{"id":960,"date":"2026-04-13T13:06:25","date_gmt":"2026-04-13T05:06:25","guid":{"rendered":"http:\/\/shr1mp.top\/?p=960"},"modified":"2026-04-13T13:06:45","modified_gmt":"2026-04-13T05:06:45","slug":"desctf","status":"publish","type":"post","link":"http:\/\/shr1mp.top\/index.php\/2026\/04\/13\/desctf\/","title":{"rendered":"DesCTF"},"content":{"rendered":"\n

wireshark<\/h2>\n\n\n\n
\n

Traffic revealed normal Modbus traffic plus \"extra\" communications\u2014all protocol-compliant, evading firewall alerts. Attacker knows the protocol.<\/p>\n\n\n\n

\u8bd1\uff1a\u6d41\u91cf\u663e\u793a\u6b63\u5e38\u7684Modbus\u6d41\u91cf\u52a0\u4e0a\u201c\u989d\u5916\u201d\u7684\u901a\u4fe1\u2014\u2014\u6240\u6709\u534f\u8bae\u90fd\u7b26\u5408\uff0c\u53ef\u4ee5\u907f\u5f00\u9632\u706b\u5899\u8b66\u62a5\u3002\u653b\u51fb\u8005\u77e5\u9053\u534f\u8bae\u3002<\/p>\n<\/blockquote>\n\n\n\n

\u9996\u5148\u62ff\u5230\u4e00\u4e2a\u6d41\u91cf\u5305\uff0c\u4e00\u5171\u6709\u56db\u4e2a\u6d41\uff081-4\uff09\uff0c\u53d1\u73b0\u662f\u5de5\u63a7\u6d41\u91cf\uff0c\u5168\u662fmodbus<\/p>\n\n\n\n

192.168.100.102\u8fd4\u56de\u7684Read Holding Registers\u54cd\u5e94\u4e2d\u5305\u542b\u4e86\u4e00\u4e2a\u5047\u7684flag<\/p>\n\n\n\n

\u68c0\u67e5\u5176\u4ed6Modbus\u54cd\u5e94\uff0c\u53d1\u73b0192.168.100.101\u7684\u54cd\u5e94\u4e2d\u5305\u542b\u7279\u6b8a\u5b57\u7b26\u4e32\uff1a<\/p>\n\n\n\n

S7COMM01<\/code><\/pre>\n\n\n\n
\u67e5\u8be2\u8d44\u6599\u5f97\u5230\uff1a\u8fd9\u662fDES\u5bc6\u94a5\uff088\u5b57\u8282\u957f\u5ea6\uff0c\u7b26\u5408DES\u5bc6\u94a5\u7279\u5f81\uff09<\/p>\n\n\n\n
#!\/usr\/bin\/env python3\nimport struct\nimport socket\nfrom typing import List, Dict, Optional\nfrom Crypto.Cipher import DES\nPCAP_FILE = \"challenge.pcapng\"\ndef parse_pcap(path: str) -> List[Dict]:\n    \"\"\"Parse PCAP file and extract Modbus\/TCP packets.\"\"\"\n    with open(path, \"rb\") as f:\n        data = f.read()\n    # Detect endianness from magic number\n    endian = {\n        b\"\\xd4\\xc3\\xb2\\xa1\": \"<\",\n        b\"\\xa1\\xb2\\xc3\\xd4\": \">\"\n    }.get(data[:4])\n    if endian is None:\n        raise ValueError(\"Unsupported pcap format\")\n    offset = 24  # Skip global header\n    packets = []\n    pkt_index = 0\n    while offset + 16 <= len(data):\n        # Parse packet header\n        ts_sec, ts_usec, incl_len, orig_len = struct.unpack(\n            f\"{endian}IIII\", data[offset:offset + 16]\n        )\n        offset += 16\n        \n        # Extract packet data\n        raw = data[offset:offset + incl_len]\n        offset += incl_len\n        pkt_index += 1\n        # Skip non-IPv4 packets\n        if len(raw) < 14 or struct.unpack(\"!H\", raw[12:14])[0] != 0x0800:\n            continue\n        # Parse IP header\n        ip = raw[14:]\n        if len(ip) < 20 or ip[9] != 6:  # Skip non-TCP packets\n            continue\n        src_ip = socket.inet_ntoa(ip[12:16])\n        dst_ip = socket.inet_ntoa(ip[16:20])\n        # Parse TCP header\n        tcp = ip[(ip[0] & 0x0F) * 4:]\n        if len(tcp) < 20:\n            continue\n        src_port, dst_port = struct.unpack(\"!HH\", tcp[:4])\n        if 502 not in (src_port, dst_port):  # Skip non-Modbus packets\n            continue\n        payload = tcp[(tcp[12] >> 4) * 4:]\n        if len(payload) < 8:  # Skip