grocid / ctf

🏴 Collection of CTF solutions

Collection of CTF solutions

This is an index of the CTF solutions I have published on my blog. Most of them are complete.

0CTF

2016

Equation (2 points)
RSA? (2 points)

BackdoorCTF

2016

Collision course (350 points)
Baby (200 points)

BCTF

2016

Hyper RSA (500 points)

BKPCTF

2016

HMAC_CRC (5 points)

DefconCTF

2016

b3s23

IceCTF

2016

A few collected solutions

Internetwache

2016

Crypto 60 (60 points)
Crypto 50 (50 points)

LabyREnth

2016

Threat tier

Nuit du Hack

2016

Toil33t (400 points)

PlaidCTF

2016

Tonerre (200 points)

2015

Curious (70 points)
Strength (110 points)

SCTF

2016

Space Friends (200 points)
Schensted (110 points)
Ed25519 (40 points)

TU CTF

2016

Secure Auth (150 points)
Hash'n'bake (200 points)
Pet Padding Inc. (150 points)

TWCTF

2016

Palindrome (30 points)
Twin Primes (50 points)
Super Express (100 points)
Pinhole Attack (500 points)

VolgaCTF

2016

XXY (300 points)
Lazy (250 points)

CTF problems I have created

Midnightsun 2019 Quals

tulpan257
open-gyckel-krypto
EZDSA

Midnightsun 2018 Quals

snurre80
Hm4c

Midnightsun 2018 Finals

snurre128
badchair
zilch-konstigheter

SEC-T 2018

Matsign
Marty0ska1 (broken)
gsh

SEC-T 2017

qproximity
madlog

ctf's Issues

[IceCTF 2016] bad decrypt - Attack of the Hellman!

Hi~
I use

openssl aes-256-cbc -a -d -in flag.enc -out flag.txt
according to your writeup and use the key, but what I get is bad decrypt.
I don't know why. Could you help me?

LabyREnth threat4 (yara1)

Lool, really really lucky.

The intended solution was that you again have appended data in each file, 3 different in 6 files. After messing with it a bit you get XOR key 0x26 and decrypted texts:

icbx2A/Ozsb7if/bu-e62BvGzsbO1Mb-zxr/yRq6ywLGihz72A/73g/J1Bm61Qy63g7D2Mb/1AnJzgv-igr73geIdqDv2Q/IzMbO0gu63gvH2gL73gu6yAvG1R262AvK1gf9zsb/ywnCiciEk8i62gfD28bR0xrCihrCzsbA0xjN3caLm8b84xr/2NCnc6Ok2BvGzsb/1An5zgX9idC6zwT9xQrJyKOk4KOkica6ica6icbN3hjD1A3No6Okcq-nc6-jcsrA0xjN3caXihG6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C65qOkcq-jjhn/yQXIzcaXihG6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C65qOkcq-jjhrC0xj-idO64MaEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8bXdqCjcq-jdqC6ica6ica6ignJ1ArD3g/J19Cnc6-jcte61Qy63g7/1qOk5qaaROTXOR\x00\x00\r\n
vRjD3gu6ysbzqvjbihjP1gu63gW6zgvOzwnOigfG1cbQyxjDyxrD1QTNigXAihrC0xm6zwT91Qr/zcb-yxr7l6OkvxnD1A263g7/ihr/1xbGyxr/igj/1gXRihj/2gL7yQu6zwf90ca8k8C8ihb70xi63Q/O0cbO0gu6zA/M2Rq6mti6yB/OzxmUdqCncBjP1gu6zwT9xQrJyMaUigvIyPX-1QmncBGnc8a6ica6ica62RrM0wTB2NCnc6-jdqCjcq--zA/M2Rq6psbVicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEihOnc6-jcsrNzwnJ1Aq6psbVicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEihOnc6-jcsrO0g/MzcaXihG6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C6k8C65qOkcq-jcqOkica6ica6icb91QT-0xrD1QSUdqCjcq-LigXAihrCzwOncBOaROTXOR\x00\x00\r\n
if3M0xr/ige6wufsqsbM3wL/ihrJigr/3gv93cb71gK63AfM0wfO0wXI2MbJz8bO0g/NigvIyQX-zwq6zgfOysSnc/vN0wTBihrCzsbOzwPK1gfOzsb8zwLJ3MbMzxbGywn/igv7yQ66i8CEi8bKyw/Mih3D3g663g7/igzD2BnOideMigjT3gvNo6OkdqDM3wL/igvIyPX-1Qm6o8b/1An5zgX9dqDVdqC6ica6ica6ihnO2A/IzRmUdqCjcqOkcq-jjgzD2BnOidO64MaEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8bXdqCjcq--2Qv91QT-idO64MaEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8aEk8bXdqCjcq--3g7D2Aq6psbVicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEicCEihOnc6-jcq-nc8a6ica6ica6yQXIzg/O0wXIo6Okcq-jmsbJz8bO0gvHdqDXROTXOR\x00\x00\r\n

Next, it looks like base64 but decrypts to nonsense. There is ROTXOR in end of each text, after a waaay long thinking and trying you get that you actually need to rotate base64 alphabet by XOR key (also need to replace + with -). Code:

print(base64.b64decode(enc.replace('-', '+').translate(str.maketrans(alph, alph[0x26:] + alph[:0x26]))).decode())

gets you this text:

Write a YARA rule to detect all variations of this encoded data.
Using the template below replace each "**" pair with the first 12 bytes:

rule enc_doc : enc_doc
{
        strings:

                        $first = { ** ** ** ** ** ** ** ** ** ** ** ** }
                        $second = { ** ** ** ** ** ** ** ** ** ** ** ** }
                        $third = { ** ** ** ** ** ** ** ** ** ** ** ** }

        condition:
                        1 of them
}

So after filling the rule like this:

rule enc_doc : enc_doc
{
        strings:

                        $first = { 50 74 4C 62 15 41 53 10 5F 55 44 5C }
                        $second = { 4F 40 15 6B 16 5E 54 09 4F 41 43 10 }
                        $third = { 4F 45 44 5E 14 67 09 69 5C 55 44 11 }

        condition:
                        1 of them
}

you get the answer.

After all, your solution is horribly wrong and still you got the answer, very lucky :) And the challenge itself is broken very hard, my rule didn't work when I swapped two strings in it.