Byte Bandits CTF 2020 Writeup - よっちんのブログ

この大会は2020/4/12 0:30(JST)～2020/4/13 0:30(JST)に開催されました。
今回もチームで参戦。結果は5734点で318チーム中6位でした。
自分で解けた問題をWriteupとして書いておきます。

meet me there (crypto)

鍵を2つ使って、AES暗号を2回行い、サンプルの平文と暗号文がわかっている。こういう場合は平文を1回暗号化したものと、暗号文を1回復号したものを突合し、合っているものを探す。その際暗号文を1回復号したものはhex文字列である必要があるということから絞ることができる。まずは平文を1回暗号化したものと、暗号文を1回復号したもののリストをファイルに保存する。

from Crypto.Cipher import AES
from string import printable, hexdigits
from itertools import product

def is_hexdigits(s):
    for c in s:
        if c not in hexdigits:
            return False
    return True

pt = 'aaaaaaaaaaaaaaaa'
ct = 'ef92fab38516aa95fdc53c2eb7e8fe1d5e12288fdc9d026e30469f38ca87c305ef92fab38516aa95fdc53c2eb7e8fe1d5e12288fdc9d026e30469f38ca87c305'

for c in product(printable, repeat=3):
    key1 = '0' * 13 + ''.join(c)
    cipher1 = AES.new(key1, AES.MODE_ECB)
    c1 = cipher1.encrypt(pt.encode('hex'))
    with open('meet1.txt', 'a') as f:
        f.write(key1 + ',' + c1.encode('hex') + '\n')

for c in product(printable, repeat=3):
    key2 = ''.join(c) + '0' * 13
    cipher2 = AES.new(key2, AES.MODE_ECB)
    p2 = cipher2.decrypt(ct.decode('hex'))
    if is_hexdigits(p2):
        with open('meet2.txt', 'a') as f:
            f.write(key2 + ',' + p2 + '\n')

試した結果、暗号文を1回復号したものがhex文字列である鍵は1種類。

key2 = '}=v0000000000000'

復号データは以下の通り。

c91dcfc4e250ee0c013fe3e6faa0d76fc91dcfc4e250ee0c013fe3e6faa0d76f

平文を1回暗号化したものがこのデータになるものを検索する。

key1 = '0000000000000t\r['

これを元にフラグを復号する。

from Crypto.Cipher import AES

ct_flag = 'fa364f11360cef2550bd9426948af22919f8bdf4903ee561ba3d9b9c7daba4e759268b5b5b4ea2589af3cf4abe6f9ae7e33c84e73a9c1630a25752ad2a984abfbbfaca24f7c0b4313e87e396f2bf5ae56ee99bb03c2ffdf67072e1dc98f9ef691db700d73f85f57ebd84f5c1711a28d1a50787d6e1b5e726bc50db5a3694f576'

key1 = '0000000000000t\r['
key2 = '}=v0000000000000'

cipher1 = AES.new(key1, AES.MODE_ECB)
cipher2 = AES.new(key2, AES.MODE_ECB)

p2 = cipher2.decrypt(ct_flag.decode('hex'))
p1 = cipher1.decrypt(p2.decode('hex'))
flag = p1.decode('hex')
print flag

flag{y0u_m@d3_i7_t0_7h3_m1dddl3}

RSyay! (crypto)

$ nc crypto.byteband.it 7002

!!! Welcome !!!
Can you encrypt some data for me :)

Plaintext (b64encoded) :  sZEnbo8LgXLIzrDIlblUSPO3imgQLDrtlFjA2R2THi0=

241ce23f7a376cf061fbf619324c904fad37311ba4b568c7ea521baf7bc657aecfa5946c88ecba763bdf415a41911154ca6e3f2d23177c5734f24ba8574211b335a5ec9845806286f80cb8c7e85cde6d5f493dbd75cfc47487795ffbe447a0581086cf63afb4a0f4136b94c257a81d760e3a906763432e13910a699698f055bad
180d87bbf1974bdf048053dd416611c2ab33277b1839964724212e39cda7fee7e8a720aad1df5adc30d162be7667639777bfd4fcfcd4e0134e9fdb5c88849080ff5fda8484da5c168383bd2874f3d77450bc6995396739c9acfea2b7ea6819e3da92f988538c7c55e6e0db388bf11df419e51a3153f4c058d8a7e38b499950bb1

Ciphertext (b64encoded) : sZEnbo8LgXLIzrDIlblUSPO3imgQLDrtlFjA2R2THi0=

No flag for you !!!

