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README.md

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+PassphraseEntropy
+=================
+
+Estimate the entropy of a passphrase. This is calculated as the number of bytes
+required to encode the passphrase on top of a Deflate stream of a preset
+dictionary.
+
+Inspired by [xkcd 936](http://xkcd.com/936/).
+
+Usage
+-----
+
+    require "passphrase_entropy"
+
+    pe = PassphraseEntropy.new
+    # or customise the dictionary:
+    pe = PassphraseEntropy.new(File.read("/usr/share/dict/words"))
+
+    pe.entropy("password") # => 6
+    pe.entropy("correct horse battery staple") # => 24
+    pe.entropy("Tr0ub4dor&3") # => 21
+
+You can decide on your acceptable level of complexity.
+
+Notes
+-----
+
+It's a bit slow: the dictionary must be deflated every time. This could be
+ameliorated by saving the state, but that would require a modified zlib
+library. (It's easy to do with a pure Ruby zlib library, but Ruby is so much
+slower in this case that the overall gain in speed is almost zero.))