crypto/encryptor-shop (#27)

* challenge

* dockerfile

* better solvepath

Co-authored by: glacialcascade <[email protected]>

* fix deploy

* change solution (#73)

---------

Co-authored-by: glacialcascade <[email protected]>

enc-shop/Dockerfile

@@ -0,0 +1,26 @@
+FROM --platform=linux/amd64 ubuntu:20.04 AS build
+
+RUN apt-get update -y && apt-get install -y gcc wget unzip && rm -rf /var/lib/apt/lists/*
+
+RUN wget -Oynetd.c https://raw.githubusercontent.com/johnsonjh/ynetd/master/ynetd.c \
+    && gcc -o ynetd ynetd.c \
+    && rm -f /tmp/ynetd.zip
+
+FROM --platform=linux/amd64 python:3.12-slim-bookworm
+
+RUN useradd -m -d /home/ctf -u 12345 ctf
+WORKDIR /home/ctf
+
+COPY server.py ./
+COPY requirements.txt ./
+COPY flag.txt ./
+
+COPY --from=build ynetd ynetd
+RUN chmod +x ynetd
+
+RUN chown -R root:root /home/ctf
+RUN pip install -r requirements.txt
+
+USER ctf
+EXPOSE 3000
+CMD ["./ynetd", "-p", "3000", "python3 server.py"]

enc-shop/chall.yaml

@@ -0,0 +1,21 @@
+name: Encryptor Shop
+categories:
+  - crypto
+value: 50
+flag:
+  file: ./flag.txt
+description: |-
+  After realizing how insecure the systems of many companies are (they're
+  always getting hacked), I decided to start offering Encryption as a 
+  Service (EaaS). With such a strong guarantee of security, I'll even 
+  give you the source code AND my encrypted super secret flag.
+hints: []
+files:
+  - src: ./server.py
+authors:
+  - Mudasir
+deploy:
+  nc:
+    build: .
+    expose: 5000/tcp
+visible: true

enc-shop/flag.txt

@@ -0,0 +1 @@
+bcactf{w0w_@lg3br@_d3in48uth934r}

enc-shop/requirements.txt

@@ -0,0 +1 @@
+pycryptodome==3.20.0

enc-shop/server.py

@@ -0,0 +1,37 @@
+from Crypto.Util.number import *
+
+p = getPrime(1024)
+q = getPrime(1024)
+r = getPrime(1024)
+n = p * q
+phi = (p - 1) * (q - 1)
+e = 65537
+d = pow(e, -1, phi)
+
+print("Welcome to the enc-shop!")
+print("What can I encrypt for you today?")
+
+
+for _ in range(3):
+    message = input("Enter text to encrypt: ")
+    m = bytes_to_long(message.encode())
+    c = pow(m, e, n)
+    print(f"Here is your encrypted message: {c}")
+    print(f"c = {c}")
+    print("Here is the public key for your reference:")
+    print(f"n = {n}")
+    print(f"e = {e}")
+
+print("Thank you for encrypting with us!")
+print("In order to guarantee the security of your data, we will now let you view the encrypted flag.")
+x=input("Would you like to view it? (yes or no) ")
+
+if x.lower() == "yes":
+    with open("flag.txt", "r") as f:
+        flag = f.read().strip()
+    m = bytes_to_long(flag.encode())
+    n = p*r
+    c = pow(m, e, n)
+    print(f"Here is the encrypted flag: {c}")
+    print("Here is the public key for your reference:")
+    print(f"n = {n}")

enc-shop/solve.sage

@@ -0,0 +1,18 @@
+#!/usr/bin/sage
+from Crypto.Util.number import long_to_bytes
+
+# sample values
+n = 25672717426820513710943205839364483916462804218548801569145802652630577782900538695973744861105496729905627921059008367561569523305960463813216527736798326653292438086191944762789172647312900551902461060352376019547331143616013304184614930906004171476480510470587909456774815137176855734430612064982542863659570395295482686120891117968898216622206615854128611416469340618239366172641009539517658878551692132791618016300846596344387852192596993173217855138296633962850557028630352495932230646527202752234601562497215822764663650767710458962369922520233739420580532819716680556394145993527683129009216404336549252330091
+e = 65537
+flag = 1083625496779970264419136305221671016117829663020044050146555349899787834011590086920869802111023991613821066623716559972755949113701501874048562224439274720251945134284083684492761219205132838372764452677281768274305372099912532025957284473071373317196230163363640098605346443685525909370527747332955548675206412274584950839734716346547094425347696118475715583261416077609393966266153722274187119120682987915292966511175589928588362998907734884276112744211599285641171140402681639667044788628645470007439249161157400845406659256582870356000464445445478989477502854810644166856995360456670068896815610415108264478323
+n2 = 27368115041311256275227024917950858454235788667021073198311116355186647544767717811973935453582324380335430354834788466153181148557334562671645375401434653132878379082952166788670952485522953276925557458943115441308780807836250927064634092981175701891508525682899992903672949137706472233445943878804298329629278526495430527687968010293979124319259469576481820335370026921880073228386101660016332613679667679937345493922150203642222273091669777337735254184786282363582190082715868568569189222774625804615657773286270123197871252891782578640230520633961840128629257859130644151809317074356832564646866829943858656213303
+
+# n and n2 share a factor p, so we can efficiently
+# find p using gcd (euclidean algorithm)
+
+p = gcd(n, n2)
+r = n2 // p
+
+d = pow(e, -1, (p-1)*(r-1))
+flag = pow(flag, d, n2)
+print(long_to_bytes(flag))