BlueHens CTF 2024 Writeup

(function () {
  let cardsElement = document.getElementById("cards");
  cardsElement.innerHTML = "";
  firebase
    .database()
    .ref("/flag")
    .limitToFirst(12000)
    .once("value", (ss) => {
      let results = ss.val();
      results.forEach((cObj) => {
        $("#cards").append(`<div>CHR: ${cObj.chr} NEXTCHR: ${cObj.next}`);
      });
    });
})();

hints = """CHR: ? NEXTCHR: 1627 - 1151
CHR: 3 NEXTCHR: 275 + 896
CHR: O NEXTCHR: 912660 / 2173
CHR: } NEXTCHR: 5918500 / 3115
CHR: \ NEXTCHR: -1963 + 3668
...
CHR: ! NEXTCHR: (191 * 1) + 46
CHR: E NEXTCHR: (2074 * 0) + 415
CHR: | NEXTCHR: (3310 * 0) + 1422
CHR: \ NEXTCHR: -1963 + 3668
CHR: o NEXTCHR: 3775 - 3429
CHR: END NEXTCHR: You got the flag"""

lines = hints.split("\n")
data = []
for line in lines:
    s1, s2 = line.split(" NEXTCHR: ")
    chr = s1[5:]
    if s2 == "exit()" or s2 == "You got the flag":
        data.append((chr, -1))
        continue
    nextpos = int(eval(s2))
    data.append((chr, nextpos))

pos = 0
while True:
    chr, nextpos = data[pos]
    if chr == "END":
        break
    print(chr, end="")
    pos = nextpos

?tCUzc-Hj,xMUjas&TF{JS0N_1n_tr33}o`JM0FYN

const tryDecode = (boardData) => {
    let encBytes = aesjs.utils.hex.toBytes(
        "ebab19591082e96b3db6d083e73b70350b008c5c33950b3b0cbb3bcb0001b643ef875cbecf0622"
    );
    let keyHex = md5(boardData.join(","));
    let keyBytes = aesjs.utils.hex.toBytes(keyHex);
    let aesCtr = new aesjs.ModeOfOperation.ctr(keyBytes);
    let message = aesCtr.decrypt(encBytes);
    let text = aesjs.utils.utf8.fromBytes(message);
    if (text.includes("ctf")) console.log(text);
};

for (let i = 0; i < 2 ** 16; i++) {
    const boardDataStr = i.toString(2).padStart(16, "0"); // 例如："1100110000110011"
    const boardData = Array.from(boardDataStr).map((bit) => parseInt(bit, 10)); // 例如：[1, 1, 0, 0, ...]
    tryDecode(boardData);
}

flag = [" "] * 51

flag[0] = 'u'
flag[1] = 'd'
flag[2] = 'c'
flag[3] = 't'
flag[4] = 'f'
flag[5] = '{'

flag[10] = chr(0x5f)  # _

flag[-1] = chr(0x7d)  # }

l1 = [0x47, 0x4a, 0x13, 0x42, 0x58, 0x57, 0x1b]
s1 = "r!zz13r"
for i in range(7):
    flag[29 + i] = chr(l1[i] ^ ord(s1[i]))
s2 = "5ki8idi"
for i in range(7):
    flag[43 + i] = chr(l1[i] ^ ord(s2[i]))

l2 = [0x40, 0x05, 0x5c, 0x48, 0x59, 0x0f, 0x5a, 0x5b, 0x00]
for i in range(8):
    flag[15 + i] = chr(l2[i] ^ ord(s2[i % 7]))

l3 = [0x62, 0x6e, 0x60, 0x75, 0x69, 0x34, 0x00]
for i in range(6):
    flag[8 + i] = chr(l3[i] - 1)

l4 = [0x05, 0x17, 0x01, 0x01, 0x1d, 0x00]
for i in range(5):
    flag[37 + i] = chr(l3[i] ^ l4[i])

flag[6] = flag[31]
flag[26] = flag[44]

