Defcon 24 OpenCTF

A CTF solution by dr_dave ­ contact me: dr_dave@hackedexistence.com

The Setup:

This challenge is an obfuscation challenge that requires you to perform conversions for the purpose of decoding the flag. The chain of conversions is as follows: base34>hex>ascii

The Challenge - Flag #43

Basic Obfuscation – 50 Points
We found this message, can you decode it?

3xvsw8su9k7xlcq39w38uv33q5u5dt260uwb81mwnjjeiku8emau2788t

Facts about the message:

The message is 58 Characters
We use this website http://www.somacon.com/p525.php to generate the frequency analysis to determine the potential base of the message:

Code: 48 0x30 '0' Count: 1
Code: 49 0x31 '1' Count: 1
Code: 50 0x32 '2' Count: 2
Code: 51 0x33 '3' Count: 5
Code: 53 0x35 '5' Count: 2
Code: 54 0x36 '6' Count: 1
Code: 55 0x37 '7' Count: 2
Code: 56 0x38 '8' Count: 6
Code: 57 0x39 '9' Count: 2
Code: 97 0x61 'a' Count: 1
Code: 98 0x62 'b' Count: 1
Code: 99 0x63 'c' Count: 1
Code: 100 0x64 'd' Count: 1
Code: 101 0x65 'e' Count: 2
Code: 105 0x69 'i' Count: 1
Code: 106 0x6A 'j' Count: 2
Code: 107 0x6B 'k' Count: 2
Code: 108 0x6C 'l' Count: 1
Code: 109 0x6D 'm' Count: 2
Code: 110 0x6E 'n' Count: 1
Code: 113 0x71 'q' Count: 2
Code: 115 0x73 's' Count: 2
Code: 116 0x74 't' Count: 2
Code: 117 0x75 'u' Count: 6
Code: 118 0x76 'v' Count: 2
Code: 119 0x77 'w' Count: 4
Code: 120 0x78 'x' Count: 2

Note that Y and Z are missing (we also found out that F is missing as well,but ignore that ;­) ) this means that the string is base 34 (0­9=10 chars, a-w=24 chars).

This website http://convertxy.com/index.php/numberbases/ is used to perform the base34 conversion:

Base 34 to Hex

77335F753565645F615F6434727462306172645F74305F7031636B5F7468335F62347365

This website is used to perform the hex to ascii conversion: http://www.rapidtables.com/convert/number/hex-to-ascii.htm

Hex to ASCII reveals the flag:

w3_u5ed_a_d4rtb0ard_t0_p1ck_th3_b4se