UNbreakable International 2024 - Individual Phase Writeup

CTF writeup

この大会は2024/3/22 19:00(JST)～2024/3/24 19:00(JST)に開催されました。
今回は個人で参戦。結果は1314点で281チーム中56位でした。
自分で解けた問題をWriteupとして書いておきます。

safe-password (OSINT)

https://haveibeenpwned.com/Passwordsで80回以上Pwnされているものをチェックしていく。

Bubblegum123!
$ echo -n "Bubblegum123\!" | sha256sum         
fdc852bc63a266c8c38db64bef90d62d53ddeef00aa85df7b941ac780b3d75d8  -

CTF{fdc852bc63a266c8c38db64bef90d62d53ddeef00aa85df7b941ac780b3d75d8}

start-enc (Cryptography)

以下の順にデコードする。

・2進数
・base64
・ASCIIコード
・16進数
#!/usr/bin/env python3
from base64 import *

with open('encoded.txt', 'r') as f:
    enc = f.read()

enc = enc.split(' ')
msg = ''
for c in enc:
    msg += chr(int(c, 2))
print('[+]', msg)

msg = b64decode(msg).decode()
print('[+]', msg)

codes = msg.split(';')[:-1]
msg = ''
for code in codes:
    msg += chr(int(code[2:]))
print('[+]', msg)

codes = msg.split(' ')
flag = ''
for code in codes:
    flag += chr(int(code, 16))
print('[*]', flag)

実行結果は以下の通り。

[+] 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
[+] 54 68 65 20 66 6c 61 67 20 69 73 3a 20 43 54 46 7b 35 38 34 62 33 31 32 62 62 35 62 62 33 34 30 65 39 34 30 38 35 63 34 33 61 62 61 30 36 33 63 35 62 35 61 38 38 30 33 39 31 33 39 33 62 61 65 63 66 37 33 37 64 38 37 32 34 36 36 39 36 63 62 37 7d
[+] 54 68 65 20 66 6c 61 67 20 69 73 3a 20 43 54 46 7b 35 38 34 62 33 31 32 62 62 35 62 62 33 34 30 65 39 34 30 38 35 63 34 33 61 62 61 30 36 33 63 35 62 35 61 38 38 30 33 39 31 33 39 33 62 61 65 63 66 37 33 37 64 38 37 32 34 36 36 39 36 63 62 37 7d
[*] The flag is: CTF{584b312bb5bb340e94085c43aba063c5b5a880391393baecf737d87246696cb7}
CTF{584b312bb5bb340e94085c43aba063c5b5a880391393baecf737d87246696cb7}

wifibasic (Network, Wireless)

$ aircrack-ng -w /usr/share/wordlists/rockyou.txt wifibasic.cap 
Reading packets, please wait...
Opening wifibasic.cap
Read 968 packets.

   #  BSSID              ESSID                     Encryption

   1  02:00:00:00:00:00  BitSentinelRulez          WPA (1 handshake)
   2  02:00:00:00:01:00  Unbreakabl3               Unknown
   3  02:00:00:00:02:00  YetAnotherHacker          WPA (0 handshake)
   4  02:00:00:00:03:00  Unbreakable               Unknown
   5  02:00:00:00:04:00  TargetHiddenSSID          WPA (1 handshake)

Index number of target network ? 5

Reading packets, please wait...
Opening wifibasic.cap
Read 968 packets.

1 potential targets


                               Aircrack-ng 1.7 

      [00:00:00] 131/10303727 keys tested (1258.33 k/s) 

      Time left: 2 hours, 16 minutes, 28 seconds                 0.00%

                          KEY FOUND! [ tinkerbell ]


      Master Key     : 58 65 AF CE 4E 69 4C 14 DD 09 27 47 EB BD 45 EB 
                       27 9A 75 79 9C D1 4D F5 AF B6 DE 01 4D C2 A8 97 

      Transient Key  : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

      EAPOL HMAC     : C1 D1 C8 EC 42 1E 31 80 61 4C FF 7B 02 8F E4 19

以下を設定して、スクリプトを実行する。

BSSID = "02:00:00:00:04:00"
ESSID = "TargetHiddenSSID"
PSK = "tinkerbell"
CTF{73841584e4c011c940e91c76bf1c12a7a4850e4b3df0a27ba8a35388c316d468}

wifiland (Network, Wireless)

$ aircrack-ng -w /usr/share/wordlists/rockyou.txt wifiland.cap 
Reading packets, please wait...
Opening wifiland.cap
Read 4594 packets.

