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[原创]KCTF 2024 第六题 WriteUp
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发表于: 2024-8-27 00:28
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首先x64dbg挂上，验证一遍给的用户名和序列号能否通过，发现可以通过，看来没加反调试或者反调试被ScyllaHide给干掉了，好事
用IDA看了看，关键的部分是这个函数，用x64dbg下硬件断点设置日志输出，把这两个地方call的地址都记录下来
其中，v13的基本上都是上面这种，调用的外部函数，或者.text段中的被虚拟化的函数
而rcx的则是零散的加了混淆的代码块，基本上就popfq的下一条指令是真实指令，别的都是保存、恢复上下文和修改控制流的指令，写个脚本通过刚才记录的call地址把代码还原出来（如果想看仔细一点就把去重的逻辑删掉，输出的内容会多很多）
代码很清楚，没有嵌套混淆，也没有奇奇怪怪的扩展指令集优化
简单描述一下算法，就是把输入的16字节用户名（不满16字节则填充0x00）和"welcome_to_fzbz,"异或，再把输入的64字节0-9、A-F序列号转换为32字节，然后进行4次大循环，每次循环处理8字节序列号，其中用到了异或后的16字节用户名，最后把处理完的序列号和"welcome_to_fzbz,my_name_is_sbzx!"判断是否相同，我写了一个每一次大循环的python等价代码
每个input、output都是4字节，两个加起来8字节，用户名的部分我硬编码到算法代码里了
一直做到这个部分，都一路顺畅，很快搞完没有卡壳，然后……求逆算法花了巨多的时间，期间试过用z3来求解，然后发现挂了半天啥也没算出来，问GPT也是一通胡言乱语，最后还是用笨办法老老实实手动搞完的
fn = ...
fp = open(..., "w")
visited = set()
for i in fn.splitlines():
ea = int(i, 16)
if ea in visited:
continue
visited.add(ea)
idaapi.create_insn(ea + 1)
fp.write(hex(ea))
fp.write("\t\t")
fp.write(generate_disasm_line(ea + 1, 0))
fp.write("\n")
fp.close()fn = ...
fp = open(..., "w")
visited = set()
for i in fn.splitlines():
ea = int(i, 16)
if ea in visited:
continue
visited.add(ea)
idaapi.create_insn(ea + 1)
fp.write(hex(ea))
fp.write("\t\t")
fp.write(generate_disasm_line(ea + 1, 0))
fp.write("\n")
fp.close()import ctypes
input_A = ...
input_B = ...
rsp_1 = ctypes.c_uint(input_A)
rsp_2 = ctypes.c_uint(input_B)
rsp_0 = ctypes.c_uint(0x2538263C)
rsp_5 = ctypes.c_uint(0x5F656D6F)
eax = ctypes.c_uint(0)
ecx = ctypes.c_uint(0)
edx = ctypes.c_uint(0)
for _ in range(0x1F):
eax.value = rsp_2.value
eax.value <<= 6
ecx.value = rsp_0.value
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_2.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value >>= 3
edx.value = rsp_0.value
edx.value >>= 8
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value += eax.value
eax.value = ecx.value
rsp_1.value = eax.value
eax.value = rsp_5.value
ecx.value = rsp_0.value
ecx.value += eax.value
eax.value = ecx.value
rsp_0.value = eax.value
eax.value = rsp_1.value
eax.value <<= 6
ecx.value = rsp_0.value
ecx.value >>= 0x18
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_1.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value >>= 3
edx.value = rsp_0.value
edx.value >>= 0x10
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value += eax.value
eax.value = ecx.value
rsp_2.value = eax.value
rsp_1.value ^= rsp_0.value
rsp_2.value ^= rsp_0.value
rsp_0 = ctypes.c_uint(0x665F6F74)
rsp_5 = ctypes.c_uint(0x2C7A627A)
for _ in range(0x1F):
eax.value = rsp_2.value
eax.value <<= 4
ecx.value = rsp_0.value
ecx.value >>= 0x18
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_2.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value >>= 5
edx.value = rsp_0.value
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value += eax.value
eax.value = ecx.value
rsp_1.value = eax.value
eax.value = rsp_5.value
ecx.value = rsp_0.value
ecx.value += eax.value
eax.value = ecx.value
rsp_0.value = eax.value
eax.value = rsp_1.value
eax.value <<= 4
ecx.value = rsp_0.value
ecx.value >>= 0x10
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_1.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value >>= 5
edx.value = rsp_0.value
edx.value >>= 8
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value += eax.value
eax.value = ecx.value
rsp_2.value = eax.value
output_A = rsp_0.value ^ rsp_1.value
output_B = rsp_0.value ^ rsp_2.value
import ctypes
input_A = ...
input_B = ...
rsp_1 = ctypes.c_uint(input_A)
rsp_2 = ctypes.c_uint(input_B)
rsp_0 = ctypes.c_uint(0x2538263C)
rsp_5 = ctypes.c_uint(0x5F656D6F)
eax = ctypes.c_uint(0)
ecx = ctypes.c_uint(0)
edx = ctypes.c_uint(0)
for _ in range(0x1F):
eax.value = rsp_2.value
eax.value <<= 6
ecx.value = rsp_0.value
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_2.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value >>= 3
edx.value = rsp_0.value
edx.value >>= 8
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value += eax.value
eax.value = ecx.value
rsp_1.value = eax.value
eax.value = rsp_5.value
ecx.value = rsp_0.value
ecx.value += eax.value
eax.value = ecx.value
rsp_0.value = eax.value
eax.value = rsp_1.value
eax.value <<= 6
ecx.value = rsp_0.value
ecx.value >>= 0x18
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_1.value
edx.value += ecx.value
ecx.value = edx.value
eax.value ^= ecx.value
ecx.value = rsp_1.value
ecx.value >>= 3
edx.value = rsp_0.value
edx.value >>= 0x10
edx.value &= 0xFF
ecx.value += edx.value
eax.value ^= ecx.value
ecx.value = rsp_2.value
ecx.value += eax.value
eax.value = ecx.value
rsp_2.value = eax.value
rsp_1.value ^= rsp_0.value
rsp_2.value ^= rsp_0.value
rsp_0 = ctypes.c_uint(0x665F6F74)
rsp_5 = ctypes.c_uint(0x2C7A627A)
for _ in range(0x1F):
eax.value = rsp_2.value
eax.value <<= 4
ecx.value = rsp_0.value
ecx.value >>= 0x18
ecx.value &= 0xFF
eax.value += ecx.value
ecx.value = rsp_0.value
edx.value = rsp_2.value
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