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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* -- more information on it can be found [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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   "source": [
    "## Solution"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# your solution here\n"
   ]
  }
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