   #  BSSID              ESSID                     Encryption

   1  02:00:00:00:00:00  BitSentinelRulez          Unknown
   2  02:00:00:00:05:00  wifiland                  WPA (1 handshake)

Index number of target network ? 2

Reading packets, please wait...
Opening wifiland.cap
Read 4594 packets.

1 potential targets


                               Aircrack-ng 1.7 

      [00:00:00] 8/14344392 keys tested (27.07 k/s) 

      Time left: 6 days, 3 hours, 11 minutes, 53 seconds         0.00%

                           KEY FOUND! [ 12345678 ]


      Master Key     : 7F 76 94 BD AC D9 1E 94 22 2F 00 BD 49 CD 4D DA 
                       8B 0C 31 16 D5 28 A4 BC C8 3F 8A 40 AE 78 D7 A5 

      Transient Key  : 2F 75 56 F7 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
                       00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

      EAPOL HMAC     : A6 01 F2 FA 2C BE BB F7 BC CF 3A 2D 83 A2 44 29

Wiresharkの[編集]>[設定]から[Protocols]>[IEEE 802.11]の画面を表示後、Decryption keysの編集から以下を設定し、通信パケットを復号する。

・Key type: wpa-pwd
・Key     : 12345678

パケットにARPの通信がある。

3304	2024-03-13 22:50:05.094651	02:00:00:00:13:00	Broadcast	ARP	102	Who has 93.184.216.34? Tell 10.0.3.19

以下を設定して、スクリプトを実行する。

ip_client = "10.0.3.19"
ip_target = "93.184.216.34"
CTF{b67842d03eadce036c5506f2b7b7bd25aaab4d1f0ec4b4f490f0cb19ccd45c70}

traffic-e (Cryptography, Network)

TLSの通信がほとんどなので、証明書をエクスポートし、内容を確認してみる。

$ openssl x509 -in 6.cer -text -pubkey -inform DER
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            12:6b:f1:48:8d:dc:4b:02:a8:6d:0a:77:51:ff:63:0a:75:ed:f1:14
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=AU, ST=Some-State, O=Internet Widgits Pty Ltd, CN=Ephvuln
        Validity
            Not Before: Mar 15 11:09:16 2024 GMT
            Not After : Mar 15 11:09:16 2025 GMT
        Subject: C=AU, ST=Some-State, O=Internet Widgits Pty Ltd, CN=Ephvuln
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (1027 bit)
                Modulus:
                    07:3c:0d:c9:3d:ff:a0:26:b8:88:e0:eb:2f:da:7a:
                    c6:e1:82:a7:bc:5f:a4:39:00:d3:61:a4:42:eb:7c:
                    db:44:02:7b:31:96:76:b5:fd:37:49:1b:ce:30:8d:
                    76:e5:be:3e:a9:61:d8:7f:ff:5d:25:52:b4:34:ed:
                    8e:e6:62:a1:05:39:ea:21:f4:37:17:0d:f9:70:ef:
                    2b:6a:9b:e1:41:09:16:53:0b:83:97:b4:35:7b:f1:
                    78:85:2a:35:c9:0b:33:20:b4:93:21:3e:05:fb:00:
                    09:66:d8:9e:ba:09:25:c3:44:22:e4:80:a5:c9:17:
                    67:37:cc:98:ab:09:99:32:87
                Exponent:
                    00:97:de:37:9a:97:9d:08:04:59:a7:ab:09:b8:48:
                    ea:eb:be:83:79:ef:79:d2:5c:0b:e7:4d:24:35:a4:
                    0c:7e:11:db:51:7b:50:59:0c:fc:be:15:9d:d3:75:
                    22:1e:0f:45:d1:a0:d4:d9:a9:40:3f:fe:34:fb:1e:
                    40:2a:85:4b:41:71:6c:8e:aa:fc:0e:66:e6:f7:09:
                    e6:54:ea:e6:83:8d:69:b6:1e:b0:00:c0:a4:6d:13:
                    5c:e6:32:36:c2:76:d5:fe:26:28:48:62:97:98:e6:
                    e5:5a:f5:57:b1:0d:4e:84:67:01:22:35:ba:b0:fe:
                    5d:cf:81:d9:fb:83:2f:cb:ef
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                E0:02:07:E4:1E:35:4A:16:07:07:58:BE:65:4E:40:AA:D3:9B:BB:95
            X509v3 Authority Key Identifier: 
                E0:02:07:E4:1E:35:4A:16:07:07:58:BE:65:4E:40:AA:D3:9B:BB:95
            X509v3 Basic Constraints: critical
                CA:TRUE
    Signature Algorithm: sha256WithRSAEncryption
    Signature Value:
        03:01:3d:cf:26:97:63:11:a1:cf:be:5a:03:c5:2b:dc:a0:e8:
        6e:35:97:16:43:9b:4d:e0:60:6e:22:59:82:fd:49:7e:7a:ed:
        d5:6a:de:46:79:53:80:38:db:b9:03:7b:17:08:f0:f0:b3:a6:
        e1:a3:46:23:2a:0e:8a:0a:12:78:33:97:75:f2:fe:d7:6e:5c:
        51:8d:a0:11:3d:93:66:81:5c:4b:7c:29:d2:6c:49:9e:44:e8:
        89:5c:b5:09:b0:55:4e:2f:73:e9:b0:45:ea:a3:4d:b9:2e:23:
        8e:10:47:ac:ad:c5:a0:fd:98:dd:56:83:2f:75:73:0c:fd:9e:
        18:35:66
-----BEGIN PUBLIC KEY-----
MIIBIDANBgkqhkiG9w0BAQEFAAOCAQ0AMIIBCAKBgQc8Dck9/6AmuIjg6y/aesbh
gqe8X6Q5ANNhpELrfNtEAnsxlna1/TdJG84wjXblvj6pYdh//10lUrQ07Y7mYqEF
Oeoh9DcXDflw7ytqm+FBCRZTC4OXtDV78XiFKjXJCzMgtJMhPgX7AAlm2J66CSXD
RCLkgKXJF2c3zJirCZkyhwKBgQCX3jeal50IBFmnqwm4SOrrvoN573nSXAvnTSQ1
pAx+EdtRe1BZDPy+FZ3TdSIeD0XRoNTZqUA//jT7HkAqhUtBcWyOqvwOZub3CeZU
6uaDjWm2HrAAwKRtE1zmMjbCdtX+JihIYpeY5uVa9VexDU6EZwEiNbqw/l3Pgdn7
gy/L7w==
-----END PUBLIC KEY-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

