+++
author = "Maik de Kruif"
title = "Twisted robot"
subtitle = "Beginners Quest 5 - Google CTF"
date = 2021-09-24T19:40:00+01:00
description = "A writeup for challenge 5 of the beginners quests of the Google CTF."
cover = "img/writeups/google-ctf/2021/beginners-quest/5/cover.png"
tags = [
    "Google CTF",
    "Beginners Quest",
    "ctf",
    "hacking",
    "writeup",
    "misc",
]
categories = [
    "ctf",
    "writeups",
    "hacking",
    "misc",
]
aliases = [
    "5"
]
+++

## Story line

### Istanbul - Bazaar

It’s a hot day, and your skin is cracking and dry. It’s difficult to make your way through the crowded bazaar. A high pitch voice pierces through the soundscape from a salesman that’s trying to sell colorful fabrics and then from another corner comes delicious smells. You spot a hand waving - it’s your contact that you’ve been waiting to meet. "Take a seat, my friend, I’m Gökhan, have you been to Istanbul before? No, really? I’m sure that you will have a great time, I’ve ordered tea for the two of us. Show me the amulet, will you?. Wow, this is really something from my younger days, this is as mysterious as it is beautiful and belongs to “The cloaked brotherhood”. They are very dangerous, and eventhough your quest is urgent, I would advise you to not continue looking for the owner of this. Go home, and forget about it." In the blink of an eye, four tough guys show up, and you start to run together with Gökhan through the crowded marketplace and then up on a rooftop. The tough guys are closing in, but the two of you climb down from the rooftop, run around a corner and are able to hide in two crates.

#### Challenge: Twisted robot (misc)

We found this old robo caller. It basically generates random phone numbers to spam. We found the last list of numbers in generated and also some weird file... Maybe it's got to do with these new beta features they were testing?

### After solving

Gökhan is pointing at a parked vehicle. He tells you that you will have to try and reach it and that if you stay where you are, that you will get captured sooner or later. The guards know the neighborhood like their own backpocket. At first you doubt the plan, it seems like a very risky option. Gökhan then finally tells you he's not going to stay there, and his last offer is for you to go with him.

#### Stay ([7]({{< ref "7.md" >}}))

Gökhan exits the crate, and makes a quick sprint for the car. The tough guys spot him, and they approach. As he enters the car he tries to start it, and the car makes an ominous sound, as the bad guys are closing in. He looks back through the rear window, and sees that the bad guys are about to jump on the back of the car, and they are pulling out guns. He tries to start the car furiously one more time and... IT WORKS! Gökhan disappears off in the distance. You overhear the tough guys when they are talking about a person, “Mesut”, that got classified information. You quickly send a message to the boss to look up the person. After a short wait you receive a response that he is currently on his private yacht in Croatia. Looks like it is time for some travel again.

#### Come with Gökhan ([8]({{< ref "8.md" >}}))

As you and Gökhan are leaving the crates to enter a car, you spot the tough guys coming after you, and they are pulling out weapons. Gökhan starts the car and the two of you take off. After a decent distance outside of the city, he gives you an invitation to a private conference that will take place in Buenos Aires.

## Attachment

[attachment.zip](/files/writeups/google-ctf/2021/beginners-quest/5/attachment.zip)

{{< code language="py" title="RoboCaller1337.py" isCollapsed="true" >}}

```py
import random

# Gots to get that formatting right when send it to our call center
def formatNumber(n):
    n = str(n)
    return f'{n[:3]}-{n[3:6]}-{n[6:]}'

# This generates random phone numbers because it's easy to find a lot of people!
# Our number generator is not great so we had to hack it a bit to make sure we can
# reach folks in Philly (area code 215)
def generateRandomNumbers():
    arr = []
    for i in range(624):
        arr.append(formatNumber(random.getrandbits(32) + (1<<31)))
    return arr

def encodeSecret(s):
    key = [random.getrandbits(8) for i in range(len(s))]
    return bytes([a^b for a,b in zip(key,list(s.encode()))])


def menu():
    print("""\n\nWelcome to the RoboCaller!! What would you like to do?
1: generate a new list of numbers
2: encrypt a super secret (in beta)
3: decrypt a super secret (coming soon!!)
4: exit""")
    choice = ''
    while choice not in ['1','2','3']:
        choice = input('>')
        if choice == '1':
            open('robo_numbers_list.txt','w').write('\n'.join(generateRandomNumbers()))
            print("...done! list saved under 'robo_numbers_list.txt'")
        elif choice == '2':
            secret = input('give me your secret and I\'ll save it as "secret.enc"')
            open('secret.enc','wb').write(encodeSecret(secret))
        elif choice == '3':
            print("stay tuned for this awesome feature\n\n")
        elif choice == '4':
            print("Thank you for using RoboCaller1337!")
    return

def main():
    while True:
        menu()

if __name__ == "__main__":
    main()
```