xの式を変形する。

x = (pow(p, m, m) + p) * (pow(q, m, m) + q) + pow(p, m-1, m) * pow(q, m-1, m)
  = (p + p) * (q + q) + 1 * 1
  = 4 * p * q + 1 = 4 * n + 1
→ n = (x - 1) / 4

xからnを計算でき、さらにeが分かっているので、暗号化データを作成できる。それを何回も繰り返し答えていけばよい。

import socket
from base64 import b64encode, b64decode
from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP

def recvuntil(s, tail):
    data = ''
    while True:
        if tail in data:
            return data
        data += s.recv(1)

e = 65537L

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('crypto.byteband.it', 7002))

data = recvuntil(s, ':)\n').rstrip()
print data

for i in range(32):
    print 'Round %d' % (i+1)
    data = recvuntil(s, '\n').rstrip()
    print data
    data = recvuntil(s, '\n').rstrip()
    print data
    text = b64decode(data.split(':  ')[1])

    data = recvuntil(s, '\n').rstrip()
    print data
    data = recvuntil(s, '\n').rstrip()
    print data
    x = int(data, 16)
    data = recvuntil(s, '\n').rstrip()
    print data

    n = (x - 1) / 4
    keys = RSA.construct((n, e))
    encryptor = PKCS1_OAEP.new(keys)
    ciphertext = b64encode(encryptor.encrypt(text))

    data = recvuntil(s, ': ')
    print data + ciphertext
    s.sendall(ciphertext + '\n')

    data = recvuntil(s, '\n').rstrip()
    print data

data = recvuntil(s, '\n').rstrip()
print data
data = recvuntil(s, '\n').rstrip()
print data

実行結果は以下の通り。

　　　　　　　　　　　　　　　　：
Round 32

Plaintext (b64encoded) :  ZBFSZUVh0l6vO4wlpStpNCl0GLGDSigtKsTaqI4Ry+U=

2e758bc09b21e2d698d8dc9d7ebe23126b952fdd4a7f5c2fd07a89745af34b8c09e1a04f89d21c36e6ad8dbba049a907be07739bef12c2c7f8e74d725fc03e698f55f8268330bbd5da8bc937da676cd0acbee9b15ecb4c24b3232ba55bf96a58aa0acc48b9bd7cbdd210eb4189ef9f3160400263494859e71b984471d84a4bb290965b57590a5165de91adb39a6df19e5e8815d20a6aa5377ae7cb5eca859b5
10e2fd990bd0d91c9bdd16d3a2c2759cd9eb161a0b9a0a6016f872492d3ee4b5df4ea9339cbd5d67698574a6f4e239c1f4fd0dfe0c8834d4c5c67cedf61f0ab03c4efe68ebfbc6e1c5b8c7c0c8392cf29c94b19e553e47c28cc8a046abd12d8c03625c9c5578fc3bbd8851e9d59cb8b3579485ea94f9ef456a823b87de056a613fe6d7f0a6549bb483c383f152770fca46e4ad1477cb0b6a30439e6ac9e565d

Ciphertext (b64encoded) : oZ4S9s7xikH6JB5kN5oA7Pa3/dgOBffMq54xaDq4P13t3KLECBrXAte1Jv76L0Vhv9/tdUR9eWazGZd0QM1loLbiK2xfTyac0xW5InrPYdnNr25yYJum4/NHIc1PmnsmHLyBk3gEKE6uJ0SzVrbXMJExDe5RUi+2mumx6G1B0tm8ZsRntLYdC8G+DW98z65pFD0jJDSB62tSLr8P1r++


Flag :  flag{RSA_1s_th3_str0ng3st_c1ph3r_ind33d_0_0}

flag{RSA_1s_th3_str0ng3st_c1ph3r_ind33d_0_0}

AESy (crypto)

$ nc crypto.byteband.it 7004

WELCOME TO ALICE's ENCRYPTION SERVICE
(Plaintext is hex-encoded before encryption)


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
1

Enter your message(plaintext):
1234

Here is your ciphertext(hex-encoded):
f9024d920a22eac3c3368ea55da658f2c04d1f22b3f5e937f1f66aecd57fbf80


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
2

Enter the ciphertext(hex-encoded):
f9024d920a22eac3c3368ea55da658f2c04d1f22b3f5e937f1f66aecd57fbf80

Alice: Got your message!!★暗号化データを渡すと、この結果。


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
2

Enter the ciphertext(hex-encoded):
f9024d920a22eac3c3368ea55da658f2c04d1f22b3f5e937f1f66aecd57fbf7f

