Tamás Koczka Reading time ~1 minute

We got an RSA public key (pub.key) encoded in PEM format and the encrypted flag (flag.enc).

These are the converted parameters:

e = 2385330119331689083455211591182934261439999376616463648565178544704114285540523381214630503109888606012730471130911882799269407391377516911847608047728411508873523338260985637241587680601172666919944195740711767256695758337633401530723721692604012809476068197687643054238649174648923555374972384090471828019
N = 2562256018798982275495595589518163432372017502243601864658538274705537914483947807120783733766118553254101235396521540936164219440561532997119915510314638089613615679231310858594698461124636943528101265406967445593951653796041336078776455339658353436309933716631455967769429086442266084993673779546522240901
c = 1624768965978244122218384915440259949773623052619109265384960524204099241405509334298217012073574245240140975823312659160847045035132501536939096089619077929998251251236783590255562951129897302725067655285503493676186062693350470482247124598766533755027440418713398509566189239815613916662987881029294277207

The public exponent is too large thus we can suspect that the private exponent is possible too small.

I tried to attack it with Wiener’s attack: https://en.wikipedia.org/wiki/Wiener%27s_attack with the following implementation, but it did not worked: https://github.com/pablocelayes/rsa-wiener-attack

So I remembered other RSA attacks from previous CTFs and how much time this page helped me: https://github.com/mimoo/RSA-and-LLL-attacks

The last attack is the Boneh Durfee attack, which is you know BO-neh DU-rfee => BODU just like the challenge’s name, so I instantly know this will solve the challenge (also a lot of other teams are already solved it, so it should be not too hard challenge either).

Running budo.sage will give us the private exponent (d):

d = 89508186630638564513494386415865407147609702392949250864642625401059935751367507

The executing pow(c,d,N) in python give us the following plaintext:

7105857801457696083098669180371125182430908825274295869462261196993232333904846182088682459845909159924079587285438988882837378435398205428800773161869836747653246664819269651173622798039814934439562046448483899123585744167522783235535219103995347045452193429764349550389498609273176996913420550906739978

Converting this to ASCII (for example with my javascript based conversion tools, hosted on https://kt.pe/tools.html) will give us the flag (it is padded with PKCS v1.5 padding, but contrary to OAEP padding the flag is readable instantly):

ASIS{b472266d4dd916a23a7b0deb5bc5e63f}
Tamás Koczka Reading time ~1 minute

This challenge was solved by and the write up was written by one of my teammates, gym.

In this challange we are provided with a pcap file, loading it in wireshark and after a quick glance at the exported objects we can see they were using 0bin pastebin (https://github.com/sametmax/0bin).

0bin encrypts the data client side and provides a decryption key. This key if appended to the URL with a hash mark ‘#’ is used to decrypt the received data. Ideally this part of the URL should not be sent to the server, thus the server operators cannot know the content of the paste.

However in the pcap 0bin is used in conjunction with piwik, witch send the entire URL in the request, thus we have the key to decrypt the data.

GET /piwik.php?action_name=0bin%20-%20encrypted%20pastebin&idsite=1&rec=1&r=776276&h=11&m=27&s=12&url=http%3A%2F%2F0bin.asis.io%2Fpaste%2FTINcoc0f%23-krvZ7lGwZ4e2JQ8n%2B3dfsMBqyN6Xk6SUzY7i0JKbpo&urlref=http%3A%2F%2F0bin.asis.io%2F&_id=dd17974841486b63&_idts=1443081356&_idvc=1&_idn=0&_refts=0&_viewts=1443081356&send_image=0&pdf=1&qt=0&realp=0&wma=0&dir=0&fla=1&java=1&gears=0&ag=0&cookie=1&res=1440x900&gt_ms=108 HTTP/1.1

We can find three such key-id pairs in the pcap, the first one results in a fake flag, the second one has expired and the third one gives us an ASCII art of the real flag.