公開鍵のパラメータは以下であることがわかる。

n = 0x073c0dc93dffa026b888e0eb2fda7ac6e182a7bc5fa43900d361a442eb7cdb44027b319676b5fd37491bce308d76e5be3ea961d87fff5d2552b434ed8ee662a10539ea21f437170df970ef2b6a9be1410916530b8397b4357bf178852a35c90b3320b493213e05fb000966d89eba0925c34422e480a5c9176737cc98ab09993287
e = 0x0097de379a979d080459a7ab09b848eaebbe8379ef79d25c0be74d2435a40c7e11db517b50590cfcbe159dd375221e0f45d1a0d4d9a9403ffe34fb1e402a854b41716c8eaafc0e66e6f709e654eae6838d69b61eb000c0a46d135ce63236c276d5fe262848629798e6e55af557b10d4e8467012235bab0fe5dcf81d9fb832fcbef

Wiener's Attackでnを素因数分解する。

#!/usr/bin/env python3
from Crypto.Util.number import *
from fractions import Fraction

def egcd(a, b):
    x, y, u, v = 0, 1, 1, 0
    while a != 0:
        q, r = b // a, b % a
        m, n = x - u * q, y - v * q
        b, a, x, y, u, v = a, r, u, v, m, n
        gcd = b
    return gcd, x, y

def continued_fractions(n,e):
    cf = [0]
    while e != 0:
        cf.append(int(n // e))
        N = n
        n = e
        e = N % e
    return cf

def calcKD(cf):
    kd = list()
    for i in range(1, len(cf) + 1):
        tmp = Fraction(0)
        for j in cf[1:i][::-1]:
            tmp = 1 / (tmp + j)
        kd.append((tmp.numerator, tmp.denominator))
    return kd

def int_sqrt(n):
    def f(prev):
        while True:
            m = (prev + n // prev) // 2
            if m >= prev:
                return prev
            prev = m
    return f(n)

def calcPQ(a, b):
    if a * a < 4 * b or a < 0:
        return None
    c = int_sqrt(a * a - 4 * b)
    p = (a + c) // 2
    q = (a - c) // 2
    if p + q == a and p * q == b:
        return (p, q)
    else:
        return None