Alice: Got your message..??★最後の1バイトだけ変えると、この結果。


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
2 

Enter the ciphertext(hex-encoded):
f8024d920a22eac3c3368ea55da658f2c04d1f22b3f5e937f1f66aecd57fbf80

Alice: Got your message!!★先頭の1バイトだけ変えると、この結果


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
3

Here is your ciphertext(hex-encoded):
7d0d5c84903683750c535c216c008a68a45878c05a2492109a42f3a67d6545dcce2f8ecfde80973ca80f196b9a40b3c8c46b24adcf5530f90984952b571ef4613f0e77926907acc99683c7c8d92fcb278367406057825e8fc9fb44c15269e9cb


1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:

たぶん、パディングが正しい状態で復号できていないと、「Alice: Got your message..??」のメッセージが表示されると推測できる。CBC Padding Oracle Attackで復号する。

import socket

def recvuntil(s, tail):
    data = ''
    while True:
        if tail in data:
            return data
        data += s.recv(1)

def str_xor(s1, s2):
    return ''.join(chr(ord(a) ^ ord(b)) for a, b in zip(s1, s2))

def unpad(s):
    return s[:-ord(s[-1])]

def is_valid(s, enc):
    data = recvuntil(s, ':\n').rstrip()
    print data
    print '2'
    s.sendall('2\n')

    data = recvuntil(s, ':\n').rstrip()
    print data
    print enc
    s.sendall(enc + '\n')

    data = recvuntil(s, '\n').rstrip()
    print data
    data = recvuntil(s, '\n').rstrip()
    print data
    if data == 'Alice: Got your message!!':
        return True
    else:
        return False

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('crypto.byteband.it', 7004))

data = recvuntil(s, ':\n').rstrip()
print data
print '3'
s.sendall('3\n')

data = recvuntil(s, ':\n').rstrip()
print data
data = recvuntil(s, '\n').rstrip()
print data
enc = data.decode('hex')

enc_blocks = []
for i in range(0, len(enc), 16):
    enc_blocks.append(enc[i:i+16])

xor_blocks = []
for i in range(len(enc_blocks)-1, 0, -1):
    xor_block = ''
    for j in range(16):
        for code in range(256):
            print '%d - %d - %d: %s' % (i, j, code, xor_block.encode('hex'))
            print '****', str_xor(xor_block, enc_blocks[i-1][-j:]), '****'
            try_pre_block = '\x00' * (16 - j - 1) + chr(code) + str_xor(xor_block, chr(j+1)*j)
            try_cipher = (try_pre_block + enc_blocks[i]).encode('hex')
            if is_valid(s, try_cipher):
                xor_code = (j+1) ^ code
                xor_block = chr(xor_code) + xor_block
                break

    xor_blocks.append(xor_block)

xor_blocks.reverse()

flag = ''
for i in range(len(xor_blocks)):
    flag += str_xor(enc_blocks[i], xor_blocks[i])

flag = unpad(flag)
print flag

flag = flag.decode('hex')
print flag

実行結果は以下の通り。

　　　　　　　　　　　　　　　　：
1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
2

Enter the ciphertext(hex-encoded):
c4e5f427d1d9a7195eedb4d05a00e4129621f1c0eb41d546ea5aa7c27abc43d0

Alice: Got your message..??
1 - 15 - 197: f5e437c1c9b7094efda4c04a10f402
**** 66c61677b746833 ****

1. Get your message encrypted.
2. Leave a message for Alice to decrypt.
3. Get Encrypted Flag.
4. Exit.
Enter your choice:
2

Enter the ciphertext(hex-encoded):
c5e5f427d1d9a7195eedb4d05a00e4129621f1c0eb41d546ea5aa7c27abc43d0

Alice: Got your message!!
666c61677b7468335f307261636c335f306e6c795f6775316465245f3768655f313333377d
flag{th3_0racl3_0nly_gu1de$_7he_1337}

flag{th3_0racl3_0nly_gu1de$_7he_1337}

Secure DES (crypto)

$ nc crypto.byteband.it 7001
Welcome!