The last url: http://0bin.asis.io/paste/1ThAoKv4#Zz-nHPnr0vGGg3s/7/RWD2pnZPZl580x9Y2G3IUehfc

The ascii art: alt

And the flag is

ASIS{e29a3ef6f1d71d04c5f107eb3c64bbbb}
Tamás Koczka Reading time ~1 minute

This challenge was a simple reversing challenge.

The first step was NOPing out the ptrace “anti-debug” call and finding out the main hash function which was at the 0x401B40 address.

alt

This accepted the flag as an input and used big integer math to calculate it’s hash. This hash was compared to a static buffer (there was a little trick that only every 4th number was used from that buffer).

The hash function can be summarized with this python code:

result = 0
for c in flag:
    result = result * 33 + (ord(c) ^ 0x8f)

And we know that the hash of the real flag was:

27221558106229772521592198788202006619458470800161007384471764

So we could calculate the flag easily with the following code snippet:

p = 'abcdef0123456789'
def f(h, l, x):
   if l == 0:
       if h == 210839978725:
           yield 'ASIS{' + x
   else:
       for c in p:
           if ((ord(c)^0x8f) % 33) == h%33:
               for r in f((h - (ord(c)^0x8f))//33, l-1, c+x):
                   yield r
h0 = 27221558106229772521592198788202006619458470800161007384471764
x = list(f((h0-(ord('}')^0x8f))//33, 32, '}'))
print x

The flag was:

ASIS{d5c808f5dc96567bda48be9ba82fc1d6}
Tamás Koczka Reading time ~2 minutes

This was a python-based DES encryptor / decryptor service which used 256 unknown DES keys.

We could encrypt / decrypt messages with exactly 3 keys in a row. So we could only get the result of Ek1(Ek2(Ek3(M))) or Dk1(Dk2(Dk3(M))) where k1,k2,k3 were the indices of the predefined keys.

We also got the encrypted flag, but it was encrypted with 10 keys and not 3:

key = 97c4b5a27177406c404f
ciphertext = 7f62a70857410e0e9c2bb283fc9807f8b1d34bcf7a2b456e965e860e5c6818b40ac596fa43492c30

We looked up some vulnerabilities of DES on wikipedia: https://en.wikipedia.org/wiki/Data_Encryption_Standard#Minor_cryptanalytic_properties

There are weak keys and semi weak keys and that’s all. I solved this challenge the latest and by that time, a lot of team solved the challenge so I thought it should not be too hard.

First I tried to check weak keys by encrypting arbitrary message (ex. \x00) with kXkXk0 and k0kXkX keys and check whether they are the same. But I got no match.

Then I wanted to check for semi weak keys. For this to happen I needed the plaintext encrypted with every possible key on the server, so to do this I decrypted the ciphertext with the first 9 keys and then decrypted with the latest and two 0000 keys. Then I encrypted with two 00 keys and with every possible predefined key.

Here are the ciphertexts:

674ceb7725ccbce67a5847065fc4f58769fbb582e1529c6525ac8f059e24288a83d33c58f06e47a1
0c683c05d53293303aebe36fdea9ad152fa8240922cf7e7bc15134149889dde209163483d33e4757
dd0662ffd94e9840211f493704af1914545ee7af0dd055a2eca317fe3d4c17b90a479cc615a30348
798e0945f777ae36b0c7904eb513300fa9b8bb6d8076bf4b25a50d9319639bbe42c884a9514e7bd8
6f04598a38c928fb073a9bf3efc1a7f87f0d843cb429e0467472df1934911d7b354491d1f6db79ff
9fffa405edda869aadd429f018d096d7f19554ddb6f273bf71a6d5f3f60d2d06938a92ec7020bd8d
a18df05d43b43986f36aa1d615ce40fd51de43c774ad018d9e059bd6bd54ea80ee08551aaa153b74
f3fc1d612ecdb31ec9f293b3bf524620873303a6740886ba1acd4c1fa65cf1112b80a3eecae261c9
97438506dcd033bb9af6121fd017db7f4dc9b9227ea9ac40a11db76cd7d6692863f953be0c08b653
b45df38fe9adb9a76f86a4cc309d4c3c64c6159cd80ae014ca173b9441e61e8775c96701e0ad7aeb
0042a952b5b82543e3725d34a40a7a88a667c6d0745072ee78fe0cf04e8b24a41a8708aa2f87b66f
cdfb693f430f9d7cc01530e257f6090b26ec133e476ef7099c7c309f878cd85302a7151cb059ced6
8a934d0c57644dfc0a844f085e6de6c02a925694af37ce86493b02a7cc5fe577007a430994a6b16d
a0000408653172567c03479332ddc24de44a5f2d20fe82d7a44a7d7d2e43e6ea5bc29f6f7786277a
89d2f4465265602be5b8f0b49e38c052f07a1805eaa894c44792aa7f5b541ab110c6d01ae9c0c100
b0f0b889cba338c05d948473b6ee5fdd322248bae1d924368a8be3e00ee19547a19a5be285422152
5ca5c617b710b484574b969be1876b82a9cea0c8b4b634034b8e350891c94e25c118483a5d31c4a0
be27cf28ca88587c79304fa1c320b91ee7fa58e0c897e622426ecddf536924c59cb104e3d42829e3
e3230c835353fda4397ec87bec547e9b3f0f7f7239a04466128c4f4cbd86ad1fe082475db0edfeb8
b6fc82b1e9e1260a3baa7154530853f0f77417b7c75163bc44fa96c9564d775aae2b23d59c6855b8
c3854d5419e8aa75d25c1ab6e3195c9240527e56c4e23a7328c464aa5a5739d00794ac18a77c03bd
c85baf1a30da5f2ec75f02bb65edbeabcc33def79b7e5a7208ed6911eb123bb6786f40fc6b1cedfd
391d9ffb35916e430f01f03bee80a8632b618e2101c5974bd3ab8113cbe33485e254c3a3079b798d
82ef2f5ef8d0eb32d25734ff31661141d9ecf2bc91fcdef6e205ecfcec178d12ef2a865d21a3a6ee
316b7a28f8336e5641e4aa9eae458fd48d6881a3ae5e97ed73606f309503668498684bece62ac8b4
0e6c948c7df62f60a8405034c991ee7abb6c9fd0436c648605859a216c9204a2746646820dc9ecfa
29f87d0c0f639741c09be27b9c7cd089015a5127d1a69f0a6f3082fed7d2bce8d1ae2b9a042993aa
2fe1869d02be57b4a2180a9212bda077109efc3c17a01982ec0e2f82ff2e7e49036de5b4e8cc078a
16fa74f63d51719e250b2fb84a39f9d12a8bb5d640b0350998d7e354a999f676f40feabab51feeaf
47bcced21c1af40e92233b3b9b6a82dfff0413b0e0e851b79da8d74f7affc12ece19d2ab25efe3aa
0e300d28da9efe43c3fa15a9c8024777ec3d1e41b83fea612a87aeb2115fbb0392014e08ebd0c756
ca242feb4709fb082bda92ccd3120217b4c6eecb216a7ecd12422a4111a242732aa6887563e395ee
3a8355cceaf9e5d7f208ef0a66ba5b5662788c1a3fe53115188839a2be0807f428bbd4bb215fff7d
b64854af0e7cb5fbdfeb6a660ef81cd34c090f8d12e2e3a715c5659f505ad964dfad00f56a84f61d
9586cb7212ed0507d8658ab03a8ac0c011ecd585fc13a5b7e59b3b8786c7ea682e1b721592ff1f51
ac06279dc188e4c471310e3014195d3659394d0e043af8da1507a028d3dbad434a10029d49690837
2a47b3adc8733aec0510fdfcffbfe77b35f612822cabbbb3260e84adc096e51c66b85c6a2d4b51b3
8bb11c2ef8bc2645f86ebca3cbbcd1a3da6520d6e7c802923bd0b35c996d4b7c98683b8c89e73ab7
213768956645664f4e1bf3bcdb59cc1658519261ad9479f1f0fa68ca8dc31e8b2ae2dfb3ebf415eb
02667dd72157bd14480b6f3700567c80d66af98fe88ef541c5d6f5155abe65098920d49317957d1f
