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Writeups - Crypto - Oups it's all mixed

noraj (Alexandre ZANNI)
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Information#

Version#

Date By Version Comment
03/05/2016 noraj 1.0 Creation

Name#

Oups it's all mixed

Category#

Cryptography

Wording#

Decipher the following flag: eGqaelr557dK4BbK47dS17dK68cu2PVPdydK2TtR5lKC

Data(s)#

+aAzZbByYcCxXdDwWeEvV9876543210fFuUgGtThHsSiIrRjJqQkKpPlLoOmMnN/

Equipment(s)#

A computer with an OS (preferably UNIX)

Hint(s)#

The name title is a hint.

Help#

Base64

Difficulty#

Easy / Medium

Solution#

Methodology#

  1. Identify that is a base64 string
  2. Decode the base64 string and observe only garbage
  3. Look at the provided data and analyse it
  4. Identify the data as a base64 alphabet (64 characters = 26 lowercase letters + 26 uppercase letters + 10 number + + + /)
  5. Realize that the alphabet is non standard but a mixed alphabet
  6. Write a script that will translate one alphabet to the other and pick up the output standard base64 string and then decode it

Some scrip examples below.

Python 2#

#!/usr/bin/python2                                                                               
# made with python 2.7.9

import base64
import sys
import string

base64_mixed_alphabet =     '+aAzZbByYcCxXdDwWeEvV9876543210fFuUgGtThHsSiIrRjJqQkKpPlLoOmMnN/'
base64_standard_alphabet =  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

s = 'eGqaelr557dK4BbK47dS17dK68cu2PVPdydK2TtR5lKC'

s_unmixed = s.translate(string.maketrans(base64_mixed_alphabet, base64_standard_alphabet))

s_unb64 = base64.urlsafe_b64decode(s_unmixed)

Python 3#

#!/usr/bin/python3                                                      
# made with python 3.4.1

import base64

base64_mixed_alphabet =     '+aAzZbByYcCxXdDwWeEvV9876543210fFuUgGtThHsSiIrRjJqQkKpPlLoOmMnN/'
base64_standard_alphabet =  'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

s = 'eGqaelr557dK4BbK47dS17dK68cu2PVPdydK2TtR5lKC'

s_unmixed = s.translate(str.maketrans(base64_mixed_alphabet, base64_standard_alphabet))

s_unb64 = base64.urlsafe_b64decode(s_unmixed.encode('utf-8'))
s_unb64 = s_unb64.decode('utf-8') # byte to string

Bash#

# made with bash 4.2.47
echo "eGqaelr557dK4BbK47dS17dK68cu2PVPdydK2TtR5lKC" | tr '+aAzZbByYcCxXdDwWeEvV9876543210fFuUgGtThHsSiIrRjJqQkKpPlLoOmMnN/' 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/' | base64 -di && echo "\n"

Flag#

FLAG{Yesthatisjustabase64string}

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