers, many clients}
72 In this section we propose a slighly "weaker" set of cipher suites. There are
73 some known weaknesses of for example SHA-1 which is included in this this set.
74 However, the advantage of this set of cipher suites is its wider compatibility
78 \textbf{In the following document, all further examples in this paper will use Configuration B}.
81 We arrived at this set of cipher suites by selecting
84 \item TLS 1.2, TLS 1.1, TLS 1.0
88 This results in the string:
90 \begin{lstlisting}[breaklines]
91 'EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EDH+CAMELLIA256:EECDH:EDH+aRSA:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:!AES128:!CAMELLIA128:!ECDSA:AES256-SHA'
97 \begin{tabular}{| l | l | l | l | l| l | l |}
99 ID & OpenSSL name & Version & KeyEx & Auth & Cipher & Hash \\ \hline
100 0xC030 & ECDHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & ECDH & RSA &AESGCM(256) & AEAD \\ \hline
101 0xC028 & ECDHE-RSA-AES256-SHA384 & TLSv1.2 & ECDH & RSA &AES(256) & SHA384 \\ \hline
102 0x009F & DHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & DH & RSA &AESGCM(256) & AEAD \\ \hline
103 0x006B & DHE-RSA-AES256-SHA256 & TLSv1.2 & DH & RSA &AES(256) & SHA256 \\ \hline
104 0x0088 & DHE-RSA-CAMELLIA256-SHA & SSLv3 & DH & RSA &Camellia(256)& SHA1 \\ \hline
105 0xC014 & ECDHE-RSA-AES256-SHA & SSLv3 & ECDH & RSA &AES(256) & SHA1 \\ \hline
106 0x0039 & DHE-RSA-AES256-SHA & SSLv3 & DH & RSA &AES(256) & SHA1 \\ \hline
107 0x0035 & AES256-SHA & SSLv3 & RSA & RSA &AES(256) & SHA1 \\ \hline
112 \textbf{Compatibility}
114 Note that this cipher suites will not work with anything using Windows XP's
115 crypto stack (IE, Outlook), Java 6, Java 7 and Android 2.3. Java 7 could be
116 made compatible by installing the "Java Cryptography Extension (JCE) Unlimited
117 Strength Jurisdiction Policy Files" (JCE). We could not verify yet if
118 installing JCE also fixes the Java 7 DH-parameter length limitation (1024 bit).
122 \subsection{Chosing your own cipher suites}
123 \label{section:ChosingYourOwnCipherSuites}
124 \todo{ Adi... you want to describe how to make your own selection of cipher suites here. The text below was simply the old text, still left here for reference.}
126 %%% NOTE: we do not need to list this all here, can move to an appendix
127 %At the time of this writing, SSL is defined in RFCs:
130 %\item RFC2246 - TLS1.0
132 %\item RFC4132 - Camelia
133 %\item RFC4162 - SEED
135 %\item RFC4346 - TLS 1.1
137 %\item RFC4785 - PSK\_NULL
138 %\item RFC5246 - TLS 1.2
139 %\item RFC5288 - AES\_GCM
140 %\item RFC5289 - AES\_GCM\_SHA2\_ECC
141 %\item RFC5430 - Suite B
142 %\item RFC5487 - GCM\_PSK
143 %\item RFC5489 - ECDHE\_PSK
144 %\item RFC5932 - Camelia
145 %\item RFC6101 - SSL 3.0
146 %\item RFC6209 - ARIA
147 %\item RFC6367 - Camelia
148 %\item RFC6655 - AES\_CCM
149 %\item RFC7027 - Brainpool Curves
152 \subsubsection{Overview of SSL Server settings}
155 Most Server software (Webservers, Mail servers, etc.) can be configured to prefer certain cipher suites over others.
156 We followed the recommendations by Ivan Ristic's SSL/TLS Deployment Best Practices\footnote{\url{https://www.ssllabs.com/projects/best-practices/index.html}} document (see section 2.2 "Use Secure Protocols") and arrived at a list of recommended cipher suites for SSL enabled servers.
