1 %%\subsection{Recommended cipher suites}
3 In principle system administrators who want to improve their communication security
4 have ton 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}
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.
75 For example, there are some known weaknesses for the SHA-1 hash function that
76 is included in this set. However, the advantage of this set of cipher suites
77 is it's higher compatibility with a more diverse set of clients as well as
78 less computational overhead.\\
80 \textbf{All further examples in this publication use Configuration B}.\\
82 We arrived at this set of cipher suites by selecting:
85 \item TLS 1.2, TLS 1.1, TLS 1.0
88 \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}
92 This results in the OpenSSL string:
94 \begin{lstlisting}[breaklines]
95 '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'
98 \todo{make a column for cipher chaining mode}
100 \begin{tabular}{lllllll}
102 \textbf{ID} & \textbf{OpenSSL Name} & \textbf{Version} & \textbf{KeyEx} & \textbf{Auth} & \textbf{Cipher} & \textbf{MAC}\\\cmidrule(lr){1-7}
103 \verb|0xC030| & ECDHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & ECDH & RSA & AESGCM(256) & AEAD \\
104 \verb|0xC028| & ECDHE-RSA-AES256-SHA384 & TLSv1.2 & ECDH & RSA & AES(256) (CBC) & SHA384 \\
105 \verb|0x009F| & DHE-RSA-AES256-GCM-SHA384 & TLSv1.2 & DH & RSA & AESGCM(256) & AEAD \\
106 \verb|0x006B| & DHE-RSA-AES256-SHA256 & TLSv1.2 & DH & RSA & AES(256) (CBC) & SHA256 \\
107 \verb|0x0088| & DHE-RSA-CAMELLIA256-SHA & SSLv3 & DH & RSA & Camellia(256) & SHA1 \\
108 \verb|0xC014| & ECDHE-RSA-AES256-SHA & SSLv3 & ECDH & RSA & AES(256) (CBC) & SHA1 \\
109 \verb|0x0039| & DHE-RSA-AES256-SHA & SSLv3 & DH & RSA & AES(256) (CBC) & SHA1 \\
110 \verb|0x0035| & AES256-SHA & SSLv3 & RSA & RSA & AES(256) (CBC) & SHA1 \\
114 \todo{add AES128, CAMELIA128!}
116 \textbf{Compatibility}
118 Note that these cipher suites will not work with Windows XP's crypto stack (e.g. IE, Outlook),
119 %%Java 6, Java 7 and Android 2.3. Java 7 could be made compatible by installing the "Java
120 %%Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files"
121 %%(JCE) \footnote{\url{http://www.oracle.com/technetwork/java/javase/downloads/jce-7-download-432124.html}}.
122 We could not verify yet if installing JCE also fixes the Java 7
123 DH-parameter length limitation (1024 bit).
128 For a detailed explanation of the cipher suites chosen, please see
129 \ref{section:ChoosingYourOwnCipherSuites}. In short, finding the perfect cipher
130 string is impossible and must be a tradeoff between compatibility and security.
131 On the one hand there are mandatory and optional ciphers defined in a few RFCs,
132 on the other hand there are clients and servers only implementing subsets of the
135 Straight forward, the authors wanted strong ciphers, forward secrecy
136 \footnote{\url{http://nmav.gnutls.org/2011/12/price-to-pay-for-perfect-forward.html}}
137 and the best client compatibility possible while still ensuring a cipher string that can be
138 used on legacy installations (e.g. OpenSSL 0.9.8).
140 Our recommended cipher strings are meant to be used via copy and paste and need to work
144 \item TLS1.2 is preferred over TLSv1.0/SSLv3 (while still providing a useable cipher
146 \item AES256 and CAMELLIA256 count as very strong ciphers at the moment; preferrably in
148 \todo{add a reference here please}
149 \item AES128 and CAMELLIA128 count as strong enough ciphers at the moment
150 \item DHE or ECDHE for forward secrecy
151 \item RSA as this will fit most of todays setup
152 \item AES256-SHA as a last ressort (with this cipher at the end, even systems with
153 very old versions of openssl like 0.9.8 will just work. Just forward secrecy
154 will not be used. On systems that do not support elliptic curves, that cipher
155 offers support for the Microsoft crypto libraries that only support ECDHE.
158 \todo{Adi: review "justification" when next section is written}