{{< /code >}}

{{< code language="txt" title="robo_numbers_list.txt" isCollapsed="true" >}}

```txt
263-170-6234
467-553-7030
220-146-1293
630-328-6023
413-553-0465
528-403-6609
454-641-6157
396-906-1900
445-477-6643
578-095-2052
216-516-7520
314-161-3710
628-223-8174
512-530-6992
464-456-5861
631-453-4232
377-004-3494
328-898-2557
231-249-9423
420-346-3494
605-091-2725
595-329-3910
576-809-4557
425-030-9549
389-181-5710
621-627-6043
423-199-4297
579-089-8585
589-493-0846
375-597-0939
541-648-2941
453-069-7058
543-829-0828
417-111-8475
348-244-2759
290-882-6713
522-462-5631
296-565-4379
586-916-6837
603-365-6384
476-088-1715
526-794-4364
570-480-9107
427-082-3513
418-582-5172
294-148-5624
591-766-9391
581-588-1423
554-956-1539
339-448-0163
633-317-6710
529-345-6802
510-151-9942
589-325-3637
291-288-6435
536-342-5711
431-379-8656
306-181-2272
287-750-2264
435-289-4194
609-844-8168
254-346-9938
477-230-3596
224-779-5283
351-591-9900
472-433-7441
259-370-4161
390-390-2128
271-570-0282
532-522-3403
231-987-9206
273-558-2616
232-775-4473
286-546-5820
567-085-6134
303-874-3098
517-707-7333
439-636-1845
641-978-4477
221-606-7704
356-456-0141
621-732-2415
569-522-9393
404-995-4514
573-986-7745
407-677-1104
225-688-0071
219-031-4274
355-266-3258
624-119-4589
313-157-7824
518-033-0420
462-462-1679
435-580-6776
232-015-6685
585-746-5975
359-937-2568
426-271-0216
618-678-9397
318-533-9973
560-054-9472
251-899-2041
295-475-7765
590-304-8246
535-896-4676
538-266-6918
282-578-1932
621-085-1982
471-750-3026
605-061-1357
491-478-9362
531-518-2579
542-766-2906
391-955-0814
271-424-2279
429-855-1832
598-055-6110
429-626-4468
559-114-4811
582-184-7627
499-756-4897
412-170-2198
551-289-1076
397-901-9532
367-175-9646
418-401-3012
223-706-1395
556-314-7884
427-512-9355
441-717-0237
586-758-7240
639-239-7961
563-296-1195
532-034-4947
493-797-3707
240-611-7275
286-614-0798
421-788-3182
292-386-6585
216-386-1373
594-193-2268
337-029-6841
444-662-2067
375-664-3202
293-053-1463
284-235-3914
453-792-3489
266-783-6052
336-422-4671
460-035-3225
470-633-3867
234-048-5223
479-812-5684
606-457-0299
604-025-5219
584-232-4973
534-831-8849
488-941-7239
569-682-8387
250-288-5751
548-842-8852
537-471-2881
473-732-5683
600-901-1360
244-545-5850
611-587-7286
493-004-5626
541-097-0083
318-276-8582
245-121-2701
253-657-5692
587-739-7213
346-259-4605
358-583-6937
636-360-3508
335-700-9223
443-401-2276
580-846-2364
414-725-9375
528-421-5892
606-581-0093
549-745-9361
286-689-9360
218-754-0615
321-168-4565
392-771-7530
292-211-8722
338-038-3332
583-603-7610
472-413-9015
510-699-6253
327-105-5061
347-747-7710
532-690-4353
307-141-9393
614-124-9061
431-024-6717
574-101-4075
632-578-6542
277-867-6340
324-715-7328
374-913-2900
221-589-8179
229-100-8677
306-006-3850
572-818-6626
477-129-5073
568-463-6658
536-661-1868
307-094-0858
248-972-7972
507-465-6313
592-140-8643
635-756-2115
231-603-0653
310-669-1732
577-741-6941
327-413-3011
484-314-6788
398-283-4866
333-140-1452
372-012-4273
265-303-9517
389-626-2935
227-237-4448
302-214-1402
257-043-3569
339-178-6072
467-085-9034
370-830-3049
527-417-1646
360-260-6692
418-021-9682
224-191-9935
475-252-8920
258-028-1912
485-558-4912
475-641-7490
312-575-1076
221-080-0996
565-758-4057
606-410-6813
380-364-3878
454-647-2580
558-276-5241
454-930-4133
425-279-0788
634-055-1448
387-308-6704
297-447-2024
325-867-9896
366-669-5395
370-728-9888
515-812-6899
290-897-7319
608-784-7731
306-673-3231
248-002-8127
556-900-1349
558-045-6325
330-120-0416
503-768-1631
315-471-5872
287-925-3595
597-434-5185
500-420-1998
276-255-9286
260-165-4499
634-488-7067
550-656-9222
364-996-1908
393-231-0222
314-836-5961
278-146-7285
258-469-2984
458-789-6127
376-740-8946
244-618-6086
587-168-0361
275-523-6075
605-784-9728
377-514-5609
263-776-1318
609-187-9670
243-602-9787
253-775-4101
408-681-2234
406-675-5201
496-579-4417
566-496-2682
503-553-5948
439-293-2040
383-486-6162
323-283-9234
305-479-8863
478-975-6735
354-724-0291
330-167-6218
237-014-6927
332-972-5118
336-818-8016
232-442-6385
381-560-9510
491-498-2317
491-354-6757
492-988-1457
504-965-5273
623-178-2982
