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-rw-r--r--sites/www/installing-1.x.rst26
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@@ -48,6 +48,32 @@ Tools on the Mac, or the ``build-essential`` package on Ubuntu or Debian Linux
-- basically, anything with ``gcc``, ``make`` and so forth) as well as the
Python development libraries, often named ``python-dev`` or similar.
+Slow vs fast crypto math
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+PyCrypto attempts to use the ``gmp`` C math library if it is present on your
+system, which enables what it internally calls "fastmath" (``_fastmath.so``).
+When those headers are not available, it falls back to "slowmath"
+(``_slowmath.py``) which is a pure-Python implementation.
+
+Real-world tests have shown significant benefits to using the C version of this
+code; thus we strongly recommend you install the ``gmp`` development headers
+**before** installing Paramiko/PyCrypto. E.g.::
+
+ $ apt-get install libgmp-dev # or just apt
+ $ yum install gmp-devel # or dnf
+ $ brew install gmp
+
+If you're unsure which version of math you've ended up with, a quick way to
+check is to examine whether ``_fastmath.so`` or ``_slowmath.py`` appears in the
+output of::
+
+ from Crypto.PublicKey import RSA
+ print(RSA._impl._math)
+
+Windows
+~~~~~~~
+
For **Windows** users we recommend using :ref:`pypm`, installing a C
development environment such as `Cygwin <http://cygwin.com>`_ or obtaining a
precompiled Win32 PyCrypto package from `voidspace's Python modules page