incomplete Modbus headers\n            continue\n        # Parse Modbus\/TCP MBAP\n        tid, pid, length = struct.unpack(\"!HHH\", payload[:6])\n        unit_id = payload[6]\n        func_code = payload[7]\n        pdu = payload[8:]\n        packets.append({\n            \"index\": pkt_index,\n            \"ts\": ts_sec + ts_usec \/ 1_000_000,\n            \"src\": src_ip,\n            \"dst\": dst_ip,\n            \"sport\": src_port,\n            \"dport\": dst_port,\n            \"tid\": tid,\n            \"unit\": unit_id,\n            \"func\": func_code,\n            \"pdu\": pdu,\n            \"raw\": payload,\n        })\n    return packets\ndef printable(bs: bytes) -> str:\n    \"\"\"Convert bytes to printable ASCII with dots for non-printable chars.\"\"\"\n    return \"\".join(chr(b) if 32 <= b < 127 else \".\" for b in bs)\ndef des_ecb_decrypt(ciphertext: bytes, key: bytes) -> bytes:\n    \"\"\"Decrypt DES-ECB using pycryptodome.\"\"\"\n    cipher = DES.new(key, DES.MODE_ECB)\n    return cipher.decrypt(ciphertext)\ndef find_fake_flag(packets: List[Dict]) -> Optional[str]:\n    \"\"\"Find fake flag in Modbus Read Holding Registers responses.\"\"\"\n    for pkt in packets:\n        if (pkt[\"func\"] == 3 and pkt[\"src\"] == \"192.168.100.102\" \n                and len(pkt[\"pdu\"]) >= 1):\n            text = printable(pkt[\"pdu\"][1:])\n            if \"flag{\" in text:\n                return text[text.index(\"flag{\"):]\n    return None\ndef find_des_key(packets: List[Dict]) -> Optional[Dict]:\n    \"\"\"Find DES key in Modbus Read Holding Registers responses.\"\"\"\n    for pkt in packets:\n        if (pkt[\"func\"] == 3 and pkt[\"src\"] == \"192.168.100.101\" \n                and len(pkt[\"pdu\"]) >= 1 and b\"S7COMM01\" in pkt[\"pdu\"][1:]):\n            return {\n                \"key\": b\"S7COMM01\",\n                \"packet_index\": pkt[\"index\"]\n            }\n    return None\ndef extract_cipher_blocks(packets: List[Dict], after_index: int) -> List[bytes]:\n    \"\"\"Extract cipher blocks from Diagnostics\/Return Query Data packets.\"\"\"\n    blocks = []\n    for pkt in packets:\n        if pkt[\"index\"] <= after_index:\n            continue\n            \n        if not (pkt[\"src\"] == \"192.168.100.10\" \n                and pkt[\"dst\"] == \"192.168.100.101\" \n                and pkt[\"func\"] == 8 \n                and len(pkt[\"pdu\"]) >= 10):\n            continue\n            \n        subfunc = struct.unpack(\"!H\", pkt[\"pdu\"][:2])[0]\n        data = pkt[\"pdu\"][2:]\n        \n        # Filter out obvious test messages and collect valid blocks\n        if subfunc == 0x0000 and len(data) == 8 and data not in (b\"LINK\", b\"PING\", b\"TEST\", b\"ECHO\"):\n            blocks.append(data)\n            if len(blocks) == 6:\n                break\n                \n    return blocks\ndef main():\n    packets = parse_pcap(PCAP_FILE)\n    # Step 1: Find fake flag and DES key\n    fake_flag = find_fake_flag(packets)\n    if fake_flag:\n        print(\"[+] Fake flag found:\", fake_flag)\n    key_info = find_des_key(packets)\n    if not key_info:\n        raise RuntimeError(\"DES key not found\")\n        \n    des_key = key_info[\"key\"]\n    key_packet_index = key_info[\"packet_index\"]\n    print(f\"[+] DES