158 Following Ivan Ristic's adivce we arrived at a categorisation of cipher suites.
161 \begin{tabular}{| l | l | l | l | l|}
163 & Version & Key\_Exchange & Cipher & MAC \\ \hline
164 \cellcolor{green}prefer & TLS 1.2 & DHE\_DSS & AES\_256\_GCM & SHA384 \\ \hline
165 & & DHE\_RSA & AES\_256\_CCM & SHA256 \\ \hline
166 & & ECDHE\_ECDSA & AES\_256\_CBC & \\ \hline
167 & & ECDHE\_RSA & & \\ \hline
169 \cellcolor{orange}consider & TLS 1.1 & DH\_DSS & AES\_128\_GCM & SHA \\ \hline
170 & TLS 1.0 & DH\_RSA & AES\_128\_CCM & \\ \hline
171 & & ECDH\_ECDSA & AES\_128\_CBC & \\ \hline
172 & & ECDH\_RSA & CAMELLIA\_256\_CBC & \\ \hline
173 & & RSA & CAMELLIA\_128\_CBC & \\ \hline
176 & SSL 3.0 & NULL & NULL & NULL \\ \hline
177 & & DH\_anon & RC4\_128 & MD5 \\ \hline
178 & & ECDH\_anon & 3DES\_EDE\_CBC & \\ \hline
179 & & & DES\_CBC & \\ \hline
181 \cellcolor{blue}{\color{white}special }
182 & & PSK & CAMELLIA\_256\_GCM & \\ \hline
183 & & DHE\_PSK & CAMELLIA\_128\_GCM & \\ \hline
184 & & RSA\_PSK & ARIA\_256\_GCM & \\ \hline
185 & & ECDHE\_PSK & ARIA\_256\_CBC & \\ \hline
186 & & & ARIA\_128\_GCM & \\ \hline
187 & & & ARIA\_128\_CBC & \\ \hline
188 & & & SEED & \\ \hline
192 A remark on the ``consider'' section: the BSI (Federal office for information security, Germany) recommends in its technical report TR-02102-2\footnote{\url{https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR02102/BSI-TR-02102-2_pdf.html}} to \textbf{avoid} non-ephemeral\footnote{ephemeral keys are session keys which are destroyed upon termination of the encrypted session. In TLS/SSL, they are realized by the DHE cipher suites. } keys for any communication which might contain personal or sensitive data. In this document, we follow BSI's advice and therefore only keep cipher suites containing (EC)DH\textbf{E} (ephemeral) variants. System administrators, who can not use forward secrecy can still use the cipher suites in the ``consider'' section. We however, do not recommend them in this document.
194 %% NOTE: s/forward secrecy/perfect forward secrecy???
196 Note that the entries marked as ``special'' are cipher suites which are not common to all clients (webbrowsers etc).
199 \subsubsection{Tested clients}
201 Next we tested the cipher suites above on the following clients:
203 %% NOTE: we need to test with more systems!!
205 \item Chrome 30.0.1599.101 Mac OS X 10.9
206 \item Safari 7.0 Mac OS X 10.9
207 \item Firefox 25.0 Mac OS X 10.9
208 \item Internet Explorer 10 Windows 7
209 \item Apple iOS 7.0.3
213 The result of testing the cipher suites with these clients gives us a preference order as shown in table \ref{table:prefOrderCipherSuites}.
214 Should a client not be able to use a specific cipher suite, it will fall back to the next possible entry as given by the ordering.
