The Birthday paradox / attack

Authors: Zademn, ireland Reviewed by:

Prerequisites

Probability theory (for the main idea)
Hashes (an application)

Motivation

Breaking a hash function (insert story)

The birthday paradox

*insert story / introduction about why it's called a paradox + use*

Birthday version

Question 1

What is the probability that 1 person has the same birthday as you?

Solution

Let $A$ be the event that someone has the same birthday as you and $\bar{A}$ be the complementary event
- The events are mutually exclusive => $Pr(A) = 1 - Pr(\bar{A})$
Let $E_i$ be the events that person $i$ does not have your birthday

Then

Question 2

Suppose the birthdays are distributed independently and uniformly

Solution

General case

Question 1

Question 2

Code examples

def my_birthday(n, d):
    return 1 - pow((d-1)/d , n)

def same_birthday(n, d):
    p = 1
    for i in range(1, n): #1 -> n-1
        p*=(1-i/d)
    return 1 - p

print(same_birthday(23, 365), same_birthday(32, 365), same_birthday(100, 365))
# (0.5072972343239854, 0.7533475278503207, 0.9999996927510721)

An useful approximation

def approx_same_birthday(n, d):
    return 1 - pow(e, -pow(n, 2) / (2*d)).n()
    
print(approx_same_birthday(23, 365)) 
print(approx_same_birthday(32, 365))
print(approx_same_birthday(100, 365))

# 0.515509538061517
# 0.754077719532824
# 0.999998876014983

Finding a collision

Algorithm

Example:

Consider the following hash function:

import hashlib
import random
from Crypto.Util.number import long_to_bytes, bytes_to_long

def small_hash(m, hash_bits):
    '''
    Arguments
        m: bytes -- input
        hash_bits: int -- number of bits of the hash
    Returns:
        {bytes} - hash of the message of dimension `hash_bits`
        '''
    assert hash_bits > 0, "no negative number of bits"
    
    mask = (1 << hash_bits) - 1 # Mask of bits
    t = hashlib.sha256(m).digest() # the hash in bytes
    t = bytes_to_long(t)
    t = t & mask # get the last 12 bits
    return long_to_bytes(t)

We make the following function to find the hashes:


def small_hash_colision(M_bits, T_bits):
    '''
    Arguments
        M_bits: int -- dimension of the message space
        T_bits: int -- dimension of the hash space
    Returns:
        {(bytes, bytes, bytes)} -- message1, message2, hash
        or
        {(b'', b'', b'')} -- if no collision found
    '''
    N = 1<<T_bits 
    print('Hash size: ', N)
    # This is `s`
    num_samples = 1 * isqrt(N)
    num_samples += num_samples//5 + 1 # num_samples = 1.2 * isqrt(N) + 1
    print(f'Making a list of {num_samples} hashes')
    print(f'Probability of finding a collision is {same_birthday(num_samples, N)}')
    
    m_list = []
    t_list = []
    
    # Make a list of hashes
    for i in range(num_samples):
        m = random.getrandbits(M_bits) # Get random message
        #m = random.randint(0, M_bits-1) 
        m = long_to_bytes(m)
        
        t = small_hash(m, T_bits) # Hash it
        if m not in m_list:
            m_list.append(m)
            t_list.append(t)
            
    # Check for collisionn
    for i in range(len(t_list)):
        for j in range(i+1, len(t_list)):
            if t_list[i] == t_list[j]:
                print('Collision found!')
                return m_list[i], m_list[j], t_list[i]
    else:
        print('Collision not found :(')
        return b"", b"", b""

We use it as follows:

M_bits = 30
T_bits = 20

m1, m2, t = small_hash_colision(M_bits, T_bits)
print(m1, m2, t)
print(small_hash(m1, T_bits) == small_hash(m2, T_bits))

# Hash size:  1048576
# Making a list of 1229 hashes
# Probability of finding a collision is 0.513213460854798
# Collision found!
# b'!\xafB\xc5' b'4F\xb6w' b'\x01y\xf7'
# True

Eliminating Storage Requirements with Pollard's Rho

This is done by choosing the next input based on the hash of the previous input, according to the following sequence:

# TODO:
# have the two hash functions used in this chapter be the same

from Crypto.Hash import SHA256
from Crypto.Util.number import bytes_to_long, long_to_bytes

from tqdm.autonotebook import tqdm

# bits in hash output
n = 30

# H
def my_hash(x):
    x = bytes(x, 'ascii')
    h = SHA256.new()
    h.update(x)
    y = h.digest()
    y = bytes_to_long(y)
    y = y % (1<<n)
    y = int(y)
    return y

# g
def get_message(r):
    x = "Crypto{" + str(r) + "}"
    return x

# f
def f(r):
    return my_hash(get_message(r))

And finally, we can locate the first place in the sequence where the collision occurred, which will let us determine what the colliding inputs to the hash function are. This is done by determining how many iterations apart the colliding inputs are, and then stepping together one iteration at a time until the collision occurs.

This is implemented in the following code snippet.

"""
initialization

This routine will find a hash collision in sqrt time with constant space.
"""

seed = 0

x0 = get_message(seed)
x = x0
y = f(x)

"""
detect collision
using Floyd / tortoise and hare cycle finding algorithm

expected number of iterations is ~ sqrt(pi/2) * 2^(n/2),
we run for up to 4 * 2^(n/2) iterations
"""
for i in tqdm(range(4 << (n//2))):
        
    if f(x) == f(y):
        break
        
    x = f(x)
    
    y = f(y)
    y = f(y)
    

"""
locate collision
"""
x = x0
y = f(y)


for j in tqdm(range(i)):
    if f(x) == f(y):
        break
    
    x = f(x)
    
    y = f(y)
    

m1 = get_message(x)
m2 = get_message(y)

assert my_hash(m1) == f(x)
assert my_hash(m2) == f(y)

print("[+] seeds for messages: {}, {}".format(x, y))
print("[+] messages: {}, {}".format(m1, m2))
print("[+] collided hashes of messages: {}, {}".format(my_hash(m1), my_hash(m2)))


# 31%    40391/131072 [00:03<00:08, 10666.20it/s]
# 30%    12032/40391 [00:00<00:02, 13112.15it/s]
# 
# [+] seeds for messages: 404842900, 254017312
# [+] messages: Crypto{404842900}, Crypto{254017312}
# [+] collided hashes of messages: 1022927209, 1022927209

Resources

PreviousIntroduction / overview NextIntroduction to Isogeny Cryptography

Last updated 4 years ago

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