1. Encrypt String
2. Get Key
3. Get encrypted flag
4. Exit

> 1
String : hoge
Xd8MvjZmWLM=
> 2
0xb4e814ab5f737f395af33f800908463fb27ff67e1fd9ef7d2fd81c185f1dc34316d04bc7a5d9c99576f2ab414bbcf8ae819a073bbe8e473ea85cfd99f2307dc7
['0x43575cc90b034910f680e694098e42b310956cae702e787ed801cdbc7c2bcabc18f08a6f5b52aa98cb27edf7ed03b59c935d9899faf55e24ac15a32c58a61047', '0x9a7b75c2f6bd8caeb13e3eb678399ad73d621d8e304536a5648d83f6ccbba78fdbfa21b935d825584e5bd90ad3cc61f16dc4eb679c1db235a3bad5f6e7ae8040', '0x15a8a2a3f34d54da64aa6f932b1a717d0783512d2f2c8c256ffb5169c7f698a8f68f4386a4b72572f31fdffc33fef6bf951d867c7f571a366c77728570b8973b', '0xa86d423a87cb7213ff983f395cd142a4b5c298c66cf96befc9b4bf7dc38ea4759565bb1d49553e618c894da66bcdb9d602f2bcc428a46cc3cad1bb5ee63e00de', '0x660282ff1ac8c7ab351897c6ed70dc6e38f712b762ad0c3f5070d4dea84a8cde92e101cfedafd986e71ec4e0cfe9af528c440b0d0c72c74c65c16f35ed7a9b45', '0x9e8bca900b610120a052d0db42dfab08b0932ed6e2b9f58f05ba532d234ad6f2dc9f1b7ef1f88ccdd17643a60a2145d0fcf7bb039e58471a12d29438418ac037', '0x58d06b7674bec7982cc42b6e1abc79a007e053e14ef0ab2871bfee35c0a432d7b370f779ba3d8677b5855fc8ffa7b2c7457447ab73c315b8007ab668b57cd346', '0x3c26769a2363b83c3a69f48b371b30775aac3a5ba3966e8795181f848127ef9d0b35091453620f7c32785ad1713f1a67d5fc10f23b2c153d3c0ceb83a7d3a964', '0x46b35f9f3511d7c498eeabbd18edd22050c2d03909c1ddb4bfb59e9da5f253e968729d910f798b06e83cda52a0150a04f4607307d02e19b5c29539a1b24afbf3', '0xdf57eb1273f990a60c1d44574ac57efa4ecea52c61c020f3ab1ffa2f2729890111c108e29a376001a2a26d9c76064d66fe01edc27203b7bd51a62af81b1fc22', '0xb40bb30b424fac1d5937b4347ac1a0503583fc98baa693ae0a7de458466eff48eb823f5fd27b549dd9c3eb90aa51746e78921b896b954fb1fac5399904ce022f', '0x3f835f0bfd13dc88179dc6a792f505f971f602a957622b10af0746a8b40e40de58905d3edffa98839401f6bc40c38977333ff4fd817ac038db23861a8359d5f3', '0x10f302d7ebda0c480fa59889651d7a2f590dd2f54537144b762ce629162411f231d914b29432f7678144e8c3a24337b1b59dc53969dd330ed647dba5ef3be79', '0xb4129905ac577a556b92e5484d9299077ac96f585e2f3b85d25520204152671b26749cbacd8b2012c0722c3b2bcb12c526e1a2083f45ee9e5a7c6e44e9e0864b', '0x3cbabd445073aaf0cb876d857dfc0672cfcbf8c45b83d7762d33c0bb71e7a63c32164597449f1848000e0234935e95598e3ecf15d522994963aaa1a556c49354', '0x6166017774c89279f6fcf413192bb8fd6cb3404e104148542b474390cfb2663167ba0dfc77741781569644da18645f1a0a9a6d826d2f868b92784c0323d91ba8', '0x909fe481fde99853bc0878a1ab2463b214b45310e94a1f748b709cbe420fb6502c4b2be387af93a98b09d30671ee715cf315c55329c616db0e0d10d0feb5e6f6', '0x432abef9e92cb5349d60ef246050986b4aefbe25b4213488c1b051dd767702efc034523dd2562022ae1fe5bb630e715e136042521d324b9859a0d6be5606ae27', '0x8073ddc015537d4d24e367b54edda7f71e57d9d2c0b85feaa0a38a9546d6cc08761cf19a6f4f4d8b1f03cc60487b82d92a1d4a3b600dc299949a978f09d8f9fa', '0x6b6a28658161532b904464cab276bc689c34e5765686f23e200c9bd44c781090ccf9b5f4085eb8dcb4fbf1b3b6b400728b537f716e4870b8dc970079d86f5470', '0x6b67fac8fdb53a9b30afbe9a1c10a7fa4e2a59a8964af7b93b4e6b046ce6c09e90fefd459e0130e5abc25da9077714a2235ad656412c91011a50e9656ebba543', '0x91196fb37862f7ba0a4393f61a8d5b6eff5e0a4f3e77222bc12665049a98e507ea0189963c