[IceCTF 2016] [Crypto 140 – Sand Castle] Write Up

Description

We found this very mysterious image, it doesn’t look complete and there seems to be something hidden on it… does this mean anything to you?
sandcastle_d4ba9f13fe0b0da8778f07f068c7bc2179494459d38d4a25ba5af1beb01f1201
This flag is not in the standard flag format. The flag contains digits and no special characters, convert the message to lowercase and then add IceCTF{message} to it.

Resolution

Using Google Images, we have found the image is a part of this book:
51FP5XY2X2L._AC_UL320_SR230,320_

Looking in EXIF data we got:

58-7-9
210-13-2
67-3-16
85-17-15
305-18-4
83-12-18
75-8-15
47-4-2
83-20-11
208-6-6
85-11-6
75-7-3
106-9-14

The code used here is a variant of the Book Cipher:
58-7-9 = 58th page, 7th line, 9th word.

With multiples tries we got:

58-7-9        not  
210-13-2      0s
67-3-16       one
85-17-15      becomes
305-18-4      zero
83-12-18      routed
75-8-15       3
47-4-2        authenticity
83-20-11      David
208-6-6       be
85-11-6       of
75-7-3        0
106-9-14      x

Flag was IceCTF{n0obzr3adbo0x}

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