W4501 Capture the Flag I
This page will serve as a writeup for the CTF I competition problems.
Cryptography[edit]
ABC... It's as easy as 123...[edit]
[25 Points] Gabe passes secret notes to Gabby in class without the teacher noticing, but I managed to intercept one. The problem is, there are no words! What does this mean?
6-12-1-7 { 5-1-19-25 _ 1-19 _ 15-14-5 _ 20-23-15 _ 20-8-18-5-5 }
This flag is encrypted using an A1Z26 cipher. An A1Z26 cipher simply converts each letter into the number of its position in the alphabet. For example, A is 1, B is 2, C is 3, and Z is 26. We can tell that this flag uses an A1Z26 cipher because no numbers exceed 26 and the challenge name references letters and their number counterparts. After putting the flag in an online A1Z26 decryptor we get that the flag is flag{easy_as_one_two_three}.
Ass Key[edit]
[25 Points] I stole this straight off the server’s hard drive, but it doesn’t make any sense. Maybe these numbers mean something? 0110011001101100011000010110011101111011010000010101001101000011010010010100100101011111011010010111001101011111010001010101101001111101 Hint: Computers encode legible text as binary numbers using a certain format.
This flag is the binary representation of ASCII characters. We know that it is in binary because only 0s and 1s are used, and it's likely encoded using ASCII because ASCII is the most common way to encode text. The challenge name and hint also imply that it uses ASCII. We can simply put this into an online binary to ASCII converter and we get that the flag is flag{ASCII_is_EZ}.
It's all about that base[edit]
[75 Points] The CS I students are getting really good at base conversions! But this number doesn’t look like any of the bases they’ve seen before… why don’t you convert it to decimal so the students understand it: 888888888 Hint: Look at the largest digit in the provided number.
Quantities can be represented in different ways called bases. We are used to base 10, which includes the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. However, there are many other bases. For example, computers use base 2, more commonly known as binary. Base 2 only includes the digits 0 and 1. We don't know the base that 888888888 is in, but the hint implies that it is in base 9 because base 9's largest digit is 8. When using an online tool to convert from base 9 to decimal (base 10), we get that the base 10 equivalent is 387420488.
Caesar Salad[edit]
[50 Points] My letters got jumbled 3 times! Unjumble them please: iodj{glg_Mxolxv_Pdnh_Wklv_Vdodg}
Hint: The names of these challenges help…
This flag is encrypted using a Caesar cipher. This cipher shifts letters in the alphabet by a certain amount. For instance, encoding the message “hello” with a shift of 3 results in “khoor.” We know that this flag is encrypted using a Caesar cipher because the flag appears to already use letters and the challenge name references Caesar. We know that the shift is 3 because of the challenge description, but we can also brute force every possible shift if nessesary. When using an online converter, we get that the flag is flag{did_Julius_Make_This_Salad}.