9a1c12927dec5d7699200f2d98b1934d36e0df86c13a1f26f16fb2', '0x280e60d279724cd4ce625bbea08e01737d369cae5fc82036f30c7062800ad4d4300ee12cdf69c05371692eb2c6496eb7ef4bf664bcb518954c7d918a571793f', '0x43fa1f0b9f6fda78944c60b1ef7ffef1772c156e53f4c88ea34d39bc34236217f5ac2a8e342a0ed8904f60964690d1e40b733c3e881a3da6973738b9722b7dd8', '0x3794dfb5b5d020c1256a2b045f2764165b7f98a446b33be0c0ac2f1e4973410781ea3adb34d49afbdec896fd5f366b0d90fb695cea9c1fbb71197066cd9bd93c', '0x521a5387f675a7953a89321aebc649a8b30b6a509f6100e025b8c77f71573731e5d8b095a56b0549719c861117afbefbcb0ec421a1d4aead9ac5d9c1fcf32ea0', '0x97f81c76c03c70cd1925111b7e851bff3a66ed22849210c46d387f2fc56c5bf1443ad847eea8f0c45f443f8a15829bdc9606190bcc5b7cd5bf748a2474ad14a', '0x55bacb8e8ac0bd302a611b198e07c7d6129c9bede1af04e570187f32890b674f78d90434420c4172bf0ca3919e045e135b39e618b7d963b1c74d572122c569bc', '0x3ca1bc3aebade401596c25dbd8ddc6d0f4615c2a6edd77087beb78a904fe2d3d3c2bd43de1da797c1a6f0f5e068bb3317442f634f4ff5009400a072b920df41', '0xa33ff8522fd6be1a0fda28d3f89bcb38ed1636c9eef922b635b28bfbecb0a102863893d2eb9e798b1dbf01f854113947c722f7778ad42af7d5365ba4b711e140', '0x7ff9d1ccbabeeacb82dfce388e84d90dfe55ad400cbf097b5ab2ea8e55671a0a8a6c1bd5285723c8efa77630d1e0f74d34aa239dc175cb87aaaa1fab1dcd4761', '0x8779398167ba405d0fb6343cd15bd0b8d1310a58d695b1ce871c66f6b4bb497d5a78c9b95786c42740ddb0f49bc98656ef10cc9c1a9aa39018938f058463dcb6', '0x8c07fa10ceea2fa521d41d41c85b98619091f778f9c82b4b581c308cfbef67745891d851891c48ae0c3748900ba82c487befb1517d2a22cce8e666fb1f5d04aa', '0x6d53d59b4f9685c9e46ca0f951b824e49337ae93980a4d56ed4cb68fb3a80f7979387a0bbd3c36670eb01a35c69ca1625579ed12328b2cd1c560422567c302b1', '0x64a32f7bbd63119d48a61714dad8f1143cce2325a645f3f294d4ec23efd3c4d493681a09d9e2e385f8b386d25d8f78c3c2027b49495e5b1722b05f16fd6242da', 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'0xad8b368c39ef55a6e4842ba3001e9e7e049f74161e7333859feab241c2b70a27a2c8db4925ba4025b834e51da06039e425a3f7913c2c08aa02ce97243ea33feb', '0xac60a70c68a4e8ac4e7065bd91e7e032082c4422d014816eeaac4d5f3bd838751e09f97fc11df8ac421d6e7b7202a3e22037998f82972ec1dbfd966e824f0cfa', '0x948895f09daba4a63a9bda3e865a4cd58002ef422b8948769bd43d672f1f46ba605bf2abdcc66581f0081d5afb20518415497905f3abbc2ca5655d8deb14758b', '0x73cfdca97c5fbc29959f4fe9f2aa4b31ebb85cde5e857a3b6c08a65e6bfe36124e1cbbb9fc79db3ac224f13225d7956c07330d9a66b22dde7cb74c0e944a0fab', '0x9b8bcc6296010c081783dd7a05febbe763bb2e0bedad24ce2d96abca7f70ba3d99b32bd83a5ef85b02404cbf9873aa56f47a8efb858dabfe59716056a2de59c3', '0x7dd04842d416a38d432f3776263fc5be417f5aee57b15679ffb9a3c83da9ac5470aae79fbf63fd342e138510c99d1925165e4bb5cc0b092dffac86b0a3bd0d96', '0x177824f73dcd848f8829f2525b13a47c2d74bb52364f86c069eada724eb2f997e85eca89ebefdf5cad8bab85e611d2df9ac60f31ececacb0306aa7f9780d1baa', '0x70542a156c45998292d6cc8f0498db35e1be7ec279e6990e4283c0bb1316cb2b52f45d149583fcaba7a0129adafb9b5479a583d658f7000c271281bb72c2fdb0', '0x8517632bf8ab9af61729ba92277e8071ffd81e964cc1239ec4ae661376392b631001925021a798416bde7e6140cab6ceac8b7d3f7155116230fd46851187fb5a']
> 3
HVvDXneFY8wlhYnrgvf/wrXhprTIQRgsfw5At7qm1Of2FzANjfwLhw==