634-591-5132
331-525-1342
404-793-5431
319-217-4107
426-481-1776
586-057-4203
545-939-6860
222-490-9288
265-820-2560
214-953-0167
643-906-2918
240-098-8109
234-229-5525
520-069-2571
333-323-9940
574-581-5631
620-982-0443
376-752-7970
491-787-0757
613-684-5525
440-860-3533
256-223-7638
637-243-1657
216-758-4109
603-205-8094
641-246-9999
444-144-1846
355-714-3959
638-675-0199
377-418-0369
586-516-3786
466-160-7684
585-714-3750
249-234-7101
515-494-7889
595-840-9246
566-590-0137
358-175-7059
597-287-7017
287-942-8287
442-059-3333
575-505-6136
582-311-6644
619-262-5832
641-788-3280
500-425-4419
218-860-8515
584-013-1982
635-304-8233
413-023-9360
479-795-4224
617-013-8823
417-989-2352
599-480-4023
240-440-2193
299-079-3077
486-349-6987
520-519-6531
606-723-6888
604-485-3290
397-591-6728
222-732-4786
579-723-9610
536-385-6396
328-207-1008
312-228-8345
644-212-3015
470-159-3454
224-910-8039
390-956-1685
335-913-5456
347-509-5657
627-631-4607
299-531-9650
460-460-4197
295-431-0008
318-347-7116
361-830-5578
296-014-3473
592-647-0418
255-674-6356
274-042-5312
325-171-2177
477-736-2939
431-571-1160
305-047-9411
630-334-9642
381-695-7832
298-958-0065
625-500-4706
529-536-9523
383-823-4781
414-338-6319
264-259-0104
478-344-9117
377-027-4085
511-706-8104
613-897-8628
473-831-4406
356-775-0560
332-288-4564
352-970-1774
292-376-4341
319-525-3358
312-514-8535
320-070-0911
290-445-6791
498-406-2432
571-079-8429
276-991-8864
481-943-3126
300-932-1931
546-824-1119
246-372-8126
457-455-4498
288-703-2121
564-818-7304
385-921-6819
629-576-4994
445-109-7495
234-199-7440
632-582-4939
502-998-8220
447-225-4830
498-776-9758
512-819-2534
593-249-1838
346-730-7389
387-061-3438
612-634-6490
304-590-7107
336-236-8058
280-109-9842
532-456-4518
494-312-1472
576-075-3393
502-770-3549
314-251-4866
395-081-9884
362-320-4067
550-221-6228
578-292-6373
560-076-7938
376-384-4159
408-610-4527
407-474-3128
504-027-5065
401-434-6766
511-795-6354
244-790-9831
370-882-6561
464-032-5501
335-056-6491
535-523-9560
563-151-5751
310-650-9154
293-427-5198
227-854-7364
298-835-7781
273-952-9012
372-909-4330
598-402-8660
243-015-8194
383-775-3972
580-873-1801
603-389-4616
435-894-4631
221-659-0436
421-068-3372
576-550-9159
600-740-4753
304-667-9863
398-463-0613
228-119-7649
632-929-1125
418-318-7960
481-314-3005
220-193-6628
376-247-2252
230-189-8583
443-754-1310
307-629-5088
443-234-5273
604-417-4882
371-452-6491
244-192-9948
436-547-5170
255-052-6193
589-472-9545
422-443-9210
471-193-0096
326-900-8982
534-113-4669
370-370-7079
236-477-6842
582-225-8431
330-375-2592
258-253-0724
352-635-3974
277-093-0733
276-827-6629
399-408-4024
299-332-2510
369-955-9431
270-411-8250
505-556-4268
509-193-1267
390-556-9212
545-455-8494
621-160-7841
400-314-8126
301-366-4666
566-601-5258
497-181-3783
475-458-5134
231-329-9853
643-929-0930
467-709-3820
476-757-4475
220-068-1085
306-235-7886
600-225-3007
463-008-1308
542-224-9895
490-906-7623
439-771-8632
528-612-0493
527-246-8898
358-761-7567
555-591-5964
340-723-1623
616-706-2286
479-042-1523
567-628-3190
237-335-5616
342-661-9978
511-003-2993
365-015-3536
579-912-9244
277-699-2447
572-716-1892
432-103-7130
500-710-9000
244-105-1867
395-361-9492
531-841-0316
576-664-6128
363-937-1524
570-451-8880
407-256-2659
550-243-2375
629-676-6662
238-241-4419
595-426-7736
605-009-1897
268-368-8879
276-606-9527
351-839-4116
479-653-9492
549-268-4096
223-918-1181
311-654-3871
464-091-8325
507-686-1330
326-205-4956
438-871-2036
611-564-1201
324-442-1210
547-150-7680
237-378-9286
365-038-6522
262-981-4774
414-149-0176
220-121-1826
423-447-3437
487-112-2749
224-428-8478
251-934-9033
274-700-5893
469-421-0585
344-241-4166
401-960-8975
261-506-6354
589-256-3123
305-932-3116
418-063-9114
600-969-4231
363-050-1450
627-535-7555
487-543-9495
461-161-1043
512-384-1050
234-199-2812
541-513-3632
630-049-9049
585-881-3200
460-978-5106
635-887-0620
298-611-3587
272-932-0243
554-772-3962
618-711-7118
426-926-4284
596-851-6696
306-766-1867
445-639-1594
467-786-0153
515-688-5843
291-664-2547
295-859-0241
605-833-5133
476-067-3223
356-715-8717
625-145-6351
439-027-8936
446-996-9104
```