74234a4542100db00056f7a77a2e5bc7856686d95f0509ab9c796512292bff9ec0c8f06127a25b20
2cf96a56e3fc409ef3f0f00da146620a1ad1f011314a4d3cd035dcbbd04bfa59ffe3904139c42ff7
0c5bb0cb40029eb5ec2c92d0a52609351bf5b2d68b60e473fb5a0a02528733592049bf0e7625a876
3860be91e21db6303958dac8882c65ada4e0aed679ea588f154126278602c7948c8ce62cb6e88977
67b096e2a08d1d75f060c953e0ba12665a5c070ef438837f8ce27d90943f4aeea5c64ac755eb8e6c
10a0f516ce58e616545d4d917a73bfe6ddc5f7ecf6e926dd4a6ac8d0d44b9227a0d2c3862c95a7ca
dcf9544c08a5f52ff3f637c0dd2a8651e86aa98235732d0febc0243a23741594362bbee96e45d507
51267cdf44914ab955a88f6314ff1f5fd0a98d6e9b4844b44aef2cba41d933afac6c5bb738d50ced
23882b9e863a466c87efa19145b48aab2e977c4cd33e5da4d407bc7b2485b1ba13a8316bfc5926aa
3a12e7e48b6780821c347785aa1c0b4b6925630eb830d40e7c4c5ead4deb8b579646868af84191ab
255800f7e87f66aacf1a3386ac099fc19c9426e29df1bf5ef5e7c829aeb33a706a32e5b69d3f260b
b6f9c82d07d9349bdd302d551fd592ba3f7e930116db134dfebc24957e24ba3705f721f151dda54e
5c04aff64a08d55037cbbfde3cc7259ebd9693a0d9d960c7a075513660c4c2fa5d2f8e952e86dc4c
bde1b6ab17a66e160527be43cfae1a5647a46a6221098e726d4ff14f27648a7fcd892ef051b3291b
623f487f4554628df3e18df1fd92de6122c8cfceb44852c17b309eb2e96c44e65070d49250a84153
ab67d559aad58fd3312735aa181de0fbeb4ca35190cb78f5f59a9b0bf8331c062802750193ce437a
8ec2fcae0b5332c9ea1462f966fed692e58a7e46b74f3794ba13039b29a81874995aa9765a2b5b7a
8f3a2cd4bc695ef6b60c110abcf3b31e1833bc1a13b4061e887e41b4d10d9b85a3ef8f8d738f1f86
2f8645560e600193dac20877b64257279ac48216b9939756e191d6552d52556f6896cee21091d1ad
f0bd305daaafefcea9eed7fdf9a2b1b1bb0fa41efc7a2d909c23bb9229bed008eac826cc1430d24c
f623d4c97a08df4d335b319605dd8e5daa493d89fa3c1eadad26d61b39307bc4a994341e35f8cd44
4281876e02512ac2e34170af5d9c75b80712c24f4e3b7a4b8757ce8f0f8d15bf3f80bfffc01e1cf3
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be0473bcac1d7293b72ea4a9ab0b30f00b6cd899241a2e15221127884f852c915cb017eb55060c33
c3c8a7742063a9c0f56635bc64c5882bc25683d41a04d51d39728130ca7ad7603225785e884f7c1d
1bc42bf6678e2089dd4f5ef5adc21b337e70a240048356c1c7071169da5858d53deb5a183acf075c
a310323eba4f1b2c7811471e413f36a3d6c70aca3e7187ff204917e7243a45d145e3a12e417f8dc9
8d1c19a6e80e4b115e01183dd246dee3361b0fc50119eb6af70b8656cba4e02b7c751297f38c34b6
5a1d66c2fddb4559de3334c9f4a8fa252a71eb4348e632f0e737f21d347365923c9342db1ea533ab
d32188801b589001188ab8afbac49e251f5299ae3df0512c1f32ff157b07f7e730a6b8105f7fa8de
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a40f47ddcad28f53b4f72493f2ea082a62b953404f22f4cacb905041f63628ef89b0de37e91c45d8
898037c3d079e442206e4787a00206c5954a2e20cd2c6501ac3b0b9f2fabb2a80a435eb5039d3806
2a2e80e1d4cef290893e104e9c58c52cf10073d25776615c5c384ca263ed9e3d534585e1bdaf05b9
3b5d6a2818a910d6cbf68959fed9ecb39cfb95f10d18a6f0bb60bc5540270b8d1708e7efcc85b5e2
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4bf311e9ad62d8d6fb55f23a69cb289d3e393f7f846a840e3d2f55506b1c189b5ebfcd96188594f1
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462d859d367aebf607b71f7d7b4af8d46d1e2a7fa701c72c3a781f8021fe87f96131c7299cb13773
0c45fc2b65db639cba59075c4d2375c1fb98a6b405935bb331084d07c4e6fcb58c8f7ebe40b809f2
2ef4fe524b8d4b1cbb972e6a8cb069437c07cdb045f0f4dd0c5f5181aaad55a06e924a85a64442b4