def wiener(n, e):
    kd = calcKD(continued_fractions(n, e))
    for (k, d) in kd:
        if k == 0:
            continue
        if (e * d - 1) % k != 0:
            continue
        phin = (e * d - 1) // k
        if phin >= n:
            continue
        ans = calcPQ(n - phin + 1, n)
        if ans is None:
            continue
        return (ans[0], ans[1])

n = 0x073c0dc93dffa026b888e0eb2fda7ac6e182a7bc5fa43900d361a442eb7cdb44027b319676b5fd37491bce308d76e5be3ea961d87fff5d2552b434ed8ee662a10539ea21f437170df970ef2b6a9be1410916530b8397b4357bf178852a35c90b3320b493213e05fb000966d89eba0925c34422e480a5c9176737cc98ab09993287
e = 0x0097de379a979d080459a7ab09b848eaebbe8379ef79d25c0be74d2435a40c7e11db517b50590cfcbe159dd375221e0f45d1a0d4d9a9403ffe34fb1e402a854b41716c8eaafc0e66e6f709e654eae6838d69b61eb000c0a46d135ce63236c276d5fe262848629798e6e55af557b10d4e8467012235bab0fe5dcf81d9fb832fcbef

p, q = wiener(n, e)
print('p =', p)
print('q =', q)
print('e =', e)

実行結果は以下の通り。

p = 50762598711424764578309835161028536505017062779535728598193739021422991384075097619364837920473739477192741993467746530937273178568830967465304495915565021
q = 25620367680132110555879075164092814080992627707664402686174667544938777823227941746742946229140990674686598986153493963260846509398173854491013932770379699
e = 106645361573597107845396067866499068630105849159408665310862014583870062061704662230754284832387896920427209753236862548800746662398609212688373613186979102970308417884832531601035544107102590028211579550508699494971288803583755640940424098301425895738898909222425910339731329121362635050810847489912118168559

これで、p, qがわかり秘密鍵を生成できる。

$ rsatool.py -f PEM -o secret.pem -p 50762598711424764578309835161028536505017062779535728598193739021422991384075097619364837920473739477192741993467746530937273178568830967465304495915565021 -q 25620367680132110555879075164092814080992627707664402686174667544938777823227941746742946229140990674686598986153493963260846509398173854491013932770379699 -e 106645361573597107845396067866499068630105849159408665310862014583870062061704662230754284832387896920427209753236862548800746662398609212688373613186979102970308417884832531601035544107102590028211579550508699494971288803583755640940424098301425895738898909222425910339731329121362635050810847489912118168559
Using (p, q) to calculate RSA paramaters

n =
73c0dc93dffa026b888e0eb2fda7ac6e182a7bc5fa43900d361a442eb7cdb44027b319676b5fd374
91bce308d76e5be3ea961d87fff5d2552b434ed8ee662a10539ea21f437170df970ef2b6a9be1410
916530b8397b4357bf178852a35c90b3320b493213e05fb000966d89eba0925c34422e480a5c9176
737cc98ab09993287

e =
97de379a979d080459a7ab09b848eaebbe8379ef79d25c0be74d2435a40c7e11db517b50590cfcbe
159dd375221e0f45d1a0d4d9a9403ffe34fb1e402a854b41716c8eaafc0e66e6f709e654eae6838d
69b61eb000c0a46d135ce63236c276d5fe262848629798e6e55af557b10d4e8467012235bab0fe5d
cf81d9fb832fcbef

d =
288ab34527ecad227bd3ec9e847ca8942f388b65d3a759397c5faa9ee7ffe007

p =
3c93a69f7a213fa1a8da9903d92c278553b67525beded842fcef7bfc4884679a808f94fee5f155bf
d0c211b0d19eec38420918065cb4086ed426c543d24ec7fdd

q =
1e92dbfa3f67f1525e1fa427e571434a9dc64e05de873cce671820e83932e7d31633bd16cea7b0a4
835c70d55ff36accc6749a0c7744c8d328dd30f1af243c7b3

Saving PEM as secret.pem

このファイルをWiresharkで設定して通信を見てみる。

IPアドレス：127.0.0.1
ポート番号：4433
プロトコル：ssh

TLSストリームを見る。

whoami
root
cert.pem
flag.txt
priv.pem
server.sh
test.py
cat flag.txt
CTF{46b1d043b3d2d98a267455affce276c47a1f2bfb940881d1e9725c798373f532}
exit
CTF{46b1d043b3d2d98a267455affce276c47a1f2bfb940881d1e9725c798373f532}