219 \begin{tabular}{|l|l|l|l|l|}
221 Pref & Cipher Suite & ID & Browser \\ \hline
222 1 & TLS\_DHE\_RSA\_WITH\_AES\_256\_GCM\_SHA384 & 0x009f & OpenSSL command line client \\ \hline
223 2 & TLS\_ECDHE\_ECDSA\_WITH\_AES\_256\_CBC\_SHA384 & 0xC024 & Safari \\ \hline
224 3 & TLS\_ECDHE\_RSA\_WITH\_AES\_256\_CBC\_SHA384 & 0xC028 & Safari \\ \hline
225 4 & TLS\_DHE\_RSA\_WITH\_AES\_256\_CBC\_SHA256 & 0x006B & Safari, Chrome \\ \hline
226 5 & TLS\_ECDHE\_ECDSA\_WITH\_AES\_256\_CBC\_SHA & 0xC00A & Safari, Chrome, Firefox, IE \\ \hline
227 6 & TLS\_ECDHE\_RSA\_WITH\_AES\_256\_CBC\_SHA & 0xC014 & Safari, Chrome, Firefox, IE \\ \hline
228 7 & TLS\_DHE\_RSA\_WITH\_AES\_256\_CBC\_SHA & 0x0039 & Safari, Chrome, Firefox \\ \hline
229 8 & TLS\_DHE\_DSS\_WITH\_AES\_256\_CBC\_SHA & 0x0038 & Firefox, IE \\ \hline
230 9 & TLS\_DHE\_RSA\_WITH\_CAMELLIA\_256\_CBC\_SHA & 0x0088 & Firefox \\ \hline
231 10 & TLS\_DHE\_DSS\_WITH\_CAMELLIA\_256\_CBC\_SHA & 0x0087 & Firefox \\ \hline
233 \caption{Preference order of cipher suites}
234 \label{table:prefOrderCipherSuites}
239 Table \ref{table:prefOrderOpenSSLNames} shows the same data again with specifying the corresponding OpenSSL name.
244 \begin{tabular}{|l|l|l|}
246 Cipher Suite & ID & OpenSSL Name \\ \hline
247 TLS\_DHE\_RSA\_WITH\_AES\_256\_GCM\_SHA384 & 0x009f & DHE-RSA-AES256-GCM-SHA384 \\ \hline
248 TLS\_ECDHE\_ECDSA\_WITH\_AES\_256\_CBC\_SHA384 & 0xC024 & ECDHE-ECDSA-AES256-SHA384 \\ \hline
249 TLS\_ECDHE\_RSA\_WITH\_AES\_256\_CBC\_SHA384 & 0xC028 & ECDHE-RSA-AES256-SHA384 \\ \hline
250 TLS\_DHE\_RSA\_WITH\_AES\_256\_CBC\_SHA256 & 0x006B & DHE-RSA-AES256-SHA256 \\ \hline
251 TLS\_ECDHE\_ECDSA\_WITH\_AES\_256\_CBC\_SHA & 0xC00A & ECDHE-ECDSA-AES256-SHA \\ \hline
252 TLS\_ECDHE\_RSA\_WITH\_AES\_256\_CBC\_SHA & 0xC014 & ECDHE-RSA-AES256-SHA \\ \hline
253 TLS\_DHE\_RSA\_WITH\_AES\_256\_CBC\_SHA & 0x0039 & DHE-RSA-AES256-SHA \\ \hline
254 TLS\_DHE\_DSS\_WITH\_AES\_256\_CBC\_SHA & 0x0038 & DHE-DSS-AES256-SHA \\ \hline
255 TLS\_DHE\_RSA\_WITH\_CAMELLIA\_256\_CBC\_SHA & 0x0088 & DHE-RSA-CAMELLIA256-SHA \\ \hline
256 TLS\_DHE\_DSS\_WITH\_CAMELLIA\_256\_CBC\_SHA & 0x0087 & DHE-DSS-CAMELLIA256-SHA \\ \hline
258 \caption{Preference order of cipher suites, with OpenSSL names}
259 \label{table:prefOrderOpenSSLNames}
263 Note: the tables \ref{table:prefOrderOpenSSLNames} and \ref{table:prefOrderCipherSuites} contain Elliptic curve key exchanges. There are currently strong doubts\footnote{\url{http://safecurves.cr.yp.to/rigid.html}} concerning ECC.
264 If unsure, remove the cipher suites starting with ECDHE in the table above.
267 Based on this ordering, we can now define the corresponding settings for servers. We will start with the most common web servers.