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     "source": [
      "Practice Problem - Cryptography"
     ]
    },
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     "source": [
      "Cryptography is the study of how to make messages secret or how to read secret messages. A very simple encryption technique is called the *Caesar cipher*, which you can read up more about [here](http://en.wikipedia.org/wiki/Caesar_cipher). The basic idea is that each letter is replaced by a letter that is a certain number of letters away, so for example if the shift was 2, then A would become C, B would become D, etc. (and Z will become B).\n",
      "\n",
      "Write a function that given a string and a shift, will produce the encrypted string for that shift. Note that the same function can be used to decrypt a message, by passing it a negative shift. \n",
      "\n",
      "The rules are: you should only accept and return lowercase letters, and spaces should not be changed.\n",
      "\n",
      "Then, decrypt the following message, which was encrypted with a shift of 13:\n",
      "    \n",
      "    pbatenghyngvbaf lbh unir fhpprrqrq va qrpelcgvat gur fgevat    \n",
      "    \n",
      "Now if you are up for a challenge, try and decrypt this **and** find the shift:\n",
      "    \n",
      "    gwc uivioml bw nqvl bpm zqopb apqnb\n",
      "    \n",
      "Hint: there are several ways you can convert between letters and numbers. One is to use the built-in functions ``chr`` and ``ord`` (and remember you can find out more about a function by using ``?`` in IPython). Another is to set up the alphabet in a string and use item access (``[4]``) to convert from numbers to letters, and the ``index`` method to convert from letters to numbers."
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      "Solution"
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      "\n",
      "# your solution here\n"
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