Crackme-py

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A simple python challenge

Challenge Info

crackme.py

The code for your convenience:

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# Hiding this really important number in an obscure piece of code is brilliant!
# AND it's encrypted!
# This want our biggest client to know his information is safe with us.
bezos_cc_secret = "A:4@r%uL`M-^M0c0AbcM-MFE0g4dd`_cgN"

# Reference alphabet
alphabet = "!\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ"+ \
            "[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~"



def decode_secret(secret):
    """ROT47 decode

    NOTE: encode and decode are the same operation in the ROT cipher family.
    """

    # Encryption key
    rotate_const = 47

    # Storage for decoded secret
    decoded = ""

    # decode loop
    for c in secret:
        index = alphabet.find(c)
        original_index = (index + rotate_const) % len(alphabet)
        decoded = decoded + alphabet[original_index]

    print(decoded)



def choose_greatest():
    """Echo the largest of the two numbers given by the user to the program

    Warning: this function was written quickly and needs proper error handling
    """

    user_value_1 = input("What's your first number? ")
    user_value_2 = input("What's your second number? ")
    greatest_value = user_value_1 # need a value to return if 1 & 2 are equal

    if user_value_1 > user_value_2:
        greatest_value = user_value_1
    elif user_value_1 < user_value_2:
        greatest_value = user_value_2

    print( "The number with largest positive magnitude is "
        + str(greatest_value) )

choose_greatest()

Understanding crackme.py

Let’s break down step-by-step what each part of crackme.py does:

  • Reference alphabet is defined:
    • A string called alphabet is defined, this includes special characters, numbers, and letters. This will be used as the foundation for encoding and decoding
  • decode_secret function is defined:
    • The decode_secret function is made to decode a message, bezos_cc_secret, using ROT47
    • ROT47 works by shifting each character in the string by 47 positions (hence the 47) within the alphabet string
    • For each char in bezos_cc_secret, decode_secret will:
      • Find the char’s position (index) in the alphabet string
      • Add 47 to the index, then wrap around the alphabet if needed, and find the new character
      • Append this new char to the decoded message
      • Then print the decoded message to reveal the hidden content of bezos_cc_secret (AKA our flag)
  • choose_greatest function is defined:
    • Prompts the user to enter a first and a second number (user_value_1 and user_value_2)
    • Start with the assumption that user_value_1 is the greatest
    • Compare user_value_1 and user_value_2 as strings:
      • If user_value_1 is larger, keep that as the largest
      • If user_value_2 is larger, update greatest_value to user_value_2
    • Finally, print the greater number
  • Run choose_greatest:
    • The script finally calls choose_greatest, thus prompting the user for input and then displaying the largest number based on string comparison

Let’s give a quick summary, just to simplify:

  • decode_secret is defined, it decodes bezos_cc_secret
  • choose_greatest is defined to find and print the larger of two numbers entered by the user (us) is defined -
  • HOWEVER… only choose_greatest is called

The solution

So, the code already gives us our solution, but it isn’t called. Instead, only choose_greatest is called. Why is this?

Simple, it’s a red herring:

Something that misleads or distracts from a relevant or important question

So, all we have to do is edit the python file to call decode_secret!

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# Hiding this really important number in an obscure piece of code is brilliant!
# AND it's encrypted!
# This want our biggest client to know his information is safe with us.
bezos_cc_secret = "A:4@r%uL`M-^M0c0AbcM-MFE0g4dd`_cgN"

# Reference alphabet
alphabet = "!\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ"+ \
            "[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~"



def decode_secret(secret):
    """ROT47 decode

    NOTE: encode and decode are the same operation in the ROT cipher family.
    """

    # Encryption key
    rotate_const = 47

    # Storage for decoded secret
    decoded = ""

    # decode loop
    for c in secret:
        index = alphabet.find(c)
        original_index = (index + rotate_const) % len(alphabet)
        decoded = decoded + alphabet[original_index]

    print(decoded)



def choose_greatest():
    """Echo the largest of the two numbers given by the user to the program

    Warning: this function was written quickly and needs proper error handling
    """

    user_value_1 = input("What's your first number? ")
    user_value_2 = input("What's your second number? ")
    greatest_value = user_value_1 # need a value to return if 1 & 2 are equal

    if user_value_1 > user_value_2:
        greatest_value = user_value_1
    elif user_value_1 < user_value_2:
        greatest_value = user_value_2

    print( "The number with largest positive magnitude is "
        + str(greatest_value) )

decode_secret(bezos_cc_secret) ### WHAT WE ADDED.

choose_greatest()

flag: picoCTF{1|\/|_4_p34|\|ut_8c551048}