{{< /code >}}

## Recon

The attachment contains tree files: `RoboCaller1337.py`, `secret.enc` and `robo_numbers_list.txt`.

Looking at the python script, we can assume the other two files are generated with it. It contains a function `generateRandomNumbers()`, which generates 624 random phone numbers. It also has a function `encodeSecret()`, which encodes a key with a random number.

`robo_numbers_list.txt` looks like a big list with 624 phone numbers.

`secret.enc` appears to contain just raw bytes.

## Solving

When starting on this challenge I was actually clueless how to solve this. I though the key was random. But then I remembered why you don't use the default random methods. They're not actually random, but rather pseudorandom.

Pseudorandom numbers are generated using an algorithm that generates a sequence of numbers whose properties approximate the properties of sequences of random numbers. This sequence is determined by an initial value, called a seed. Although this seed could be actually random, once you know it, you can calculate all the values it would output.

From reading [the python documentation of `random.py`](https://github.com/python/cpython/blob/main/Lib/random.py), we can see it uses the Mersenne Twister algorithm with a period of 2^19937-1. Also known as MT19937.

When searching for an algorithm to predict its results, I came across [this library](https://github.com/kmyk/mersenne-twister-predictor). It seems that if you know the preceding 624 numbers, you can predict the next numbers.

So, let's install the package first:

```sh
pip install mersenne-twister-predictor
```

Then, we need a script to read the numbers and enter them in the predictor. Let's start out by creating an instance of the predictor:

```py
from mt19937predictor import MT19937Predictor

predictor = MT19937Predictor()
```

Then we need the numbers to enter. To get them, I reversed the algorithm in the `RoboCaller1337.py` file:

```py
with open("robo_numbers_list.txt", "r") as file:
    for line in file:
        number = int(line.replace("-", "")) - (1<<31)
        predictor.setrandbits(number, 32)
```

Now that the numbers are added, we can read the `secret.enc` file, and try to decode it. Again, I reversed the algorithm in the original file.

```py
with open("secret.enc", "rb") as file:
    secret = list(file.read())

flag= ""
key = [predictor.getrandbits(8) for _ in range(len(secret))]
for a,b in zip(key, secret):
    flag += chr(a^b)

print(flag)
```

{{< code language="py" title="Full code" isCollapsed="true" >}}

```py
from mt19937predictor import MT19937Predictor

predictor = MT19937Predictor()

with open("robo_numbers_list.txt", "r") as file:
    for line in file:
        number = int(line.replace("-", "")) - (1<<31)
        predictor.setrandbits(number, 32)

with open("secret.enc", "rb") as file:
    secret = list(file.read())

flag= ""
key = [predictor.getrandbits(8) for _ in range(len(secret))]
for a,b in zip(key, secret):
    flag += chr(a^b)

print(flag)
```

{{< /code >}}

## Solution

After executing this code, we get the flag! It's `CTF{n3v3r_3ver_ev3r_use_r4nd0m}`.