サーバの処理概要は以下の通り。

m: 素数、4で割って3余る
l: 0～1016の8の倍数が入った配列(シャッフル)
l2: pow(key: 8バイト(不明) + long_to_bytes(l[i]), 65536, m)、128個の配列

■1. Encrypt String
・指定した文字列を暗号化して表示

■2. Get Key
・m、l2を表示

■3. Get encrypted flag
・フラグを暗号化して表示

□暗号化
・文字列長が8の倍数になるようにそのときの文字列長%256の文字をパディング
・128回以下を繰り返し
　・DES.new(key[l[i]:l[i]+8], DES.MODE_ECB)
　・暗号化

mは素数のため、l2からkeyとシャッフルしたlの値を割り出せる。あとは逆順で鍵を回しながら、DES暗号データを復号する。

import socket
from Crypto.Cipher import DES
from Crypto.Util.number import *
from sympy import sqrt
import base64

def recvuntil(s, tail):
    data = ''
    while True:
        if tail in data:
            return data
        data += s.recv(1)

def unpad(s):
    l = len(s)
    for i in range(8):
        if ord(s[-1]) == len(s) - 1:
            s = s[:-1]
        else:
            return s

def decrypt(ciphertext):
    plaintext = ciphertext
    for i in range(127, -1, -1):
        cipher = DES.new(key[l[i]:l[i]+8], DES.MODE_ECB)
        plaintext = cipher.decrypt(plaintext)
    return unpad(plaintext)

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('crypto.byteband.it', 7001))

data = recvuntil(s, '> ')
print data + '2'
s.sendall('2\n')

data = recvuntil(s, '\n').rstrip()
print data
m = int(data, 16)
data = recvuntil(s, '\n').rstrip()
print data
l2 = eval(data)

e = 65536
phi = m - 1
d = inverse(e, phi)

key = ''
l = []
for i in range(128):
    ct = int(l2[i], 16)
    pt = long_to_bytes(sqrt(pow(ct, d, m)))
    key += pt[:8]
    l.append(bytes_to_long(pt[8:]))

data = recvuntil(s, '> ')
print data + '3'
s.sendall('3\n')
data = recvuntil(s, '\n').rstrip()
print data
enc_flag = base64.b64decode(data)

flag = decrypt(enc_flag)
print flag

実行結果は以下の通り。

Welcome!

