Zip Line
by Doug Zongker
Answer: ROANOKES
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:
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.