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Guide to Deploying Diffie-Hellman for TLS

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Guide to Deploying Diffie-Hellman for TLS

We have three recommendations for correctly deploying Diffie-Hellman for TLS:

  1. Disable Export Cipher Suites. Even though modern browsers no longer support export suites, attacks allow a man-in-the-middle attacker to trick browsers into using export-grade cryptography, after which the TLS connection can be decrypted. Export ciphers are a remnant of 1990s-era policy that prevented strong cryptographic protocols from being exported from United States. No modern clients rely on export suites and there is little downside in disabling them.
  2. Deploy (Ephemeral) Elliptic-Curve Diffie-Hellman (ECDHE). Elliptic-Curve Diffie-Hellman ECDH) key exchange avoids all known feasible cryptanalytic attacks, and modern web browsers now prefer ECDHE over the original, finite field, Diffie-Hellman. The discrete log algorithms we used to attack standard Diffie-Hellman groups do not gain as strong of an advantage from precomputation, and individual servers do not need to generate unique elliptic curves.
  3. Generate a Strong,Unique Diffie Hellman Group. A few fixed groups are used by millions of servers, which makes them an optimal target for precomputation, and potential eavesdropping. Administrators should generate unique, 2048-bit or stronger Diffie-Hellman groups using “safe” primes for each website or server.

Steps (1) and (2) can be accomplished simultaneously by configuring your server to only use modern, secure cipher suites. We describe how to define modern ciphers and to generate a unique Diffie-Hellman group for popular servers below. You can test your server using the tool below, or by using the Qualsys SSL Server Test. If you have information on how to patch other software, please let us know.

Generating a Unique DH Group

You will first need to generate a new, unique Diffie-Hellman group, regardless of the server software you use. Modern browsers, including Google Chrome, Mozilla Firefox, and Microsoft Internet Explorer have increased the minimum group size to 1024-bit. We recommend that you generate a 2048-bit group. The simplest way of generating a new group is to use OpenSSL:

openssl dhparam -out dhparams.pem 2048

Common Server Products

For each server product, we provide two configuration options: (1) safe cipher suites that you should use, and (2) how to specify the Diffie Hellman parameters you generated above. The selected
ciphers are based on Mozilla’s Moderate Cipher List.

Apache HTTP Server (mod_ssl)

SSL parameters can globally be set in httpd.conf or within specific virtual hosts.

Cipher Suites

Disable support for SSLv2 and SSLv3 and enable support for TLS, explicitly allow/disallow specific ciphers in the given order :

DH Parameters

In newer versions of Apache (2.4.8 and newer) and OpenSSL 1.0.2 or later,you can directly specify
your DH params file as follows:

SSLOpenSSLConfCmd DHParameters "{path to dhparams.pem}"

If you are using Apache with LibreSSL, or Apache 2.4.7 and OpenSSL 0.9.8a or later, you can append the DHparams you generated earlier to the end of your certificate file.

Reload configuration

sudo service apache2 reload

nginx

To be placed in the website configuration server block in /etc/nginx/sites-enabled/default :

Cipher Suites

DH parameters

ssl_dhparam {path to dhparams.pem}

Reload configuration

sudo nginx -s reload

Microsoft IIS

  1. Open the Group Policy Object Editor (i.e. run gpedit.msc in the command prompt).
  2. Expand Computer Configuration, Administrative
    Templates, Network, and then click SSL Configuration
    Settings.
  3. Under SSL Configuration Settings, open the SSL
    Cipher Suite Order setting.
  4. Set up a strong cipher suite order. See this list of Microsoft’s supported ciphers and Mozilla’s TLS configuration instructions.

Lighttpd

Changes should be made in /etc/lighttpd/lighttpd.conf
Cipher Suites

DH parameters

ssl.dh-file="{path to dhparams.pem}"

Reload configuration

sudo service lighttpd restart

Apache Tomcat

In the server.xml file (for JSSE)
Cipher Suites

Note: To be able to use the 256 bit AES Ciphers, it is necessary to install the JCE Unlimited Strength Jurisdiction Policy Files,which can be found here.

DH Parameters

SSLDHParametersFile {path to dhparams.pem}

Postfix SMTP

Both parameters should be set in /etc/postfix/main.cf.

Cipher suites

smtpd_tls_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, aECDH, EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CDC3-SHA, KRB5-DE5, CBC3-SHA

DH params

smtpd_tls_dh1024_param_file = ${config_directory}/dhparams.pem

Reload configuration

sudo postfix reload

Sendmail

These changes can be made in the LOCAL_CONFIG section of your
/etc/mail/sendmail.mc

Cipher Suites

DH Parameters

O DHParameters={path to dhparams.pem}

Reload configuration

sudo service sendmail restart

Dovecot

These changes should be made in /etc/dovecot.conf

Cipher Suites

DH parameters

#regenerates every week
ssl_dh_parameters_length = 2048
                

Reload configuration

sudo doveadm reload

HAProxy

These changes should be made in the global section of your configuration.

Cipher Suites

DH Parameters

Append the DH parameter file generated using OpenSSL
to your certificate (crt file).

Note: while there is configuration option
named tune.ssl.default-dh-param to set the
maximum size of primes used for DHE, placing custom
parameters in your certificate file overrides it.

Reload configuration

sudo haproxy -f haproxy.cfg -p $(</var/run/haproxy-private.pid) -st $(</var/run/haproxy-private.pid)

Amazon Elastic Beanstalk

The latest set of predefined SSL parameters (2015-05) use ECDHE ciphers, not DHE, and are therefore not vulnerable to Logjam. See the Amazon documentation.

OpenSSH

The SSH protocol is safe from the LogJam attack in which an active attacker can force the connection down to export-grade cryptography. However, many SSH implementations, including OpenSSH use fixed primes, including the 1024-bit Oakley Group 2. There are a couple of options. The first and easiest option is to force clients to use elliptic-curve Diffie-Hellman. Specificially, Curve 25519. This can be accomplished by setting your Key Exchange algorithms as follows:

KexAlgorithms curve25519-sha256@libssh.org

If you want to continue to support non-elliptic-curve Diffie-Hellman, at the very least,you should disable Group 1 support, by removing the diffie-hellman-group1-sha1 Key Exchange. It is fine to leave diffie-hellman-group14-sha1, which uses a 2048-bit prime.

It is also an option to use generate new, unique Diffie-Hellman groups:

ssh-keygen -G moduli-2048.candidates -b 2048
ssh-keygen -T moduli-2048 -f moduli-2048.candidates

You then need to install module-2048 to your system’s moduli file. In Debian/Ubuntu, this is located at /etc/ssh/moduli. SSH chooses (practically randomly) from this file, so you should replace your existing moduli file with the new groups you generated instead of appending these new groups.

Written by spectre

June 1st, 2015 at 11:05 am

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