1 %%\subsection{Recommended cipher suites}
3 In principle system administrators who want to improve their communication security
4 have to make a difficult decision between effectively locking out some users and
5 keeping high cipher suite security while supporting as many users as possible.
6 The website \url{https://www.ssllabs.com/} gives administrators and security engineers
7 a tool to test their setup and compare compatibility with clients. The authors made
8 use of ssllabs.com to arrive at a set of cipher suites which we will recommend
9 throughout this document.\\
11 \textbf{Caution: these settings can only represent a subjective
12 choice of the authors at the time of writing. It might be a wise choice to
13 select your own and review cipher suites based on the instructions in section
14 \ref{section:ChoosingYourOwnCipherSuites}}.
17 \subsubsection{Configuration A: Strong ciphers, fewer clients}
19 At the time of writing we recommend the following set of strong cipher
20 suites which may be useful in an environment where one does not depend on many,
21 different clients and where compatibility is not a big issue. An example
22 of such an environment might be machine-to-machine communication or corporate
23 deployments where software that is to be used can be defined freely.
26 We arrived at this set of cipher suites by selecting:
30 \item Perfect forward secrecy / ephemeral Diffie Hellman
31 \item strong MACs (SHA-2) or
32 \item GCM as Authenticated Encryption scheme
35 This results in the OpenSSL string:
37 \begin{lstlisting}[breaklines]
38 'EECDH+aRSA+AES256:EDH+aRSA+AES256:!SSLv3'
41 %$\implies$ resolves to
44 %openssl ciphers -V $string
49 %\todo{make a column for cipher chaining mode} --> not really important, is it?
52 \begin{tabular}{lllllll}
54 \textbf{ID} & \textbf{OpenSSL Name} & \textbf{Version} & \textbf{KeyEx} & \textbf{Auth} & \textbf{Cipher} & \textbf{MAC}\\\cmidrule(lr){1-7}
55 \verb|0xC030| & ECDHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & ECDH & RSA & AESGCM(256) & AEAD \\
56 \verb|0xC028| & ECDHE-RSA-AES256-SHA384 & TLSv1.2 & ECDH & RSA & AES(256) (CBC) & SHA384 \\
57 \verb|0x009F| & DHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & DH & RSA & AESGCM(256) & AEAD \\
58 \verb|0x006B| & DHE-RSA-AES256-SHA256 & TLSv1.2 & DH & RSA & AES(256) (CBC) & SHA256 \\
64 \textbf{Compatibility}
66 Only clients which support TLS 1.2 are covered by these cipher suites (Chrome 30,
67 Win 7 and Win 8.1 crypto stack, Opera 17, OpenSSL $\ge$ 1.0.1e, Safari 6 / iOS
68 6.0.1, Safari 7 / OS X 10.9).
72 \subsubsection{Configuration B: Weaker ciphers, more compatability}
74 In this section we propose a slightly weaker set of cipher suites. For
75 example, there are some known weaknesses for the SHA-1 hash function that is
76 included in this set. The advantage of this set of cipher suites is not only
77 the compatibility with a broad range of clients, but also less computational
78 workload on the provisioning hardware.
