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services/phew/Dockerfile

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+FROM python:3.12-slim
+
+ARG PASSWORD=root
+ENV DEBIAN_FRONTEND=noninteractive
+ENV HOME=/home/ctf
+WORKDIR /home/ctf/chall
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    openssh-server \
+    build-essential \
+    libffi-dev \
+    libssl-dev \
+    python3-dev \
+    bash \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN useradd -m -d /home/ctf -s /bin/bash ctf && \
+    echo "ctf:${PASSWORD}" | chpasswd
+
+RUN mkdir -p /var/run/sshd
+
+COPY requirements.txt /tmp/requirements.txt
+RUN pip install --no-cache-dir -r /tmp/requirements.txt
+
+COPY ./src /home/ctf/chall/src
+COPY ./start.sh /start.sh
+RUN chmod +x /start.sh /home/ctf/chall/src/run.sh
+
+RUN chown -R root:root /home/ctf/chall && chmod -R 555 /home/ctf/chall
+
+EXPOSE 8000 22
+CMD ["/start.sh"]

services/phew/Pailier.py

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+from Crypto.Util.number import *
+from math import lcm
+import random
+
+class pailier:
+    def __init__(self):
+        self.primes = [getPrime(512) for _ in range(2)]
+        self.n = 1
+        self.phi = 1
+        self.mul = 1
+        for i in range(2):
+            self.n  *= self.primes[i]
+            self.phi *= (self.primes[i] - 1)  
+        self.n2 = self.n * self.n
+        self.g = [pow(random.randrange(1, self.n2), self.primes[i], self.n2) for i in range(2)]
+        for x in self.g:
+            self.mul = (self.mul * x) % self.n2
+        self.miu = inverse(self.L(pow(self.mul, self.phi, self.n2)), self.n)
+        self.alpha = [None, None]
+        for idx in range(2):
+            while True:
+                a = random.randrange(2, self.n - 1)
+                if GCD(a, self.n) == 1:
+                    self.alpha[idx] = a
+                    break
+        self.beta  = [random.randrange(0, self.n), random.randrange(0, self.n)]
+
+    def L(self, val):
+        return (val - 1) // self.n
+
+    def pubkey(self):
+        return (self.n, self.g)
+
+    def encrypt(self, msg: int) -> int:
+        r  = random.randrange(0, self.n - 1)
+        gb = self.g[random.randrange(0, 2)]
+        gm = pow(gb, msg, self.n2)
+        rn = pow(r, self.n, self.n2)
+        return (gm * rn) % self.n2
+
+    def decrypt(self, ct: int) -> int:
+        raw = self.L(pow(ct, self.phi, self.n2)) % self.n
+        raw = (raw * self.miu) % self.n
+        t = pow(ct % self.n, (self.n - 1) // 2, self.n) if (self.n % 2 == 1) else 0
+        idx = 0 if t == 1 else 1
+        return (self.alpha[idx] * raw + self.beta[idx]) % self.n
+

services/phew/chall.py

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+#!/usr/bin/env python3
+
+from Pailier import *
+from Crypto.Util.number import *
+with open("/flag.txt", "rb") as f:
+    flag = f.read()
+flag = bytes_to_long(flag)
+cipher = pailier()
+while True:
+    print("1. encrypt")
+    print("2. bingo")
+    print("3. decrypt")
+    print("4. exit")
+    inp = int(input("> "))
+    if inp==1:
+        print("pt (hex)")
+        inp = input("> ")
+        ct = cipher.encrypt(int(inp,16))
+        print('ct : ','{0:x}'.format(ct))
+    elif inp==2:
+        ct = cipher.encrypt(flag)
+        print('ct : ','{0:x}'.format(ct))
+    elif inp==3:
+        print("ct (hex)")
+        inp = input("> ")
+        pt = cipher.decrypt(int(inp,16))
+        print('pt : ','{0:x}'.format(pt))
+    else:
+        exit()

