第七届浙江省大学生网络与信息安全竞赛决赛reverse-wp

请注意，本文编写于 277 天前，最后修改于 243 天前，其中某些信息可能已经过时。

Reverse1

思路：

64位elf

ida分析

分析这几个函数

init函数初始化了一个table，一看就是rc4加密

继续看crypt1 和 crypt2, 是魔改的rc4

before_main函数加密key，秘钥是keykey

after_main函数使用加密之后的key作为秘钥加密了flag

exp：

def crypt1(s,key, key_len):
    v5 = 0
    v6 = 0
    res = []
    for i in range(key_len):
        v5 = (v5 + 1) % 256
        v6 = (v6 + s[v5]) % 256
        v4 = s[v5]
        s[v5] = s[v6]
        s[v6] = v4
        res.append(key[i] ^ (s[(s[v5] + s[v6]) %256]))
    return res

def crypt2(s,enc,enc_len):
    v5 = 0
    v6 = 0
    res = []
    for i in range(enc_len):
        v5 = (v5 + 1) % 256
        v6 = (v6 + s[v5]) % 256
        v4 = s[v5]
        s[v5] = s[v6]
        s[v6] = v4
        res.append(enc[i] + s[(s[v5] + s[v6])%256])
    return res

def init(s,key,key_len):
    v8 = [0]*258
    for i in range(256):
        s[i] = i
        v8[i] = key[i % key_len]
    v6 =0
    for j in range(256):
        v6 = (v8[j] + v6 + s[j]) % 256
        v4 = s[j]
        s[j] = s[v6]
        s[v6] = v4

s = [0]*256
key1 = [ord(b) for b in "keykey"]
key = [ord(b) for b in "ban_debug!"]
init(s,key1,len(key1))

res = crypt1(s,key,len(key))
print(res)
s2 = [0]*256
key2 = init(s2, res,len(res))

enc = [0x4E, 0x47, 0x38, 0x47, 0x62, 0x0A, 0x79, 0x6A, 0x03, 0x66, 
  0xC0, 0x69, 0x8D, 0x1C, 0x84, 0x0F, 0x54, 0x4A, 0x3B, 0x08, 
  0xE3, 0x30, 0x4F, 0xB9, 0x6C, 0xAB, 0x36, 0x24, 0x52, 0x81, 
  0xCF]
flag = crypt2(s2,enc,len(enc))

for i in flag:
    print(chr(i%256),end="")

    
'''
运行结果
[105, 13, 90, 178, 64, 234, 25, 63, 47, 106]
flag{1237-12938-9372-1923-4u92}
'''