Introduction

1. Finding Flags (2 points)

   Answer:: crypto{y0ur_f1rst_fl4g}

2. Great Snakes (3 points)

   The given Python script has the following code:

   #!/usr/bin/env python3
   
   import sys
   # import this
   
   if sys.version_info.major == 2:
       print("You are running Python 2, which is no longer supported. Please update to Python 3.")
   
   ords = [81, 64, 75, 66, 70, 93, 73, 72, 1, 92, 109, 2, 84, 109, 66, 75, 70, 90, 2, 92, 79]
   
   print("Here is your flag:")
   print("".join(chr(o ^ 0x32) for o in ords))
   

   Running the script gives us the flag: crypto{z3n_0f_pyth0n}

3. ASCII (5 points)

   Solution:

   print(''.join([chr(int(x)) for x in [99, 114, 121, 112, 116, 111, 123, 65, 83, 67, 73, 73, 95, 112, 114, 49, 110, 116, 52, 98, 108, 51, 125]]))
   

   Answer:: crypto{ASCII_pr1nt4bl3}

4. Hex (5 points)

   Solution:

   bytes.fromhex('63727970746f7b596f755f77696c6c5f62655f776f726b696e675f776974685f6865785f737472696e67735f615f6c6f747d')
   

   Answer:: crypto{You_will_be_working_with_hex_strings_a_lot}

5. Base64 (10 points)

   Solution:

   import base64
   data_bytes = bytes.fromhex('72bca9b68fc16ac7beeb8f849dca1d8a783e8acf9679bf9269f7bf')
   data_base64 = base64.b64encode(data_bytes)
   print(data_base64.decode('utf-8'))
   

   Answer:: crypto/Base+64+Encoding+is+Web+Safe/

6. Bytes and Big Integers (10 points)

   Solution:

   # python3 -m pip install pycryptodome
   from Crypto.Util.number import *
   long_to_bytes(11515195063862318899931685488813747395775516287289682636499965282714637259206269)
   

   Answer:: crypto{3nc0d1n6_4ll_7h3_w4y_d0wn}

7. XOR Starter (10 points)

   Solution:

   string = 'label'
   chars = [ord(c) for c in string]
   key = 13
   encrypted = [chr(c ^ key) for c in chars]
   print(''.join(encrypted))
   

   Answer:: crypto{aloha}

8. XOR Properties (15 points)

   We are given the following information:

   Commutative: A ⊕ B = B ⊕ A
   Associative: A ⊕ (B ⊕ C) = (A ⊕ B) ⊕ C
   Identity: A ⊕ 0 = A
   Self-Inverse: A ⊕ A = 0
   
   KEY1 = a6c8b6733c9b22de7bc0253266a3867df55acde8635e19c73313
   KEY2 ^ KEY1 = 37dcb292030faa90d07eec17e3b1c6d8daf94c35d4c9191a5e1e
   KEY2 ^ KEY3 = c1545756687e7573db23aa1c3452a098b71a7fbf0fddddde5fc1
   FLAG ^ KEY1 ^ KEY3 ^ KEY2 = 04ee9855208a2cd59091d04767ae47963170d1660df7f56f5f
   

   Solution:

   import operator
   
   # Source: https://stackoverflow.com/a/66662724/6456163
   xor = lambda a, b: bytes(map(operator.xor, a, b))
   key1 = bytes.fromhex('a6c8b6733c9b22de7bc0253266a3867df55acde8635e19c73313')
   key2 = xor(bytes.fromhex('37dcb292030faa90d07eec17e3b1c6d8daf94c35d4c9191a5e1e'), key1)
   key3 = xor(bytes.fromhex('c1545756687e7573db23aa1c3452a098b71a7fbf0fddddde5fc1'), key2)
   flag = xor(xor(xor(bytes.fromhex('04ee9855208a2cd59091d04767ae47963170d1660df7f56f5f'), key1), key2), key3)
   print(flag.decode('utf-8'))
   

   Answer:: crypto{x0r_i5_ass0c1at1v3}

9. Favourite byte (20 points)

   Solution:

   # decode hex string to bytes
   ciphertext = bytes.fromhex("73626960647f6b206821204f21254f7d694f7624662065622127234f726927756d")
   
   # try all possible byte values for the key
   for key in range(256):
       # initialize an empty list to store the decrypted message
       decrypted = []
       # xor the ciphertext with the key byte
       for byte in ciphertext:
           decrypted.append(byte ^ key)
       # try to print the decrypted message as a string
       try:
           print("Key:", key, "Message:", bytes(decrypted).decode())
       except:
           pass
   

   The answer was found for key 16: crypto{0x10_15_my_f4v0ur173_by7e}

10. You either know, XOR you don't (30 points)