{"componentChunkName":"component---src-templates-post-template-js","path":"/ctf-k3rn3lctf-2021-en","result":{"data":{"markdownRemark":{"id":"8f2d88fc-4ea8-5c0e-9b64-2706574a154f","html":"
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This page has been machine-translated from the original page.

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I participated in K3RN3LCTF, which ran from November 13, 2021, with team 0neP@dding (though I actually competed solo).

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I only solved 2 Rev challenges this time, but I still managed to finish 139th out of 501 teams — which might mean the competition was tougher than I expected.

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This was an eccentric CTF that did not hesitate to distribute real malware in its Rev challenges. Here is a brief writeup.

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Table of Contents

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Zabomb (Rev)

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Description\nYou received a suspicious file from the k3rn3l4rmy hacking group, the title says 'Not a Zip Bomb, Please Open', you decide NOT to open it and instead try to reverse it.\n\nIt is recommended that you do NOT open this, it will fill your entire disk.
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I was handed a genuine malware Zip Bomb.

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Reference: New Zip Bomb Packs 4.5 PB of Data into a 46 MB File

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Looking at the contents of the ZIP file, I found that it contained two types of compressed files: one with an enormous size and one with a small size.

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Using 7-ZIP or similar to extract only the specific file was all that was needed.

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7za.exe x -y -oC:\\output\\ -ir!filename bomb.zip\n# flag{w0w_c0mpres51on_&_d3comp53ssi0N_!s_s0_c3wl_ju5t_d0n7_gO_b0OM}
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Reference: Extracting Only Specific Files from a ZIP on the Command Line (Windows)

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WiRE (Rev)

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Description\n\nWe wire an encryption message that contains flag from remote server and dumped it out to kernelCTF_dump.pcapng file, i'm pretty sure that client has implementation of algorithm to decrypt data and get flag, will you take up the challenge?
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I was given a mysterious pcap file and a PE binary.

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It turned out that the PE binary was a client program that encrypts and transmits messages, and the pcap file was a recording of the encrypted FLAG being exchanged.

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Decompiling the binary revealed that the client operates in the following sequence:

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Analyzing the packets in the pcap file according to this flow, I found that the encrypted FLAG data d33411044a6202726302656e6901636e637462017d6702756e760101756e7b0173104c0a was received.

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Here is the encryption logic:

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void main.safeWrapMessage(void)\n\n{\n  byte *pbVar1;\n  byte *pbVar2;\n  code *pcVar3;\n  byte *pbVar4;\n  byte **local_res8;\n  \n  pbVar1 = *local_res8;\n  pbVar2 = local_res8[1];\n  for (pbVar4 = (byte *)0x0; (longlong)pbVar4 < (longlong)pbVar2; pbVar4 = pbVar4 + 1) {\n    if (local_res8[1] <= pbVar4) goto LAB_004dbf48;\n    (*local_res8)[(longlong)pbVar4] = pbVar1[(longlong)pbVar4] ^ 0x31;\n  }\n  if ((longlong)local_res8[1] < 5) {\n    return;\n  }\n  **local_res8 = **local_res8 ^ 0x84;\n  if ((byte *)0x1 < local_res8[1]) {\n    (*local_res8)[1] = (*local_res8)[1] ^ 0x69;\n    if ((byte *)0x2 < local_res8[1]) {\n      (*local_res8)[2] = (*local_res8)[2] ^ 0x41;\n      if ((byte *)0x3 < local_res8[1]) {\n        (*local_res8)[3] = (*local_res8)[3] ^ 0x52;\n        return;\n      }\n      runtime.panicIndex();\n    }\n    runtime.panicIndex();\n  }\n  runtime.panicIndex();\nLAB_004dbf48:\n  runtime.panicIndex();\n  pcVar3 = (code *)swi(3);\n  (*pcVar3)();\n  return;\n}
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The entire FLAG is XOR-encrypted with 0x31, and then the first 4 characters are XOR-encrypted again.

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So, XORing d33411044a6202726302656e6901636e637462017d6702756e760101756e7b0173104c0a with 0x31 and then replacing the first 4 characters with the known “flag” prefix yielded the FLAG.

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Summary

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With WiRE, Ghidra’s decompiled output was not great, and I had trouble reaching the Flag.

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When I tried IDA Free for decompilation, I found that the output was more readable and useful for understanding the program’s behavior compared to Ghidra, which allowed me to solve it.

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It seems that readability of decompiler output can vary depending on the binary, so when I cannot solve something, it is worth trying both Ghidra and IDA going forward.

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