1. Encrypt String
2. Get Key
3. Get encrypted flag
4. Exit

> 2
0xdaf69fa8264ee5d22fcdfcc185b33150235d2a25e16ad31fa7767db43f8937cf9a3538cb59e83e5e094987d2806e143e6e71e72d2f3d17ab842df806b51f067f
['0x95bb63bde3407439644eb4d8f07ed639e9d40e9b1faf8b9c674527516700411d8090f3d009b85bf95aa0166475b9650deaac26656071ac96bb4cee2060be2c0a', '0xe57d271e6ec906b9c3821edca070a4acd75da3aa5f03f663fce4d310e6f790e59d31b979028b07d205c071ea468bdb307f09a2bbb5e72bf2f6161882cfe9e12', '0xb1a9b8f4777f6e3f1720f27dce733366b6b696d831cadc42fe6740b8ce449ad457b5d7544a2d7ae45feec6630efd0f00469170e3b83b3eeeaa5f61c6a556ade9', '0x448f9a1d9b4e0528eb79a83922539b3782b84f0baf980a78bd7f2fbbbb72a59cbdc94ff684d0d24c48095e3ce7c998a737e855403fd415995d43168ea05fabb7', '0xbfa8bb6d8a346d97d20c3ddd4e25928dfcb39038925107ebbdce263968469dffc420ad2438b24fd85d832dc3b2ce0485cfe0d5e6130e7b2572e53def0bafc123', '0xcd28c22c4b53a1f3242f05605d2f68d4af17fd701b6becd290f910789cea0aace5d87d353cce38bfabfb7be16974e85882fc8fbb48e1e9f88950c1716258847e', '0xb469b18ead6916e89813da9bea3f24df7bff2011ea4fc8c474d5cd703da14612b01f36f0af9750ff58a96da961105acba3b02ae824618ceb1ca0a73ab5ec191b', '0x54749a7e4fc4e491175e783761332aadb9a847f628b2baff9f21070a2bcb7e9897ce86674b3bf3b098dafdd9a8445216afb2ad6a219aa37e192c13b114095f95', '0x891780c9a9f29c42b5a5fa55d5a1c578349f409022bb4debae98f6632ee5d197c39267ed1b4906b0d7986701a404c7348d0b61c57175a50ae6761659b2629a8f', '0x1210d8421d3c3a8f2ee62857e304e171f17d39fee2a71ba457338ae97db905084777edf8d5a8c6b7f9b9bac2f59812437247e15be9a8680d5947f098f2e5200c', '0x6fba783344e4b3bbd0862aa3b67ff8873b12b458c96a549aedfe7562ad583b2c7f29540b67b2fe2c3a9da7d02a1b19e0a06afe2acc63677cd8fe93c07a61af54', '0x73f791c6b14805bbd96697782bc60319c7e29edd52aec050ca93fefc5532ff9b86c8990cf53d89dfb78741ee55cd4e2ca173afae1e9d19f8ac0b165c6a8d3a2c', '0xd33199a9b848763d9b83a659cba5c29a8bac45153d7ece262fc182dbc776f46689cbb7be7481c4a413b70dd123469936e4d7b008ad65cdc766f53c847cf02309', '0x962b69bde2900f4d79485c01985cc33cdc5d735f28d913bdde9c3fd4af667a418e171f42c591629b6b156dc9097560c1459d5f4dbb6151889b1b8b07b1dc6efc', '0xd3a4fb6e2e1afe48023d1ba5ba3aaea0fd479039a219a2e8e8665a7a14e17974c21e48c74a29797fa9c4458b0ff1de0d41f31b19bcba11ac41ad7f45652e0748', '0x46b4241bedaf2eae273e3b85b5203d0258813047f69ea8b842cfe47b29d9958ac27e56706938bcba9c06eed6cb77e9bd6d5f05c27b42e1976bb3b2a28bfd228a', '0xd611fc5fd948c73b207da0529b4ef4f10aabc61caee7335649655b1a4e65e655e29840cc12e4fee25570f007044008091d2a1746b854f5d781954ccc18714ab8', '0xcd58db47ced4e31001061235d19a9736ee5959ea1ed2d8e02569e4f450c144fba9d5672f3daf1b54f9661a3211f33e7c286bb8d44bb0989db5ae779e70387514', '0x591ddb676ce02ca9540c6a4d1b38c5ee894586d74cc66cbac3381f25aafcd3bed0640b19f15a3615d3b251ddd84e152de6ef9be32069d92dbc3996e4aa0701b', '0xcb7d839f36eeab826d4e3eb5c9a9f57178bd137c8e73c0f441a52aa16d528c09fc8775e6336fd35b8ea58e40cd5d3592aaebdf9045bcd380de2ce0a0f006180c', '0xb3bfade03b726bb2caa61f01e40ec5c99e4ee62a79564042224e933ca8838d120c7498c9c81f5bf34e89b618118a72ba469cb798af92725adb6a05c0c7df99d0', '0x1f29ef4e8b7e80fabec3be56d7f9465ea1e2be4cfc40430a4653233d08ef278ed099b864f05cb0618a539308d3aa3a28b2b0bb65f9f43d92dcff0d3e784a4777', '0x8bb10b88ab8ebf4c4d5b7f240eb84cb0c8776d24cec30587a1e9152bfed5bbe0be623c64b17499c765fd5c47a484f365183bbb07a0a2fa11099c3e4a16f6a3cc', '0xa072c875ddd786a33c43dc94d9db84c3f221cda6590d062773f63907e6c496ab856e14c3c6705cfba0ef72047c6cce3325fe6679025cbb6d4232cba4903d1a79