flag[24] = "0"
flag[27] = "0"

flag[7] = flag[10]
flag[42] = flag[10]
flag[28] = flag[42]
flag[36] = flag[28]
flag[23] = flag[36]
flag[14] = flag[23]

flag[25] = "h"  # guess from "ohio"
print("".join(flag))

udctf{i_am_th3_un5p0k3n_0h!0_5ki8idi_gyatt_r!zz13r}

# In [15]: (flag*32)[::17][:32]
# Out[15]: 'u_cnfrj_sr_b_34}yd1tt{0upt04lbmb'

out = "u_cnfrj_sr_b_34}yd1tt{0upt04lbmb"
flag = [" "] * 100

for i in range(len(out)):
    flag[i] = out[(i * 17) % 32]

print("".join(flag))

udctf{just_4_b4by_1ntr0_pr0bl3m}

ciphers = [
    '43794c9c8faa2cff24edc8afe507a13f62837c7e166f428cab5aff893225ff19104bc8754c1c09',
    '5d315e8786e62cf763e9d4afe80ca13b649a717e11615986b642f3952f76b71b0342c4',
    '46785a8bcae62aeb60a5deeef107a1256ed7792752695886ff50f5886171ff1717',
    '5d315e819fe621b966e08dfae906e43a78837b31162e5e8cff46e8953275f20a0d5ad23d4712144c',
    '557f4dce9ee220b967e4dfffe616e9216a9934291b7d5690bb45ba922e6afc',
    '55315a868fef35f16beac6afe810a1206a81717e1e6b5690b152ba953462ff0c424acd6e0307055a81b93590c1fe',
    '557d489dcafd2df870a5cfe0e816f268628334291b7a5fc2aa58f99f3276f616160fc27c5116',
    '557f4dce8bee21fc24f1c5eaa712ee3f6e853431142e448db216fb9e2b70e5110c48816b46011e5a',
    '407e099783ef29fd24edc4fca704f33d6283343f1c6a178ab645ba962464f1581147c0714f530350d5f53690dee6',
    '40785ace93e530b970edccfba711e0312b9e607e1c6143c2b616e3953425f317425bc9780317085ac5a6',
]

# target:
target = "41754a9a8cf13da976dac4e1d810b1253f994b6f47514387b106e8a57175a40a0370d22c4d14084d9ea8"


def cipher_to_byte_list(cipher):
    return [int(cipher[i:i + 2], 16) for i in range(0, len(cipher), 2)]


ciphers = [cipher_to_byte_list(c) for c in ciphers]
target = cipher_to_byte_list(target)

stats = [[] for _ in range(len(target))]


def leak(cipher):
    for i in range(min(len(target), len(cipher))):
        if target[i] ^ cipher[i] >= 64:
            stats[i].append(cipher[i] ^ target[i] ^ ord(' '))


for cipher in ciphers:
    leak(cipher)


def mode(ls):
    if ls:
        return max(set(ls), key=ls.count)
    else:
        return 0


message = ""
for stats_ch in stats:
    if len(stats_ch) > 4:
        message += "."
    else:
        m = mode(stats_ch)
        if len(stats_ch) == 1 or stats_ch.count(m) > 1:
            message += chr(m)
        else:
            ch = "."
            for c in stats_ch:
                if 65 <= c <= 90 or 97 <= c <= 122:
                    ch = chr(c)
            message += ch

print(message)

.dctf{..r_in_..m.n_.._ten.r..p.ra_s.ng..k}

# ----- guess the rest -----

ciphers.append(cipher_to_byte_list(
    "41754a9a8cf13da976dac4e1d810b1253f994b6f47514387b106e8a57175a40a0370d22c4d14084d9ea8"  # target
    # "d c t f { x 0 r _ i n _ r 0 m 4 n _ 1 5 _ t e n 0 r _ 0 p 3 r a _ s 1 n g e r ? } "
))
key = [
    52, 17, 41, 238, 234, 138, 69, 153, 4, 133, 173, 143, 135, 98, 129, 72, 11, 247, 20, 94, 114, 14, 55,
    226, 223
]  # try byte by byte