key found in packet #{key_packet_index}: {des_key!r}\")\n    # Step 2: Extract cipher blocks after key packet\n    blocks = extract_cipher_blocks(packets, key_packet_index)\n    if len(blocks) != 6:\n        raise RuntimeError(f\"Expected 6 blocks, got {len(blocks)}\")\n    ciphertext = b\"\".join(blocks)\n    print(\"[+] Ciphertext:\", ciphertext.hex())\n    # Step 3: Decrypt and extract flag\n    plaintext = des_ecb_decrypt(ciphertext, des_key)\n    print(\"[+] Decrypted raw:\", plaintext)\n    # Remove PKCS#7 padding\n    pad = plaintext[-1]\n    if 1 <= pad <= 8 and plaintext.endswith(bytes([pad]) * pad):\n        plaintext = plaintext[:-pad]\n    print(\"[+] Real flag:\", plaintext.decode())\nif __name__ == \"__main__\":\n    main()<\/code><\/pre>\n\n\n\n
\u8f93\u51fa\uff1a<\/p>\n\n\n\n
[+] Fake flag found: flag{this_is_fake_try_harder!}\n[+] DES key found in packet #30612: b'S7COMM01'\n[+] Ciphertext: ded7825ede4fd19c9f37371c37c6fa2d54e6fe2801f0df1d763175a586db1c629efa82d0f8eacb417b4419392b4a6aa8\n[+] Decrypted raw: b'flag{a3f8e2d1-7c19-4a6b-b5e8-9d2f0c4a7e31}\\x06\\x06\\x06\\x06\\x06\\x06'\n[+] Real flag: flag{a3f8e2d1-7c19-4a6b-b5e8-9d2f0c4a7e31}<\/code><\/pre>\n\n\n\n
Real sign in<\/h2>\n\n\n\n
\n
The picture for pixls \uff0cPlease find the secret and send it to the WeChat official account background to get the flag\uff01\uff01<\/p>\n\n\n\n
\u8bd1\uff1a\u56fe\u7247\u4e3apixls\uff0c\u8bf7\u627e\u5230\u79d8\u5bc6\u5e76\u5c06\u5176\u53d1\u9001\u5230\u5fae\u4fe1\u516c\u4f17\u53f7\u540e\u53f0\u4ee5\u83b7\u53d6\u65d7\u5e1c<\/p>\n<\/blockquote>\n\n\n\n
\u62ff\u5230\u7684\u538b\u7f29\u5305\u7ecf\u8fc7\u52a0\u5bc6\uff0c\u540c\u65f6\u53d1\u73b0\u538b\u7f29\u5305\u52a0\u5bc6\u7b97\u6cd5\u4e3a\uff1aZipCrypto<\/code>\uff0c\u538b\u7f29\u7b97\u6cd5\u9700\u4e3a\uff1aStore<\/code><\/p>\n\n\n\n
\u5e76\u4e14\u91cc\u9762\u6709PNG\u6587\u4ef6\uff0c\u60f3\u5230\u8fdb\u884c\u660e\u6587\u7206\u7834<\/p>\n\n\n\n
C:\\Users\\q1388\\Desktop\\\u5de5\u5177\\bkcrack-1.8.1-win64>bkcrack.exe -C 1.zip -c challenge.png -p png_header\nbkcrack 1.8.1 - 2025-10-25\n[15:26:41] Z reduction using 9 bytes of known plaintext\n100.0 % (9 \/ 9)\n[15:26:41] Attack on 712831 Z values at index 6\nKeys: 5eb34ede c49019bf 815834b9\n42.5 % (302894 \/ 712831)\nFound a solution. Stopping.\nYou may resume the attack with the option: --continue-attack 302894\n[15:28:07] Keys\n5eb34ede c49019bf 815834b9\n\n\nC:\\Users\\q1388\\Desktop\\\u5de5\u5177\\bkcrack-1.8.1-win64>bkcrack -C 1.zip -c challenge.png -k 5eb34ede c49019bf 815834b9 -U out.zip 123\nbkcrack 1.8.1 - 2025-10-25\n[15:33:00] Writing unlocked archive out.zip with password \"123\"\n100.0 % (1 \/ 1)\nWrote unlocked archive.<\/code><\/pre>\n\n\n\n
\u6253\u5f00png\uff1a<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
\u63a5\u4e0b\u6765\u662f\uff1a<\/p>\n\n\n\n
-------------------------------------------------------------------------------------------------------------------<\/p>\n\n\n\n
Zigzag\u53d8\u6362\uff08Zigzag Transformation\uff09<\/p>\n\n\n\n
