cscg ist super

2026/cscg/crypto/zipper/Dockerfile

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+FROM sagemath/sagemath:10.6@sha256:19995db6194f4a4bab18ce9a88556fd15b9ed5e916b4504fefe618a7796ddbdb
+
+USER root
+
+ADD --chmod=0755 --checksum=sha256:c125df9762b0c7233459087bb840c0e5dbfc4d9690ee227f1ed8994f4d51d2e0 \
+    https://raw.githubusercontent.com/reproducible-containers/repro-sources-list.sh/v0.1.4/repro-sources-list.sh \
+    /usr/local/bin/repro-sources-list.sh
+RUN repro-sources-list.sh
+
+RUN apt-get update && apt-get install -y xinetd
+
+RUN sage -pip install --no-cache-dir --upgrade pip
+RUN sage -pip install --no-cache-dir pycryptodome
+
+RUN mkdir /ctf
+WORKDIR /ctf
+
+RUN echo "Connection blocked." > /etc/banner_fail
+COPY ctf.xinetd /etc/xinetd.d/ctf
+
+COPY run.sh .
+COPY challenge.py .
+
+RUN chown -R root:sage /ctf && chmod -R 750 /ctf && chmod 777 /ctf/run.sh && chmod 770 /ctf
+
+CMD [ "xinetd", "-dontfork" ]
+
+EXPOSE 1337

2026/cscg/crypto/zipper/challenge.py

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+from sage.all import *
+from Crypto.Util.number import long_to_bytes
+from Crypto.Util.Padding import pad
+import gzip, base64, os, string, random
+
+FLAG = os.getenv("FLAG", "dach2026{fake_flag}")
+REQUIRED_WINS = 25
+
+p = 0x1337
+Fp = GF(p)
+
+I = matrix.random(Fp, 4)
+
+def hash(message: bytes):
+    m = pad(message, 25)
+    assert len(m) % 25 == 0
+
+    sections = []
+    for i in range(0, len(m), 25):
+        block = int(m[i:i+25].hex(), 16)
+
+        digits = []
+        for _ in range(16):
+            digits.append(block % p)
+            block //= p
+
+        sections.append(matrix(Fp, [digits[:4], digits[4:8], digits[8:12], digits[12:]]))
+
+    A = I
+    for S in sections:
+        A *= S
+
+    hashsum = 0
+    for el in sum([list(r) for r in A.rows()], [])[::-1]:
+        hashsum *= p
+        hashsum += int(el)
+
+    return long_to_bytes(hashsum, 25)
+
+def random_hash():
+    return hash(os.urandom(25))
+
+def random_message(n):
+    return "".join([random.choice(string.ascii_letters) for _ in range(n)])
+
+def main():
+    print("Welcome to ZIPPER!")
+
+    wins = 0
+
+    while True:
+        target_message = random_message(32)
+        target_hash = random_hash() # Make this game impossible ( ͡° ͜ʖ ͡°)
+
+        print(f"I am thinking of a gzip file that decompresses to '{target_message}' and has a hash of '{target_hash.hex()}'.")
+        print("What file am I thinking of?")
+        user_input = input("> ")
+
+        try:
+            file = base64.b64decode(user_input)
+            file_hash = hash(file)
+
+            if len(file) == 0:
+                continue
+
+            if file_hash != target_hash:
+                wins = 0
+                print(f"Wrong hash: {file_hash.hex()} != {target_hash.hex()}.")
+                continue
+
+            if gzip.decompress(file).decode() != target_message:
+                wins = 0
+                print("Wrong data!")
+                continue
+
+            wins += 1
+            print(f"CORRECT! You have {REQUIRED_WINS-wins} wins more to go to get a prize!")
+
+            if wins >= REQUIRED_WINS:
+                print("Congratulations, you've outguessed me!")
+                print("\n" + FLAG + "\n")
+                wins = 0
+        except KeyboardInterrupt:
+            exit(0)
+        except:
+            print("Wrong **insert whatever here**!")
+            wins = 0
+
+if __name__ == "__main__":
+    main()

2026/cscg/crypto/zipper/ctf.xinetd

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+service ctf
+{
+    disable     = no
+    socket_type = stream
+    protocol    = tcp
+    wait        = no
+    user        = sage
+    type        = UNLISTED
+    port        = 1337
+    bind        = 0.0.0.0
+    server      = /bin/sh
+    server_args = /ctf/run.sh
+    banner_fail = /etc/banner_fail
+    passenv     = FLAG HOME
+    # safety options
+    per_source  = 10    # the maximum instances of this service per source IP address
+    rlimit_cpu  = 1     # the maximum number of CPU seconds that the service may use
+    #rlimit_as  = 1024M # the Address Space resource limit for the service
+}

2026/cscg/crypto/zipper/docker-compose.yml

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+version: "3"
+services:
+  zipper:
+    image: dach2026-cry-zipper:latest
+    build:
+      context: .
+    ports:
+      - "1337:1337"

2026/cscg/crypto/zipper/run.sh

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+#!/bin/bash
+
+cd /ctf
+sage -python challenge.py