', '0x21c596d5325857cc70d649a98252b3cb93d035568c4f6d6c7b65b843c8c6efaa1b3412941615eed764bbba0a51ab6a55c39f194b9c70655a67903f188e52b089', '0x25a55ea22f7b523b743d07b7deb9c654f08253470ccb7a164659d0dab256fe77339917a3b5ac068bce6ef844fd88ab7915d2b3259825dc902227b5df14f8b5f7', '0xb3c114e32a502b7fcbfa9ba31f3904358870cf6f88b344a12bba11a08790b42c8976dc7a519846370b4c680ec27fa460841f468b48f0f67a606e2eb6910b5593', '0x57ecbe4755e14b5ab78d820b1140b850f712a17e7a324efcef51f0bd683898c88a949ee483231f48cce1d6dc79951914a0c437f596b6e5923a20b72285551bd0', '0x12b1bf07e219762016c34553a814e27cdf6b7f0c2c836ce1c7497d9dcb7308ee118f092066894f80b137d67036d8778c15a97767fb4761f791d8c601ccd9a22e', '0xb6f5e8cc95d601777815fdab8f7513491fd5a5b9d5ed34aa64f413dec5d446c2f65872c7b8dec4a8e8e48bcfe26ef6e07334ffdb91701b6061ba65bab2b7c584', '0x96ea3f3a34d01476c1d5e5199a22758e6dc4a353e23f1d9d9e69fed80a35e30846f2b4d074b748cf034b8693dfea0956f02acfa68c6bc13ea59e21bca5d54f5e', '0x4579996f86482fdf266a7799813510bf39df6cfc9e079dc4d255bfff25ed4ce5b040abfba6b24fe25b24f8e42f50f0fe6d471bb34a53308e2738e8069fa5f85f', '0x58100762871f8ceae2c5d99d2636c3407a2781741051f6af80268751746fc6506c67a2f3c62993f80d360c1cb12e6b7ba66f60207e325a3400a5f9501e49d18b', '0x6e1376f816c41461b2ef108139d424ea2e94887fff7bc4de2c50bf1ba3771ac2f54826f0fe110197c0b7f177b5c9d284a1cbe2ae6c89ee75a3f2f06e19f670b1', '0x7e518635046c7511cfea3f4e34be9703ecc2af7b6591b744d000d7d728d00490e501949548869ae8d081211ef171e35cd4acb29aeb38bb2252c01a444bcc6586', '0x727e16dbc30192a48106cf150a87d55c09ab285efb5e27084bd10d4ad02d973fd95acf07e3b792715591bf04f306b13a2cf2230312fb11b454284160696d4083', '0x2003fb20bde8f7df43a47442ac77ec7e305e19700eb67111b7cfc21bd4e09b73d0223459c5b9e8bbdfdf5be285e829b61164a4c1a57a472a0aec6f0e8b8c3c24', '0x698d8668afe8c23633e77d297662b41a4b2c80785aeb7e23cf7cb8e2d8d61ee6022ee588665d7649820da87fffa5933bfbf3b5e98ff15e0ea506e991be21db02', '0x81b4d8c5ae223c16381be6635a320730499f9d905a09f0417bc7dbccc891334015a9992f898395870b6a0db79360bc5a1f8f5cc1b762c05485ce709680e52f1b', '0x945ef7559684ab1aaea5b9eb21079eaee4ae17f8ce156cf7e529ddfb8f639a663904174aeee93059faefaeab160d83c4caa913ec2f0bac7b9b1589f12b442000', '0x164537994d82840d6390e36a09b6627c680ae0cb638af90db939af47ab992e28e751ff7647c2753b8c420b4882b52aafaac82fad422e685d22eedc5f4b6cafdb', '0x1d65c39dd09b46849c5d6075ed28ebfe8ea8d2bcd01691de4358d6bf9d8387417d6977899084ab1ec8222969f1765feeff687d5967a8120a9bc1850acf7d75ca', '0x1ee818f0a5ecf19f2abaca755d0880a28b32ff7c8109451d9795f1546fb23f411d91372245c09e9535a80bd8b0aa4709fac6eeb7620e65a360324ee586e284b8', '0xb1ce7e054747fa3d1cb8db3c9116a321cca379493185fa5319bad969ceb56c77a1f5fff21faa6dd3a19f016af711e00f902d1cf9b4ac6a49d6e6e04be00be66', '0xd9ff32fa13a74b72e3491ab473af8b20c6771f41d9c0847a378eb2d849f41e21a53f7aeb677ef355e64c383dda2b71de1feaf73c5e6d98a5251444a0ca715179', '0xafc656c7478304650a5036f0785ff71e49f45608fbaca6761a1366912ae20fdc8e757ac5efad7893b1ae1c9dd76b2bb5378cdb64c95746727acc4375614e78e3', '0xd6254a71f494b798d6995c6c2ae23b0aae1d733c73b3c72f6080d6699f6b636057a736f4a9a9f99af1db034608dc3536106edac3e8aa70cdd88ddff48e5bf583', '0xb819a1839799e4e0fa95fcc113edf475ced5f4f3703a602afe548b5b7435fc5d9f4dab54e02ff6bc51a976ec3b8f44242defd1feecf8d78ebd32416f20f785b4', '0x5cbab1adc5e9f074905537856744153c7768c3c38522be6196d776104f1184b02709b45b623a3f6517953e654db6dfae5648f2cfed9389334d303b45108173f2', 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