def hashfun(msg):
    digest = []
    for i in range(len(msg) - 4):
        digest.append(ord(msg[i]) ^ ord(msg[i + 4]))
    return digest

The function simply does a XOR operation:

a ^ b = c

c ^ a = b

We already know the first 4 characters of flag.

XORing the character at index 0 with the first item in the list gives us the character at index 0+4 and so on.

l = [10, 30, 31, 62, 27, 9, 4, 0, 1, 1, 4, 4, 7, 13, 8, 12, 21, 28, 12, 6, 60]
flag = "CSR{"

for i in range(len(flag)+len(l)-4):
    flag += chr(l[i]^ord(flag[i]))
else:
    print(flag)