Harness SSL and JSSE: Key Size Control

Aged Bridge, Yulong River, Yangshuo, China
Aged Bridge, Yulong River, Yangshuo, China

Why Key Size Concerns


The key size is an important security parameter to determine the strength of cryptography algorithms. For example, RSA keys with fewer than 1024 bits are considered forgeable.  If RSA keys less than 1024 bits are used in X.509 certificates, the private keys used in these certificates can be derived and could allow an attacker to duplicate the certificates and use them fraudulently to spoof content, perform phishing attacks, or perform man-in-the-middle attacks.

On August 14, 2012, Microsoft offered an update to Windows that restricts the use of certificates with RSA keys less than 1024 bits in length.  As of October 9, 2012, the update is delivered via automatic update through the Microsoft update service.  Microsoft recommends that customers apply the update at the earliest opportunity.

Since JDK 7u12,  RSA keys less than 1024 bits in X.509 certificates are disabled.  This is in line with the NIST recommendations to move to a minimum of 2048-bit keys by January 1, 2014.  With this key size restriction, those who use X.509 certificates based on RSA keys less than 1024 bits will encounter compatibility issues with certification path building and validation.  This key size restriction also impacts components that based on X.509 certificates, for example signed JAR verification, SSL/TLS transportation, HTTPS connections, etc.

According to the SSL surveys on November 03, 2013, none out of around 200,000 SSL-enabled web sites based on Alexa's list of most popular sites are using keys below 1024 bits.  And according to another research targeted more wild scope, 0.96% (123,038) of 12,828,613 public live SSL hosts and 0.08%  (8,459)of  10,216,363 live SSH hosts in the world are using a key size of 512 bits.  In order to avoid any compatibility issue, applications are strongly recommended to renew their certificates with stronger keys. Or on your own risks, adjust the key size restriction property (jdk.certpath.disabledAlgorithms) to permit the smaller key sizes in case of any compatibility issue.

The impact on JSSE


Since JDK 7u12,  RSA keys less than 1024 bits in X.509 certificates are disabled.   This improvement directly impacts the behaviors of SunJSSE (Oracle JSSE provider) if certificate based authentication is used and the certification path contains RSA keys less than 1024 bits.

This restriction is limited via security property, "jdk.certpath.disabledAlgorithms" (See more "Java™ SE 7 Release Security Enhancements - Weak Cryptography Control" about this property).
jdk.certpath.disabledAlgorithms=MD2, RSA keySize < 1024

This security property was introduced since JDK 7.  There is no such handy approach in JDK 6 and 5.0 as the author wrote this blog.

Workarounds


If RSA keys less than 1024 bits in X.509 certificates have to be used in a certain circumstance, it is likely to run into compatibility because of this key size restriction.  Please adjust the key size restriction property (jdk.certpath.disabledAlgorithms) to permit small key sizes.  For example:
-    jdk.certpath.disabledAlgorithms=MD2, RSA keySize < 1024
+    jdk.certpath.disabledAlgorithms=MD2, RSA keySize < 512

Note that it is really risky.  One must understand and control the potential risks in the application runtime environment before enforcing this workaround.

Best Practices

  1. Put more weak key size restrictions.  Of course, please consider the potential compatibility issues.  For example, EC keys less than 160 bits and DSA keys less than 1024 bits may be also considered too weak to be acceptable.
       jdk.certpath.disabledAlgorithms=MD2, RSA keySize < 1024, \
                    EC keySize < 160, DSA keySize < 1024
  2. Upgrade your X.509 certificate to use strong keys. At present, the preferable security strength is 128-bits (128 bits AES/3072 bits RSA/256 bits ECC/3072 bits DSA keys). 2048 bits RSA/DSA keys is also acceptable according to NIST recommendations (See also a summary of the time frame, NIST Security Strength Time Frames).
  3. Moving forward to use EC keys, which is more performance friendly. 
  4. Encourage to enforce more flexible algorithm (and key size) constraints in JSSE with javax.net.ssl.SSLParameters.setAlgorithmConstraints(AlgorithmConstraints).

Looking forward …

  1. Enforce and improve algorithm and key constraints in more components other than PKI and JSSE.

  2. In a long run, enforce more strict, wide and fine key size restriction by default, for example RSA and DSA keys must be great than 1024 bits when apply to data, and EC keys cannot be less than 224 bits.

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