Rating:
# Rotor Rooter — Writeup
- Category: Crypto
- Value: 884 pts (117 solves)
- Author: elemental
## Challenge
```text
Spin it till it drains
KLEGCKRGGONTBNBVPIIZWXQQEZYAXXWQMGIZDNEWWUTOVZRWOMZKGWNKWZBQXOGZSTVCGU
FLAG FORMAT: CIT{underscore_between_each_word}
```
70 characters, A–Z only. "Rotor Rooter" + "spin" → Enigma-class rotor cipher.
## Recon
Index of coincidence rules out short-period polyalphabetic (Vigenère):
```text
period 1: 0.046 period 9: 0.060 period 13: 0.069
period 2: 0.048 period 10: 0.052 period 14: 0.036
period 3: 0.039 period 11: 0.052 period 15: 0.027
```
No clean peak at 5/6/7 (the popular Vigenère key lengths). Letter frequency
is suspiciously flat — `G`, `Z`, `W` tied at 10% each, `Z` impossible-high
for English. Consistent with Enigma. Default unknowns to brute force:
rotor ordering (60), reflector (B/C), and starting position (17576).
## Solution
Two stages.
### Stage 1 — find the rotor + start, ring = AAA
Brute Enigma M3 over **rotors ∈ Perm(I…V, 3)**, **reflector ∈ {B, C}**,
**start ∈ A…Z³** with **ring AAA** and empty plugboard. Rank candidates by
English letter-frequency + bigram score (no `CIT` bonus — it's only 70 chars,
random "CIT" substrings dominate honest scoring).
Top hit (score 266.7):
```text
rotors=(I, II, III) refl=B start=AAA
WECANONLYSEEASHORTDNHTANCEAHEADBUTWECANSEEPLEIGYTHERETHATNEEDSTOBEDONE
```
That's Turing's quote with **4 character errors** at positions 19, 20, 45, 46:
```text
expected: ...WECANONLYSEEASHORTD I S TANCE...PLE N T YTHERE...
got: ...WECANONLYSEEASHORTD N H TANCE...PLE I G YTHERE...
```
Errors cluster, suggesting the *ring* (not plugboard) is off — a plugboard
swap would scatter mismatches across the whole text wherever those letters
appeared, but here they're concentrated in two windows.
### Stage 2 — recover the ring with a crib
Plug in Turing's quote (`WECANONLY…NEEDSTOBEDONE`) as a known-plaintext crib
and sweep the right-rotor ring + all start positions. Exact match at:
```text
rotors=(I, II, III) refl=B ring=AAC start=AAC plugboard=∅
plaintext: WECANONLYSEEASHORTDISTANCEAHEADBUTWECANSEEPLENTYTHERETHATNEEDSTOBEDONE
```
The "spin it till it drains" hint is literal: ring **C** = position 2, two
clicks past `A`. The pun "Roto-Rooter" lands on **rotors I, II, III** — the
default wartime starter set.
Word-segment, lowercase, underscore-join:
```text
WE CAN ONLY SEE A SHORT DISTANCE AHEAD BUT WE CAN SEE PLENTY THERE THAT NEEDS TO BE DONE
```
## Flag
```text
CIT{we_can_only_see_a_short_distance_ahead_but_we_can_see_plenty_there_that_needs_to_be_done}
```
(Quote: Alan Turing, *Computing Machinery and Intelligence*, 1950.)
## Files
- [scripts/solve.py](scripts/solve.py) — direct decrypt with the recovered key
- [scripts/bruteforce.py](scripts/bruteforce.py) — full discovery pipeline (~3 min)
- [solution/flag.txt](solution/flag.txt) — recorded submission
