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# BDSEC

> Achieved Rank 3 With HCS

| Name | Category | Solved? |
| - | - | - |
| Poisoned Ledger Hex | Forensics | ✅ |
| Router | Networking | ✅ |

***

## Poisoned Ledger Hex

### TL;DR

Extract the three fragmented `OP_RETURN` payloads from blocks 101, 108, and 117, concatenate them in block‐order, XOR each byte with `0x42` (ASCII “B”), and you get the flag.

### Solution

1. Locate the `OP_RETURN` fragments

Inspecting each block’s transactions, you find three blocks carrying non‐standard data fields:

* **Block 101**: `OP_RETURN` → `[0, 6, 17, 7, 1, 57, 0, 14, 114, 1]`
* **Block 108**: `OP_RETURN` → `[9, 29, 1, 10, 3, 11, 44, 29, 6, 55]`
* **Block 117**: `OP_RETURN` → `[47, 18, 29, 115, 119, 29, 4, 55, 44, 44, 27, 63]`

2. **Reassemble the data**\ Concatenate the three arrays **in ascending block order** to get a single byte stream:

```python theme={null}
data = [
  0,6,17,7,1,57,0,14,114,1,
  9,29,1,10,3,11,44,29,6,55,
  47,18,29,115,119,29,4,55,44,44,27,63
]
```

3. **Decode with XOR**\ The hint (“Poisoned Ledger”) suggests a simple XOR cipher. Using `B` as the key:

```python theme={null}
xor_key = ord('B')  # 0x42
decoded = ''.join(chr(b ^ xor_key) for b in data)
print(decoded)
```

### Flag

`BDSEC\{BL0CK_CHAIn_DumP_15_FunnY\}`

***

## Router

### TL;DR

We need to find a company from the router that is being used from the given pcap file

### Solution

We just need to check each of the mac address that is captured from the pcap file and search it on google

![](https://260992468-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FeX7f2pvLxkrgE7gSlDCX%2Fuploads%2Fuahjeyx6pLck9SCm1dbg%2Fimage.png?alt=media\&token=b3819a1e-f508-4dc8-b5cc-e444e5dea543)

`Ethernet II, Src: NetisTechnol_47:fa:42 (64:ee:b7:47:fa:42), Dst: LiteonTechno_7e:74:6b (9c:2f:9d:7e:74:6b)`

from this [website](https://www.netis-systems.com/), we know the answer is `netis`

### Flag

`BDSEC\{netis\}`


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