# enough info to find the poem:
# https://milton.host.dartmouth.edu/reading_room/comus/text.shtml

c7 = "and adde the power of som adjuring verse"
for i in range(len(key), len(c7)):
    key.append(ord(c7[i]) ^ ciphers[7][i])

key.append(161)
key.append(213)

for cipher in ciphers:
    for i in range(min(len(key), len(cipher))):
        print(chr(cipher[i] ^ key[i]), end="")
    print()

udctf{x0r_in_r0m4n_15_ten0r_0p3ra_s1nger?}

code = ("""5308174f590b0103555810
13110d171212061d7f
0d0b112b03150a01260844
5d
5100020007
5944
510f060d
5c44187e465b4953
5116060713171d53
4844
5243
4e6e4354465b0f1c2d5818
510d43
4844
45
4e44
510d43
5308174f46
061011180315
5d
5100020007
5c5f43
510d
5e4f4a541d7149537f5810527f47
5116060713171d53
5b5943
5100020007
2e
510d
28443d54
510f060d
2e
510d43
5044
061011180315
5d
510f060d
5c39587e465b4953227210527f47075607060b3110
000a13150510
5d
572c4956
59
5116060713171d
5c3f
44
285f6954460663797f58
510f060d46
4844
52110717121d121a2c2748422d38124640000a266f422d315e444b420843
4e6e695446
51070c10035b
4844
1d0d041c0a120e1b2b27561b3302
5d
2a3b253d2a3e362c
5944
01161611
5c5f695446
510b011213080a122b1d542d3c08115653
4844
0516061339090c03331953170004145f1f11183c5b
5d6e4354465b
524b451812401a033e16182901411247482e537616152b55185a15425c420f005d2746002f195e5438134e1c0054
596e4354465b0f06311b441b3009551b
5109020005130c00
5c441607035b41
510f060d
5c44187e465b49537f581052
51070c1a121e07077f
4844
5109020005130c00
2e
47
285f6954465b49537f5810
510b011213080a122b1d542d3c081b47161d0d7f
4844
0d0b112b03150a01260844
5d
51070c1a121e0707
5944
510f060d
5c5f6954465b49537f5810003a1300411d53
52420f005d081912315f
5b
5109020005130c00
2e
44
284a
524204005d5c49
5b44
1d100e18150b0c1036195c1137060740
5d
510b011213080a122b1d542d3c081b47161d0d
5c444d54
52420f005d541a033e1616152b5c535f07481b2d16152b5517
4e6e4354465b145f555810527f
51070c1003714953
5c5f695446
5a4b4710091801073214104f7f09104453373612741d3c1b5d1151095d4d587e465b465c7b1c5f113713185f5e551e2b0b1e300f545c1b1e1a00065d5d714953
10070b1b46
510b011213080a122b1d542d3c081156
4e6e4354
5a4b13060f151d2c2d50451c2f0616585b513175125e7b0d5f105a542e553e5d5d71
4a4204005d71""")


def cipher_to_byte_list(cipher):
    return [int(cipher[i:i + 2], 16) for i in range(0, len(cipher), 2)]


code = code.split("\n")
code = [cipher_to_byte_list(line) for line in code]

key = [117, 100, 99, 116, 102, 123, 105, 115, 95, 120, 48, 114, 95, 103, 117, 51, 115, 115, 121, 95, 48, 114, 95, 110,
    48, 116, 63, 125, 117, 100]

for line in code:
    if not len(line) >= len(key):
        continue
    for i in range(min(len(key), len(line))):
        print(chr(line[i] ^ key[i]), end="")
    if len(line) > len(key):
        print(line[len(key):], end="")
    print()

udctf{is_x0r_gu3ssy_0r_n0t?}