I'm developing an XML representation for the .torrent files.  It
	was quite trivial to convert the XML bit like, e. g.:

<?xml version="1.0" ?> <!-- -*- XML -*- -->
<torrent
   xmlns="http://www.siamics.net/~ivan/xmlns/xmltorrent">

  <announce>http://TRACKER:PORT/announce</announce>
  <comment>COMMENT</comment>
  <created-by>CREATED-BY</created-by>
  <info>
    <length>31454056</length>
    <name>nomad3.ncep.noaa.gov/pub/gfs1p0/gfs20100320/gfs.t12z.pgrbf96</name>
    <piece-length>262144</piece-length>
  </info>
</torrent>

	to the b.torrent formatb.  However, one of the most important
	parts of the .torrent file is the bpiecesb field, containing the
	SHA1 digests of the chunks represented as a single long binary
	string.  To my mind, it's reasonable to have such a field
	encoded in Base64, like:

<torrent ...>
  ...
  <info>
    ...
    <pieces>
      <digest>TCvyo4Na18vWlW19+pirwGKKMoU=</digest>
      <digest>hnWGTGoiouUIZWjX861YIxyMYaM=</digest>
      ...
    </pieces>
  </info>
</torrent>

	Now, how do I convert these Base64 strings into the respective
	binary strings?

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