This page was last updated in August 2011 and is accurate for router version 0.8.7.


Using JNI (Java Native Interface), a bit of C code (thanks ugha!), a little manual work and a piece of chewing gum we have made several cryptography operations quite a bit faster.

The speedup comes from the super-fast GNU MP Bignum library (libgmp). We use a single function from libgmp - mpz_powm() as a replacement for the Java Math library's BigInteger modPow(). As modPow() is a significant computational portion of many crypto operations, this is of significant benefit.

The standard I2P installation includes about 20 versions of the library for different platforms, each about 50KB, inside the jbigi.jar file. The initialization of the JBigI library, including CPU identification, selection, and extraction of the correct loadable module, is handled by the NativeBigInteger class. If no module is available for the current platform, the standard Java Math library's BigInteger modPow() is used.

Rebuilding and Testing JBigI

Following are the instructions to build a new jbigi library for your own platform and testing its performance.


This works on Linux, and with a few changes in probably also on other platforms. FreeBSD has also been reported to work too. On Kaffee the speedup is very small, because it already uses native BitInteger internally. Blackdown seems to cause strange errors. Because you are going to do compilation, you need JDK; JRE won't work.

The required code is available in monotone database and the latest source tarball.

The GNU MP Bignum library (libgmp) needs to be installed, if it isn't included in your OS / distribution or installed already, it can be received from Even if you have already installed it as binary, it might still be worth a try to compile GMP yourself, since then it will be able to use the specific instructions of your processor. The latest GMP may also be used instead of GMP 5.0.2, but it hasn't been tested by us.

Step-by-step instructions

  1. Look at your running environment on the logs.jsp page. There should be one of two status messages for JBigI - either Locally optimized native BigInteger loaded from the library path or Native BigInteger library jbigi not loaded - using pure java. If the native BitInteger library was NOT loaded, you definitely need to compile your own. Certain platforms, such as OS X, OpenSolaris, and 64-bit systems, may require you to compile your own library. If the BigInteger library was loaded, do at least the next step to see what your performance is.
  2. Look on http://localhost:7657/stats.jsp to see what the lifetime average values for crypto.elGamal.decrypt and crypto.elGamal.encrypt are. The numbers are times in milliseconds. Copy these somewhere so you can compare them later on. The network average for encrypt time is about 20ms. If your encrypt time is less than 50ms for a relatively new processor, or less than 100ms for an older processor, and the native BigInteger library was loaded, you are probably fine.
  3. Get the latest released source code of I2P from the download page, or get the cutting-edge source out of the monotone database
  4. Inside the source tree change directory to: core/c/jbigi
  5. Read the README file. If you have a /usr/lib/ file, you do not have to download GMP. Use the 'dynamic' argument to Otherwise, you must download GMP version 5.0.2 from from, saving it to gmp-5.0.2.tar.bz2. If you decide to use a newer version, change the VER= line in core/c/jbigi/
  6. Take a look at, if your JAVA_HOME environment variable is set and you are using Linux then it might just work. Otherwise change the settings. Remember, you need the Java SDK installed.
  7. Run (if you downloaded GMP) or dynamic (if you have /usr/lib/
    Maybe the build spewed out some errors of missing jni.h and jni_md.h files. Either copy these files from your java install into the core/c/jbigi/jbigi/include/ directory, or fix $JAVA_HOME.
    You can run the from the core/c/ directory which will build all available jbigi libs into a jbigi.jar.
    A file named should be created in the current directory. If this doesn't happen and/or you get errors then please report them.
  8. Follow the instructions in core/c/README to install the library and run the speed test. Read the final lines of the speed test's output for some additional info, it will be something like this:
    native run time:  5842ms ( 57ms each)
    java run time:   41072ms (406ms each)
    native = 14.223802103622907% of pure java time
    If the native is indeed 5-7x faster (or more) then it looks all good. If not, please report.
  9. Copy to your i2p directory
  10. Restart your I2P programs.
  11. On http://localhost:7657/stats.jsp the crypto.elGamal.decrypt and crypto.elGamal.encrypt should be a lot faster.