81 \textbf{All further examples in this publication use Configuration B}.\\
83 We arrived at this set of cipher suites by selecting:
86 \item TLS 1.2, TLS 1.1, TLS 1.0
89 \todo{AK: Note that SHA1 is considered broken but if we are in DHE, we might get around it as long as you can not calculate a SHA1 collision ``live'' on the wire}
93 This results in the OpenSSL string:
95 \begin{lstlisting}[breaklines]
96 'EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA'
99 \todo{make a column for cipher chaining mode}
101 \begin{tabular}{lllllll}
103 \textbf{ID} & \textbf{OpenSSL Name} & \textbf{Version} & \textbf{KeyEx} & \textbf{Auth} & \textbf{Cipher} & \textbf{MAC}\\\cmidrule(lr){1-7}
104 \verb|0x009F| & DHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & DH & RSA & AESGCM(256) & AEAD \\
105 \verb|0x006B| & DHE-RSA-AES256-SHA256 & TLSv1.2 & DH & RSA & AES(256) & SHA256 \\
106 \verb|0xC030| & ECDHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & ECDH & RSA & AESGCM(256) & AEAD \\
107 \verb|0xC028| & ECDHE-RSA-AES256-SHA384 & TLSv1.2 & ECDH & RSA & AES(256) & SHA384 \\
108 \verb|0x009E| & DHE-RSA-AES128-GCM-SHA256 & TLSv1.2 & DH & RSA & AESGCM(128) & AEAD \\
109 \verb|0x0067| & DHE-RSA-AES128-SHA256 & TLSv1.2 & DH & RSA & AES(128) & SHA256 \\
110 \verb|0xC02F| & ECDHE-RSA-AES128-GCM-SHA256 & TLSv1.2 & ECDH & RSA & AESGCM(128) & AEAD \\
111 \verb|0xC027| & ECDHE-RSA-AES128-SHA256 & TLSv1.2 & ECDH & RSA & AES(128) & SHA256 \\
112 \verb|0x0088| & DHE-RSA-CAMELLIA256-SHA & SSLv3 & DH & RSA & Camellia(256) & SHA1 \\
113 \verb|0x0039| & DHE-RSA-AES256-SHA & SSLv3 & DH & RSA & AES(256) & SHA1 \\
114 \verb|0xC014| & ECDHE-RSA-AES256-SHA & SSLv3 & ECDH & RSA & AES(256) & SHA1 \\
115 \verb|0x0045| & DHE-RSA-CAMELLIA128-SHA & SSLv3 & DH & RSA & Camellia(128) & SHA1 \\
116 \verb|0x0033| & DHE-RSA-AES128-SHA & SSLv3 & DH & RSA & AES(128) & SHA1 \\
117 \verb|0xC013| & ECDHE-RSA-AES128-SHA & SSLv3 & ECDH & RSA & AES(128) & SHA1 \\
118 \verb|0x0084| & CAMELLIA256-SHA & SSLv3 & RSA & RSA & Camellia(256) & SHA1 \\
119 \verb|0x0035| & AES256-SHA & SSLv3 & RSA & RSA & AES(256) & SHA1 \\
120 \verb|0x0041| & CAMELLIA128-SHA & SSLv3 & RSA & RSA & Camellia(128) & SHA1 \\
121 \verb|0x002F| & AES128-SHA & SSLv3 & RSA & RSA & AES(128) & SHA1 \\
126 \textbf{Compatibility}
128 Note that these cipher suites will not work with Windows XP's crypto stack (e.g. IE, Outlook),
129 %%Java 6, Java 7 and Android 2.3. Java 7 could be made compatible by installing the "Java
130 %%Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files"
131 %%(JCE) \footnote{\url{http://www.oracle.com/technetwork/java/javase/downloads/jce-7-download-432124.html}}.
132 We could not verify yet if installing JCE also fixes the Java 7
133 DH-parameter length limitation (1024 bit).
138 For a detailed explanation of the cipher suites chosen, please see
139 \ref{section:ChoosingYourOwnCipherSuites}. In short, finding the perfect cipher
140 string is impossible and must be a tradeoff between compatibility and security.
141 On the one hand there are mandatory and optional ciphers defined in a few RFCs,
142 on the other hand there are clients and servers only implementing subsets of the
145 Straight forward, the authors wanted strong ciphers, forward secrecy
146 \footnote{\url{http://nmav.gnutls.org/2011/12/price-to-pay-for-perfect-forward.html}}
147 and the best client compatibility possible while still ensuring a cipher string that can be
148 used on legacy installations (e.g. OpenSSL 0.9.8).
150 Our recommended cipher strings are meant to be used via copy and paste and need to work
154 \item TLSv1.2 is preferred over TLSv1.0/SSLv3 (while still providing a useable cipher
156 \item AES256 and CAMELLIA256 count as very strong ciphers at the moment; preferrably in
158 \todo{add a reference here please}
159 \item AES128 and CAMELLIA128 count as strong enough ciphers at the moment
160 \item DHE or ECDHE for forward secrecy
161 \item RSA as this will fit most of todays setups
162 \item AES256-SHA as a last resort (with this cipher at the end, even systems with
163 very old versions of openssl like 0.9.8 will just work. Just forward secrecy
164 will not be used. On systems that do not support elliptic curves, that cipher
165 offers support for the Microsoft crypto libraries that only support ECDHE.
168 \todo{Adi: review "justification" when next section is written}