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

content/writeups/gemastik-2026-warmup/ch1-sixty-four.md

@@ -0,0 +1,43 @@
+---
+title: "Sixty Four — Double Base64"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Sixty Four"
+category: "crypto"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}"
+---
+
+# Sixty Four (500 pts) — Encoding
+
+**File:** `chall.txt` → `UjBWTlFWTlVTVXN4T1h0aU5ITXpYM014ZUhSNVgyWXdkWEpmZDJGeWJURnVaMTkxY0gwPQ==`
+
+Teks di-encode **Base64 dua kali** (hint: "sixty four" = base64). Decode berlapis:
+
+```bash
+$ echo "UjBWTlFWTlVTVXN4T1h0aU5ITXpYM014ZUhSNVgyWXdkWEpmZDJGeWJURnVaMTkxY0gwPQ==" | base64 -d
+R0VNQVNUSUsxOXtiNHMzX3MxeHR5X2YwdXJfd2FybTFuZ191cH0=
+$ echo "R0VNQVNUSUsxOXtiNHMzX3MxeHR5X2YwdXJfd2FybTFuZ191cH0=" | base64 -d
+GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}
+```
+
+## Solusi Python
+
+```python
+import base64
+
+with open("chall.txt") as f:
+    s = f.read().strip()
+
+# First base64 decode
+decoded1 = base64.b64decode(s)
+# Second base64 decode
+flag = base64.b64decode(decoded1).decode()
+
+print(f"Flag: {flag}")
+# Output: GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}
+```
+
+## Flag
+
+`GEMASTIK19{b4s3_s1xty_f0ur_warm1ng_up}`