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Verifies Maven SHA1 hashes against a known list to deter dependency-chain attack vectors
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Bitcoinj Enforcer Rules

Bitcoinj is the first open source library for Java that allows you to spend money without involving a third party using the Bitcoin protocol.

This means that if the Bitcoinj JAR was corrupted, or made use of a corrupted library, then there is a high probability that your Bitcoin private keys would be stolen and used to spend all funds that they controlled.

How a dependency-chain attack works

Imagine that SLF4J (the popular logging framework) was hacked because it was known that Bitcoinj (or your software) uses it. As part of the hack some code was introduced which was designed to reflectively search for objects used by Bitcoinj on its classpath as part of its private key handling code. Since the hack is right at the source it can be assumed that the signing key for Maven Central is compromised.

How would you detect that in your Maven build?

You may think that the SHA1 signature would protect you, but in the event of a successful attack against Maven Central (or a mirror) they would match the digest of the downloaded artifact. In the absence of a controlled whitelist of permitted libraries there is little that can be done within the Maven environment to protect yourself against this kind of dependency-chain attack vector.

A local whitelist

The Bitcoinj Enforcer Rules work with the Maven Enforcer Plugin to provide such a whitelist. You can choose how detailed you want it to be depending on your own security requirements, but bear in mind that every unchecked dependency is a way in for an attacker through the back door.


The configuration below shows how you would use the Bitcoinj Enforcer Rules in your projects. Of course the values used below are only examples.

In production you would get the list of URNs from the Bitcoinj project page over HTTPS or via the Bitcoinj git repository when including it into your project.

      <!-- Use the Enforcer to verify build integrity -->
                <digestRule implementation="">

                  <!-- Create a snapshot to build the list of URNs below -->

                  <!-- List of required hashes -->
                  <!-- Format is URN of groupId:artifactId:version:type:classifier:scope:hash -->
                  <!-- classifier is "null" if not present -->


                    <!-- A check for the rules themselves -->



        <!-- Ensure we download the enforcer rules -->


How to use it

For maximum effect, the rules should be triggered during the verify phase so that all the dependencies that could affect the build will have been pulled in. This has the useful side effect that as a developer you're not continuously checking yourself for every build - only when you're about to perform an install or deploy.

You may want to grep/find on a case-sensitive match for "URN" to find the verification messages.

You can try it out on itself by building it within this reactor project:

mvn clean install

This reactor will first build the Bitcoinj Enforcer Rules and then go on to build another artifact that depends on them working (the Rule Tester project). This second project demonstrates how you would include Bitcoinj Enforcer Rules in your projects.

Building the whitelist automatically

Clearly trying to manually create the list of URNs would be a painful process, particularly in a large project with many layers of transitive dependencies to explore. Fortunately, the buildSnapshot flag will cause the plugin to examine all the resolved dependencies within your project and build a list of URNs that you can copy-paste (with caution) into your build.

Does it work with third-party build systems?

Yes. One of the design goals was to allow Bitcoinj to be deployed into Maven Central with sufficient support that any compromise to either it or its supporting libraries could be detected. Now your projects that include Bitcoinj will be able to build through Travis or deploy through Heroku (once Bitcoinj arrives in Maven Central).

Don't include Bitcoinj in your project without these rules or you risk losing your private keys!

Known issues

Maven 2.2.1 incompatibility

If you get this message during your build:

java.lang.ClassCastException: org.codehaus.plexus.component.configurator.BasicComponentConfigurator cannot be cast to org.codehaus.plexus.component.configurator.ComponentConfigurator

then it is likely that you are building with Maven 2.2.1. You could spend a lot of time fiddling with the compiler and plugin settings, but overall it would be easier to just migrate to Maven 3+.

How do I check the checker?

Trust has to begin somewhere so I'm going to provide some signed declarations for each version. These can be validated against my public key 59A81D7B.

Release 0.0.1

The text below is available in a raw format in the file certificate-0.0.1.asc.

Hash: SHA1

I, Gary Rowe, hereby certify that this entry in the DigestRule configuration


will validate against the entry that is in Maven Central for version 0.0.1.
Version: GnuPG/MacGPG2 v2.0.17 (Darwin)
Comment: GPGTools -

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