add greatest common multiple

CODEGATE2024_Greatest_Common_Multiple/description.yml

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+name: Greatest Common Multiple
+authors:
+  - soon_haari
+original_ctf: "CODEGATE CTF"
+year: 2024
+description: >-
+  "My boss 1nteger_c said we will come back with a better GCM challenge. I hereby present, the **Greatest Common Multiple**."
+base64_flag: 'Y29kZWdhdGUyMDI0e0lzX2l0X25vcm1hbF9pZl9zb21lX2RhdGFfaXNfb2J0YWluYWJsZV9qdXN0X3dpdGhfdGFnPy4uLkZfMl4xMjgnc19zdXBlcl9wcm9wZXJ0eX0='
+flag_format: 'codegate2024{.*}'

CODEGATE2024_Greatest_Common_Multiple/release_files/chall.py

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+from Crypto.Cipher import AES
+from os import urandom
+from signal import alarm
+from secret import PoW, flag
+
+PoW(26)
+alarm(20)
+
+gen = lambda: urandom(12)
+randbit = lambda: gen()[0] & 1
+
+key, nonce = urandom(16), gen()
+
+new = lambda: AES.new(key, AES.MODE_GCM, nonce)
+s = [gen(), gen()]
+
+while cmd := input("> "):
+	if cmd == "tag":
+		cipher = new()
+		cipher.update(s[0])
+		cipher.encrypt(s[1])
+		print(f"tag: {cipher.digest().hex()}")
+
+	elif cmd == "u1":
+		s[randbit()] = gen()
+
+	elif cmd == "u2":
+		s[randbit()] += gen()
+
+if input(f"tag: ") == new().digest().hex():
+	print(flag)

CODEGATE2024_Greatest_Common_Multiple/server_files/Dockerfile

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+FROM ubuntu:22.04@sha256:965fbcae990b0467ed5657caceaec165018ef44a4d2d46c7cdea80a9dff0d1ea
+
+RUN apt update
+RUN apt-get install -y socat python3 python3-pip
+
+ENV TERM=linux
+
+RUN pip install pycryptodome
+
+COPY ./src/chall.py /
+COPY ./src/flag.txt /
+COPY ./src/banner.txt /
+COPY ./src/secret.py /
+
+CMD socat TCP-LISTEN:9999,reuseaddr,fork EXEC:"python3 /chall.py"
+
+EXPOSE 9999

CODEGATE2024_Greatest_Common_Multiple/server_files/src/banner.txt

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+
+  ▄████  ▄████▄   ███▄ ▄███▓
+ ██▒ ▀█▒▒██▀ ▀█  ▓██▒▀█▀ ██▒
+▒██░▄▄▄░▒▓█    ▄ ▓██    ▓██░
+░▓█  ██▓▒▓▓▄ ▄██▒▒██    ▒██ 
+░▒▓███▀▒▒ ▓███▀ ░▒██▒   ░██▒
+ ░▒   ▒ ░ ░▒ ▒  ░░ ▒░   ░  ░
+  ░   ░   ░  ▒   ░  ░      ░
+░ ░   ░ ░        ░      ░   
+      ░ ░ ░             ░   
+        ░                    

CODEGATE2024_Greatest_Common_Multiple/server_files/src/chall.py

@@ -0,0 +1,31 @@
+from Crypto.Cipher import AES
+from os import urandom
+from signal import alarm
+from secret import PoW, flag
+
+PoW(26)
+alarm(20)
+
+gen = lambda: urandom(12)
+randbit = lambda: gen()[0] & 1
+
+key, nonce = urandom(16), gen()
+
+new = lambda: AES.new(key, AES.MODE_GCM, nonce)
+s = [gen(), gen()]
+
+while cmd := input("> "):
+	if cmd == "tag":
+		cipher = new()
+		cipher.update(s[0])
+		cipher.encrypt(s[1])
+		print(f"tag: {cipher.digest().hex()}")
+
+	elif cmd == "u1":
+		s[randbit()] = gen()
+
+	elif cmd == "u2":
+		s[randbit()] += gen()
+
+if input(f"tag: ") == new().digest().hex():
+	print(flag)

CODEGATE2024_Greatest_Common_Multiple/server_files/src/flag.txt

@@ -0,0 +1 @@
+codegate2024{Is_it_normal_if_some_data_is_obtainable_just_with_tag?...F_2^128's_super_property}

CODEGATE2024_Greatest_Common_Multiple/server_files/src/secret.py

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+import hashlib
+import os
+import random
+import string
+
+colors = ['\033[94m', '\033[96m', '\033[95m']
+reset = '\033[0m'
+
+banner = ''.join(random.choice(colors) + char + reset if char != ' ' else char for char in open("banner.txt", "r").read())
+
+
+colors = ["\033[0;32m", "\033[0;33m", "\033[0;34m", "\033[0;35m", "\033[0;36m", "\033[0;37m", "\033[0;31m", "\033[38;5;248m", "\033[0m"]
+green, yellow, blue, purple, cyan, white, red, grey, reset = colors
+
+flag = ""
+
+for c in open("flag.txt", "r").read(): flag += colors[random.randrange(0, 6)] + c
+flag += reset
+
+def PoW(bits):
+	print(banner)
+	n = random.randrange(0, 2**bits)
+	a = "".join(random.choices(string.ascii_letters, k=20)).encode()
+	hsh = hashlib.md5(str(n).encode() + a).digest()
+
+	print(f"md5(str(n).encode() + {a}).hexdigest() = {bytes.hex(hsh)}")
+	if int(input(f"Input the decimal result of n within range(2**{bits}): ")) != n:
+		exit()