2026-08-21 13:51:31 +07:00
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---
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title: "Back and Forth — XOR"
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2026-08-21 14:28:04 +07:00
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type: "writeup"
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section: "writeups"
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status: "published"
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author: "asep"
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tags: ["gemastik", "ctf", "warmup", "reverse"]
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path: "writeups/gemastik-2026-warmup/ch8-back-and-forth.md"
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created_at: "2026-08-21T07:25:37.394Z"
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updated_at: "2026-08-21T07:25:37.394Z"
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2026-08-21 13:51:31 +07:00
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event: "GEMASTIK 2026 Warmup"
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2026-08-21 14:28:04 +07:00
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challenge: "Back and Forth — XOR"
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2026-08-21 13:51:31 +07:00
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category: "reverse"
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points: 500
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difficulty: "easy"
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flag: "GEMASTIK19{x0r_it_back_4gain}"
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---
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# Back and Forth (500 pts) — Reversing / XOR
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**File:** `chall.c`
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Flag di-XOR dengan kunci 1-byte `0x42`. Balikkan dengan XOR lagi:
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```python
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enc = [0x05, 0x07, 0x0f, 0x03, 0x11, 0x16, 0x0b, 0x09, 0x73, 0x7b, 0x39, 0x3a,
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0x72, 0x30, 0x1d, 0x2b, 0x36, 0x1d, 0x20, 0x23, 0x21, 0x29, 0x1d, 0x76,
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0x25, 0x23, 0x2b, 0x2c, 0x3f]
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flag = "".join(chr(b ^ 0x42) for b in enc)
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print(flag)
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# GEMASTIK19{x0r_it_back_4gain}
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```
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## Flag
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2026-08-21 14:28:04 +07:00
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`GEMASTIK19{x0r_it_back_4gain}`
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