思路

Problem: [HZNUCTF 2023 preliminary]Bytecode

先用ai救命

def check_flag(input_flag):
    if len(input_flag) != 28:
        print("wrong length")
        return
    # flag_list = [25,108,108,176,18,108,110,177,64,29,134,29,187,103,32,139,144,179,134,177,32,24,144,25,111,14,111,14]
    for i in range(len(input_flag)):
        char_value = ord(input_flag[i])
        char_value = (char_value * 39) % 196
        if char_value != flag_list[i]:
            print("wrong")
            return

    print("win")

input_flag = input("plz input your flag: ")
check_flag(input_flag)
​

然后再编写逆向代码

result=[]
def find_all_possible_as_given_bcd(b, c, d):
    possible_as = []  
    for a in range(c):
        if (a * b) % c == d:  
            possible_as.append(a)
            for k in range(1, c):  
                new_a = (a + k * c) % c 
                if (new_a * b) % c == d:  
                    possible_as.append(new_a)
    return possible_as
b_value = 39  
c_value = 196    
list=[25,108,108,176,18,108,110,177,64,29,134,29,187,103,32,139,144,179,134,177,32,24,144,25,111,14,111,14]
for i in range(28):
    possible_as = find_all_possible_as_given_bcd(b_value, c_value, list[i])
    result.append(possible_as[0])
    print(result)
​