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
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316b7a28f8336e5641e4aa9eae458fd48d6881a3ae5e97ed73606f309503668498684bece62ac8b4
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29f87d0c0f639741c09be27b9c7cd089015a5127d1a69f0a6f3082fed7d2bce8d1ae2b9a042993aa
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47bcced21c1af40e92233b3b9b6a82dfff0413b0e0e851b79da8d74f7affc12ece19d2ab25efe3aa
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da8114b2c8f9609bf9c0ec4272af4181d9e9ec3a0473a56c6d5a7008c5fbdc1a2272141bd81b74b6
0360d507aa121d0f78699073bc9c388dd4aeca2cd954010c0a0142f91339b8eef8e42de82a7d8ecf
dcb881c2f1a4cc5654b679c21d084ea5e5bd90e21edd0596f71673d7b9ee1e6de3a54f05c76bd6b9
0c99716c7e65d3edf003675cfb1646c02b960c3cb05030899447cd2ef0cf4fae2dce2387e0d7c0c6
6808fa6e76e405c172cde411109e99c3e00699710c3453dd7ae5d003e288550d290003bb9f7f5f7eBruteforcing 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, 0x1F1F1F1F0E0E0E0ESolutions:
Key = 0x1ffe1ffe0efe0efe, Ciphertext = 0c45fc2b65db639cba59075c4d2375c1fb98a6b405935bb331084d07c4e6fcb58c8f7ebe40b809f2
Key = 0xfe1ffe1ffe0efe0e, Ciphertext = 6b8fc0f9fea6586e312fabcbbb9a62f586d9e570053c316b49f943f9d7a5e7e9b7b614b52c10f429Plaintext = ASIS{90152c3d6e6658f2057bba4c889e5cda}\\x00\\x00