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f52b6a4d5edde7a1392db12033ce1c047b3d913188904b879f44cd5d58328273279f5ad2324b3b02
1f8e20c26a93a57bfb8e042e41f426dda4dece5fc71e8172deedcfacc6e43cafe1374d7487cab419
0009ea4366f49a89d43a846e805a70564811e7ab00435e62d9e22ed61a6f45a9e090ee81a43923fc
6a134c6203dd4001ac59ef645586d09e59b3c2e94f640aeda428202e2c00671f59943fd48a00809e
371332a6c9cb3ea401be38a4ed42078e88565f37c9dfd7b4057dcd20167f01cf4ee368f4d8b2b53e
260212df3c505a442e0329f15184244694e9353ba21047cd8bfb8bbd22b9937c84fae5372000f425
6e83274a8eb13defcf365533491e3ffea3fcb60427a7026d6e9d4547665aaea4fd83355ee913709d
179cae0cdb753f8db7781c80adc15d4c1acdf6068139e1df8cbf0268e37b931ecdbf31964869383b
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37c395edcc25ef968be21566cf6f497f464f8161d3cb63090a0a530cb4528b5149fd6a0ec9da3bfd
8be87da9b06ad3699e35cada7176d62273ca6320f8d0a6d8f7494ee4fbf6641e191e906c19491774
391d6bfd604f5b78f30d4f4cd3cf82ddb7ed396758c9dedc178671546da6697184017a37615b318d
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0992b81a729231389d64039bc1f0965ba63f1b3db5fe13e71ce712970b7c2ad0cee1bafdaa5ec0e9
722ab2cd71a8eda69ca3b0cad7b72a3027330640d21fe7e1f9a2b08287627830f426ca55193a8be3
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5296431c9091ca948683088bf222d2d3b986e57540dab6fd918fc3f3d5cd5302b7b5cc968a74eeba
43706fd827d09dda4e425a192838ab31cc33d81f2fa3ee3f768c1a80999b586f915089602320fae7
8e7706fcaf11bca2a5008f7a582880abd02657c98f09a4b12de5b2e370a9981d671aaa69325ce165
b674b7babae8fa1b747c65aa9b84430448a8956ad3e72bcb6acb3b5ddc213ca7539f599807dcd055
06a8a511df72166a5e27899ddcc209619c469ecfd9448754b71b47fa9617052f82e1698ccef9f382
751235d58a6d4e236dd5bdb074b6e0b7c4a3b005748b5849d3578e1c777a924f36244b7b97a9e8d7
5c6715492a8ce4802e0bac5f4d847da2ba1f6ba219eccc86749e9aa874355f1a73d96505826a724b
aaee675956d6b764157c31071ce5ac225800f58d74224778797b2b9d58fa385a8ed7689bf898537e
4b650c4e5a34a09244a3334589f51c62477168764fd59ba0221acd7adc18b4251d2d28c782d2eb8f
db3e20994e705708c88e52ee0b365902225e94a3da8b97f2a79e5f75e897d23507d414867bcf3fdf
67f97c93fe97ab29386c296b3db11511672dcf0be1316234590f63915eb5b01a93192d954b434b72
059627902f8fb3a170643443ed7814c854f884284fd42384774ce96945abe31c62600d1b5c831a6e
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6cd2c33a90729ab4b9aad0d0bed11b944717b869aad1360acbee069d48599b06ef71f3e5238ed9cf
900499844d9388993c2bfbc8d440402ab4db1b8d71f5eb2e0445ab889eea35658b119a5b47b1b72d
030e41fda90fd62e0d005e6ed5ca74c07e56be250d061673b0890fced941dc6e3f11e9884680cbb4
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263a7f25865aadf79dd9987504e2718e89d3749db0ac08355f555966bfdf4f85be4ee34922b54787
0e36c78d4580235bf8a0426cd3f43954d6cb5d00948338fa271b8baaade5e5c80e210e2201ff50cf
944ce0b8652b149648a672f5d26ca83be534d21610b7c826cdc9155fac929611a68f8ab109b9f039
da8114b2c8f9609bf9c0ec4272af4181d9e9ec3a0473a56c6d5a7008c5fbdc1a2272141bd81b74b6
0360d507aa121d0f78699073bc9c388dd4aeca2cd954010c0a0142f91339b8eef8e42de82a7d8ecf
dcb881c2f1a4cc5654b679c21d084ea5e5bd90e21edd0596f71673d7b9ee1e6de3a54f05c76bd6b9
0c99716c7e65d3edf003675cfb1646c02b960c3cb05030899447cd2ef0cf4fae2dce2387e0d7c0c6
6808fa6e76e405c172cde411109e99c3e00699710c3453dd7ae5d003e288550d290003bb9f7f5f7e