services/phew/dist/Pailier.py

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+from Crypto.Util.number import *
+from math import lcm
+import random
+
+class pailier:
+    def __init__(self):
+        self.primes = [getPrime(512) for _ in range(2)]
+        self.n = 1
+        self.phi = 1
+        self.mul = 1
+        for i in range(2):
+            self.n  *= self.primes[i]
+            self.phi *= (self.primes[i] - 1)  
+        self.n2 = self.n * self.n
+        self.g = [pow(random.randrange(1, self.n2), self.primes[i], self.n2) for i in range(2)]
+        for x in self.g:
+            self.mul = (self.mul * x) % self.n2
+        self.miu = inverse(self.L(pow(self.mul, self.phi, self.n2)), self.n)
+        self.alpha = [None, None]
+        for idx in range(2):
+            while True:
+                a = random.randrange(2, self.n - 1)
+                if GCD(a, self.n) == 1:
+                    self.alpha[idx] = a
+                    break
+        self.beta  = [random.randrange(0, self.n), random.randrange(0, self.n)]
+
+    def L(self, val):
+        return (val - 1) // self.n
+
+    def pubkey(self):
+        return (self.n, self.g)
+
+    def encrypt(self, msg: int) -> int:
+        r  = random.randrange(0, self.n - 1)
+        gb = self.g[random.randrange(0, 2)]
+        gm = pow(gb, msg, self.n2)
+        rn = pow(r, self.n, self.n2)
+        return (gm * rn) % self.n2
+
+    def decrypt(self, ct: int) -> int:
+        raw = self.L(pow(ct, self.phi, self.n2)) % self.n
+        raw = (raw * self.miu) % self.n
+        t = pow(ct % self.n, (self.n - 1) // 2, self.n) if (self.n % 2 == 1) else 0
+        idx = 0 if t == 1 else 1
+        return (self.alpha[idx] * raw + self.beta[idx]) % self.n
+

services/phew/dist/chall.py

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+#!/usr/bin/env python3
+
+from Pailier import *
+from Crypto.Util.number import *
+with open("/flag.txt", "rb") as f:
+    flag = f.read()
+flag = bytes_to_long(flag)
+cipher = pailier()
+while True:
+    print("1. encrypt")
+    print("2. bingo")
+    print("3. decrypt")
+    print("4. exit")
+    inp = int(input("> "))
+    if inp==1:
+        print("pt (hex)")
+        inp = input("> ")
+        ct = cipher.encrypt(int(inp,16))
+        print('ct : ','{0:x}'.format(ct))
+    elif inp==2:
+        ct = cipher.encrypt(flag)
+        print('ct : ','{0:x}'.format(ct))
+    elif inp==3:
+        print("ct (hex)")
+        inp = input("> ")
+        pt = cipher.decrypt(int(inp,16))
+        print('pt : ','{0:x}'.format(pt))
+    else:
+        exit()

services/phew/dist/flag.txt

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+GEMASTIK{AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}

services/phew/docker-compose.yml

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+version: "3.8"
+
+services:
+  phew:
+    container_name: phew_container
+    hostname: phew
+    restart: always
+    build:
+      context: .
+      args:
+        - PASSWORD=root
+    ports:
+      - "13000:8000"
+      - "13022:22"
+    environment:
+      - FLAG=GEMASTIK{local_flag}

services/phew/flag.txt

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+GEMASTIK{AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA}

services/phew/requirements.txt

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+pycryptodome

services/phew/run.sh

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+#!/bin/sh
+
+exec socat tcp-l:8000,reuseaddr,fork exec:"python3 ./chall.py"

services/phew/start.sh

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+#!/bin/bash
+set -e
+
+ssh-keygen -A
+
+# Configure SSH
+grep -q "^PermitRootLogin" /etc/ssh/sshd_config && \
+  sed -i "s/^PermitRootLogin.*/PermitRootLogin no/" /etc/ssh/sshd_config || \
+  echo "PermitRootLogin no" >> /etc/ssh/sshd_config
+
+grep -q "^PasswordAuthentication" /etc/ssh/sshd_config && \
+  sed -i "s/^PasswordAuthentication.*/PasswordAuthentication yes/" /etc/ssh/sshd_config || \
+  echo "PasswordAuthentication yes" >> /etc/ssh/sshd_config
+
+echo "AllowUsers ctf" >> /etc/ssh/sshd_config
+
+/usr/sbin/sshd
+
+if [ -n "$FLAG" ]; then
+    echo "$FLAG" > /flag.txt
+    chmod 644 /flag.txt
+    chown root:root /flag.txt
+fi
+
+exec su -c "cd /home/ctf/chall/src && ./run.sh" -s /bin/bash ctf