Tags: hash-collision crypto

Rating:

# chaos
## Description

Whats the fun rolling up a hash function if its not chaotic enough?

## Files
- [chaos.tar.gz](chaos.tar.gz)

Extract the file, see a challenge.py:
py
from secret import flag
def ROTL(value, bits, size=32):
return ((value % (1 << (size - bits))) << bits) | (value >> (size - bits))

def ROTR(value, bits, size=32):
return ((value % (1 << bits)) << (size - bits)) | (value >> bits)

pt+=b'\x80'
L = len(pt)
to_pad = 60-(L%64) if L%64 <= 60 else 124-(L%64)

def hash(text:bytes):
text = [int.from_bytes(text[i:i+4],'big') for i in range(0,len(text),4)]
M = 0xffff
x,y,z,u = 0x0124fdce, 0x89ab57ea, 0xba89370a, 0xfedc45ef
A,B,C,D = 0x401ab257, 0xb7cd34e1, 0x76b3a27c, 0xf13c3adf
RV1,RV2,RV3,RV4 = 0xe12f23cd, 0xc5ab6789, 0xf1234567, 0x9a8bc7ef
for i in range(0,len(text),4):
X,Y,Z,U = text[i]^x,text[i+1]^y,text[i+2]^z,text[i+3]^u
RV1 ^= (x := (X&0xffff)*(M - (Y>>16)) ^ ROTL(Z,1) ^ ROTR(U,1) ^ A)
RV2 ^= (y := (Y&0xffff)*(M - (Z>>16)) ^ ROTL(U,2) ^ ROTR(X,2) ^ B)
RV3 ^= (z := (Z&0xffff)*(M - (U>>16)) ^ ROTL(X,3) ^ ROTR(Y,3) ^ C)
RV4 ^= (u := (U&0xffff)*(M - (X>>16)) ^ ROTL(Y,4) ^ ROTR(Z,4) ^ D)
for i in range(4):
RV1 ^= (x := (X&0xffff)*(M - (Y>>16)) ^ ROTL(Z,1) ^ ROTR(U,1) ^ A)
RV2 ^= (y := (Y&0xffff)*(M - (Z>>16)) ^ ROTL(U,2) ^ ROTR(X,2) ^ B)
RV3 ^= (z := (Z&0xffff)*(M - (U>>16)) ^ ROTL(X,3) ^ ROTR(Y,3) ^ C)
RV4 ^= (u := (U&0xffff)*(M - (X>>16)) ^ ROTL(Y,4) ^ ROTR(Z,4) ^ D)
return int.to_bytes( (RV1<<96)|(RV2<<64)|(RV3<<32)|RV4 ,16,'big')

try:
m1 = bytes.fromhex(input("input first string to hash : "))
m2 = bytes.fromhex(input("input second string to hash : "))
if m1!=m2 and hash(m1)==hash(m2):
print(flag)
else:
print('Never gonna give you up')
except:
print('Never gonna let you down')


Looks like we need to find a **hash collision** for this hash function

So I do some research (Googling) found this [article](https://littlemaninmyhead.wordpress.com/2015/09/28/so-you-want-to-learn-to-break-ciphers/) when I search for hash 0x0124fdce

Actually it was an existing hash function - **Chaotic Hash Function**

In his [research paper](https://eprint.iacr.org/2005/403.pdf) stated some hash collision

I used the two shortest hash to solve:

nc crypto.zh3r0.cf 2222
input first string to hash : fedb02317654a8154576c8f50123ba10bfe54da84832cb1e894c5d830ec3c520
input second string to hash : 0124fdce89ab57eaba89370afedc45ef401ab257b7cd34e176b3a27cf13c3adf
b'zh3r0{something_chaotic_may_look_random_enough_but_may_be_not_sufficiently_secure} ,courtsey crazy contini : https://littlemaninmyhead.wordpress.com/2015/09/28/so-you-want-to-learn-to-break-ciphers/'

That's it! Easy challenge!

## Flag

zh3r0{something_chaotic_may_look_random_enough_but_may_be_not_sufficiently_secure}


Original writeup (https://github.com/Hong5489/zh3r0-ctf/tree/main/chaos).