\u77e5\u8bc6\u70b9\uff1a\nZigzag\u53d8\u6362\u901a\u8fc7\u4e4b\u5b57\u5f62\u8def\u5f84\u904d\u5386\u77e9\u9635\u5143\u7d20\uff0c\u5c06\u4e8c\u7ef4\u6570\u636e\u5c55\u5e73\u4e3a\u4e00\u7ef4\u5e8f\u5217\u3002\u8fd9\u79cd\u904d\u5386\u65b9\u5f0f\u786e\u4fdd\u4e86\u76f8\u90bb\u7684\u4e8c\u7ef4\u6570\u636e\u5728\u4e00\u7ef4\u5e8f\u5217\u4e2d\u4e5f\u4fdd\u6301\u76f8\u90bb\u3002\n\u539f\u59cb\u77e9\u9635:          Zigzag\u5e8f\u5217:\n0  1  5  6         0 \u2192 1 \u2192 2 \u2192 3 \u2192 4 \u2192 5 \u2192 6 \u2192 7 \u2192 8\n2  4  7  8         \n3  8  9  10\n\u5b9e\u9645\u904d\u5386\u8def\u5f84\uff1a\n\u2192 \u2198 \u2193 \u2198\n\u2193 \u2197 \u2192 \u2198\n\u2192 \u2198 \u2193 \u2198\n\n**\u4e3b\u8981\u5e94\u7528***\n1. JPEG\u56fe\u50cf\u538b\u7f29\uff08\u6700\u8457\u540d\uff09\n    \u5bf9DCT\uff08\u79bb\u6563\u4f59\u5f26\u53d8\u6362\uff09\u7cfb\u6570\u77e9\u9635\u8fdb\u884cZigzag\u626b\u63cf\n    \u5c06\u4f4e\u9891\u7cfb\u6570\uff08\u91cd\u8981\u4fe1\u606f\uff09\u6392\u5728\u524d\u9762\uff0c\u9ad8\u9891\u7cfb\u6570\uff08\u901a\u5e38\u4e3a\u96f6\uff09\u6392\u5728\u540e\u9762\n    \u4fbf\u4e8e\u540e\u7eed\u7684\u884c\u7a0b\u7f16\u7801\u548c\u970d\u592b\u66fc\u7f16\u7801\n2. \u89c6\u9891\u7f16\u7801\uff08H.264\/HEVC\u7b49\uff09\n    \u7528\u4e8e\u53d8\u6362\u7cfb\u6570\u7684\u71b5\u7f16\u7801\u4f18\u5316\n3. \u6570\u636e\u538b\u7f29\u4e0e\u52a0\u5bc6\n    \u6539\u53d8\u6570\u636e\u6392\u5217\u987a\u5e8f\uff0c\u589e\u5f3a\u538b\u7f29\u6548\u7387\u6216\u5b89\u5168\u6027<\/code><\/pre>\n\n\n\n
\u7b97\u6cd5\u5b9e\u73b0\u8981\u70b9\uff1a<\/p>\n\n\n\n
def zigzag(matrix):\n    rows, cols = len(matrix), len(matrix[0])\n    result = []\n    for s in range(rows + cols - 1):\n        if s % 2 == 0:  # \u5076\u6570\u5bf9\u89d2\u7ebf\uff1a\u5411\u4e0a\u904d\u5386\n            for i in range(max(0, s - cols + 1), min(s + 1, rows)):\n                j = s - i\n                result.append(matrix[i][j])\n        else:  # \u5947\u6570\u5bf9\u89d2\u7ebf\uff1a\u5411\u4e0b\u904d\u5386\n            for i in range(min(s, rows - 1), max(-1, s - cols), -1):\n                j = s - i\n                result.append(matrix[i][j])\n    return result<\/code><\/pre>\n\n\n\n
\u4f18\u70b9\uff1a<\/p>\n\n\n\n
\u7279\u6027<\/th>\u8bf4\u660e<\/th><\/tr><\/thead>
\u4fdd\u6301\u5c40\u90e8\u6027<\/strong><\/td>\u76f8\u90bb\u50cf\u7d20\/\u7cfb\u6570\u5728\u5e8f\u5217\u4e2d\u4ecd\u76f8\u90bb<\/td><\/tr>
\u80fd\u91cf\u96c6\u4e2d<\/strong><\/td>\u5c06\u91cd\u8981\u6570\u636e\u524d\u7f6e\uff0c\u5229\u4e8e\u538b\u7f29<\/td><\/tr>
\u7b80\u5355\u9ad8\u6548<\/strong><\/td>\u8ba1\u7b97\u590d\u6742\u5ea6\u4f4e\uff0c\u5b9e\u73b0\u7b80\u5355<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
--------------------------------------------------------------------------------------------------------------------<\/p>\n\n\n\n
from PIL import Image\n\ndef zigzag_decrypt_pixels(input_image, output_image, rows):\n    \"\"\"\n    PNG \u56fe\u7247\u50cf\u7d20 ZigZag \u89e3\u5bc6\n    \n    Args:\n        input_image: \u8f93\u5165\u7684\u52a0\u5bc6 PNG \u56fe\u7247\u8def\u5f84\n        output_image: \u8f93\u51fa\u7684\u89e3\u5bc6 PNG \u56fe\u7247\u8def\u5f84\n        rows: ZigZag \u884c\u6570\n    \"\"\"\n    # \u6253\u5f00\u56fe\u7247\n    img = Image.open(input_image)\n    width, height = img.size\n    pixels = list(img.getdata())\n    \n    total_pixels = len(pixels)\n    \n    if rows <= 1 or rows >= total_pixels:\n        img.save(output_image)\n        print(\"\u884c\u6570\u65e0\u6548\uff0c\u76f4\u63a5\u4fdd\u5b58\u539f\u56fe\")\n        return\n    \n    # \u8ba1\u7b97\u6bcf\u884c\u5e94\u8be5\u6709\u591a\u5c11\u50cf\u7d20\n    fence_lengths = [0] * rows\n    row = 0\n    direction = 1\n    \n    for i in range(total_pixels):\n        fence_lengths[row] += 1\n        \n        if row == 0:\n            direction = 1\n        elif row == rows - 1:\n            direction = -1\n        \n        row += direction\n    \n    # \u5206\u5272\u50cf\u7d20\u5230\u5404\u884c\n    fence = []\n    index = 0\n    for length in fence_lengths:\n        fence.append(list(pixels[index:index + length]))\n        index += length\n    \n    # \u6309 ZigZag \u8def\u5f84\u8bfb\u53d6\u5e76\u91cd\u7ec4\n    result_pixels = []\n    row = 0\n    direction = 1\n    row_indices = [0] * rows\n    \n    for i in range(total_pixels):\n        result_pixels.append(fence[row][row_indices[row]])\n        row_indices[row] += 1\n        \n        if row == 0:\n            direction = 1\n        elif row == rows - 1:\n            direction = -1\n        \n        row += direction\n    \n    # \u521b\u5efa\u65b0\u56fe\u7247\n    decrypted_img = Image.new(img.mode, (width, height))\n    decrypted_img.putdata(result_pixels)\n    decrypted_img.save(output_image)\n    \n    print(f\"\u89e3\u5bc6\u5b8c\u6210\uff01\u8f93\u51fa\uff1a{output_image}\")\n\n\n# \u4f7f\u7528\u793a\u4f8b\nif __name__ == \"__main__\":\n    # \u4fee\u6539\u4e3a\u4f60\u7684\u6587\u4ef6\u8def\u5f84\u548c\u884c\u6570\n    input_path = \"encrypted.png\"      # \u8f93\u5165\u7684\u52a0\u5bc6\u56fe\u7247\n    output_path = \"decrypted.png\"     # \u8f93\u51fa\u7684\u89e3\u5bc6\u56fe\u7247\n    num_rows = 3                      # ZigZag \u884c\u6570 (\u6839\u636e\u9898\u76ee\u8c03\u6574)\n    \n    zigzag_decrypt_pixels(input_path, output_path, num_rows)<\/code><\/pre>\n\n\n\n
\u6267\u884c\u540e\u5f97\u5230\u65b0\u7684\u56fe\u7247\uff0c\u662f\u4e2a\u4e8c\u7ef4\u7801\uff1a<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
\u626b\u63cf\u540e\u5f97\u5230\uff1a<\/p>\n\n\n\n
I_love_DesCTF<\/code><\/pre>\n\n\n\n
\u5173\u6ce8\u516c\u4f17\u53f7\uff0c\u53d1\u9001\u4fe1\u606f\u540e\u5f97\u5230flag\uff1b<\/p>\n\n\n\n
DesCTF{Have fun and enjoy the challenges ahead!!!}<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
wireshark Traffic revealed normal Modbus traffic plus “extra” com …<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_gspb_post_css":"","emotion":"","emotion_color":"","title_style":"","license":"","footnotes":""},"categories":[9],"tags":[],"class_list":["post-960","post","type-post","status-publish","format-standard","hentry","category-misc"],"_links":{"self":[{"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/posts\/960","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/comments?post=960"}],"version-history":[{"count":18,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/posts\/960\/revisions"}],"predecessor-version":[{"id":1143,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/posts\/960\/revisions\/1143"}],"wp:attachment":[{"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/media?parent=960"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/categories?post=960"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/shr1mp.top\/index.php\/wp-json\/wp\/v2\/tags?post=960"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}