Clearly, the flag is `IAJBO{ndldie_al_aqk_jjrnsxee}`.

We know, it is supposed to start with `DUCTF`. If you calculate the difference between the ASCII values of `DUCTF` and `IAJBO`, you get:

```
5,6,7,8,9...
```

The shift offset increments by 1.

```py
text = "IAJBO{ndldie_al_aqk_jjrnsxee}"

offset = ord('I') - ord('D')

for i in text.lower():
if not i.isalpha():
print(i, end = '')
else:
print(chr((ord(i) - offset - ord('a')) % 26 + ord('a')), end = '')
offset += 1
```

```
DUCTF{crypto_is_fun_kjqlptzy}
```

Original writeup (https://github.com/csivitu/CTF-Write-ups/tree/master/DUCTF/crypto/rot-i).