Dessert Time Writeup - BCACTF 6.0

Misc/Dessert Time - Juna Lee and Puhalenthi

I’m a very good planner, but my robotics competition really fried my brain! I remember eating something for the past few weeks but I just can’t seem to remember what. Can you please help me? Here’s my schedule for reference

Initial Thoughts

We are given the link to a spreadsheet entitled schedule. It has columns for each time and date from 3/6 to 4/23, and each column contains a number and a color. It seems to be tracking some sort of consumption.

Methodology

There are no discernable patterns in the data, and it seems quite arbitrary. When I click on a seemingly empty cell, I notice that it contains a number 1. This hints to me that some data could be obscured in white text.

I make a copy of the schedule so I can manipulate it further. I select all the text with Ctrl+A , and change it all to black. This reveals previously hidden letters in the spreadsheet.

Just like the hint says (I remember eating it early in the morning, but sometimes I would snack on it in the evening too.), these letters appear in the early morning and in the evening. To get the flag, I deleted all columns and rows without letters, and then put those letters together going from top to bottom and left to right.

Solution

The flag is bcactf{Apple_pieS_Are_yuMMy}. I liked the use of conditional formatting in this challenge, and I liked how the challenge creators used it to hide ones in the “blank” columns and use that as a bit of a clue.