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Cactus Canyon

Zip Line

by Doug Zongker

zip_line.zip is a .zip file containing about 40,000 files in a large directory structure of single character names. All the files are zero-length and have five-digit names.

The first thing to realize is that the five-digit numbers are all US ZIP codes. The path leading down to each file turns out to be Caesar shift of the corresponding city and state. For example:

  shift by 2  

You need to compute the shift for all 40,000 entries. (A useful resource is Wikipedia's List of ZIP Code prefixes, which maps each three-digit prefix to a state.)

All the shifts are between 1 and 10. You should notice a correlation between the ZIP code and the shift. For example:

City, State ZIP code shift
Cambridge, MA 02138 2
Cambridge, MA 02139 2
Cambridge, MA 02140 10
Cambridge, MA 02141 9
Cambridge, MA 02142 8
Cambridge, MA 02238 2

Adding the shift to the ZIP code always gives a number whose last digit is 0 or 1. So from the 40,000 entries you get 40,000 bits, which turns out to be another, much smaller, .zip file. That .zip file contains a single image:

You need to find the city name that exists in those six states and no others. That city name is Roanoke, giving the answer ROANOKES.