Bruteforcing them with every possible semi weak and weak keys and grepping for “ASIS{“ it gave me the flag.

Weak keys:

0x011F011F010E010E, 0x1F011F010E010E01
0x01E001E001F101F1, 0xE001E001F101F101
0x01FE01FE01FE01FE, 0xFE01FE01FE01FE01
0x1FE01FE00EF10EF1, 0xE01FE01FF10EF10E
0x1FFE1FFE0EFE0EFE, 0xFE1FFE1FFE0EFE0E
0xE0FEE0FEF1FEF1FE, 0xFEE0FEE0FEF1FEF1
0x0101010101010101, 0xFEFEFEFEFEFEFEFE
0xE0E0E0E0F1F1F1F1, 0x1F1F1F1F0E0E0E0E

Solutions:

Key = 0x1ffe1ffe0efe0efe, Ciphertext = 0c45fc2b65db639cba59075c4d2375c1fb98a6b405935bb331084d07c4e6fcb58c8f7ebe40b809f2
Key = 0xfe1ffe1ffe0efe0e, Ciphertext = 6b8fc0f9fea6586e312fabcbbb9a62f586d9e570053c316b49f943f9d7a5e7e9b7b614b52c10f429
Plaintext = ASIS{90152c3d6e6658f2057bba4c889e5cda}\\x00\\x00