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 AAA
 be the event that someone has the same birthday as you and Aˉ\bar{A}Aˉ
 be the complementary event
The events are mutually exclusive => Pr(A)=1−Pr(Aˉ)Pr(A) = 1 - Pr(\bar{A})Pr(A)=1−Pr(Aˉ)
​
Let EiE_iEi​
 be the events that person iii
 does not have your birthday
Then
​Pr(A)=1−Pr(Aˉ)=1−∏i=1nPr(Ei)=1−(364365)nPr(A) = 1 - Pr(\bar{A}) = 1 - \prod_{i=1}^n Pr(E_i) = 1 - \left( \dfrac {364} {365}\right)^nPr(A)=1−Pr(Aˉ)=1−∏i=1n​Pr(Ei​)=1−(365364​)n
​
Reminder: Pr(A,B)=Pr(A)⋅Pr(B)Pr(A, B) = Pr(A) \cdot Pr(B)Pr(A,B)=Pr(A)⋅Pr(B)
if A,BA, BA,B
are independent events
Question 2
What is the probability that 2 out of nnn
 people in a room share the same birthday?
Suppose the birthdays are distributed independently and uniformly
Solution
Let AAA
 be the event that 2 people have the same birthday, let Aˉ\bar{A}Aˉ
 be the complementary event (no 2 people have the same birthday)
Event 1 = Person 1 is born => Pr(E1)=365365Pr(E_1) = \dfrac {365} {365}Pr(E1​)=365365​
​
Event 2 = Person 2 is born on a different day than Person 1 => Pr(E2)=364365Pr(E_2) = \dfrac {364} {365}Pr(E2​)=365364​
  
​⋮\vdots⋮
​
Event n = Person n is born on a different day than Person   1,...,n−1⇒1,...,n-1 \Rightarrow1,...,n−1⇒
 ⇒Pr(En)=365−(n−1)365\Rightarrow Pr(E_n) = \dfrac {365-(n-1)} {365}⇒Pr(En​)=365365−(n−1)​
​
​Pr(Aˉ)=Pr(E1)⋅Pr(E2)⋅⋯⋅Pr(En)=365365⋅364365⋅⋯⋅365−(n−1)365=(1365)n⋅365!(365−n)!=∏i=1n−1(1−i365)Pr(\bar{A}) = Pr(E1) \cdot Pr(E_2) \cdot \dots \cdot Pr(E_n) = \dfrac {365} {365} \cdot \dfrac {364} {365} \cdot \dots \cdot \dfrac {365-(n-1)} {365} = \left( \dfrac {1} {365} \right) ^{n} \cdot \dfrac {365!} {(365-n)!} = \prod_{i=1}^{n-1} \left(1 - \dfrac i {365}\right)Pr(Aˉ)=Pr(E1)⋅Pr(E2​)⋅⋯⋅Pr(En​)=365365​⋅365364​⋅⋯⋅365365−(n−1)​=(3651​)n⋅(365−n)!365!​=∏i=1n−1​(1−365i​)
​
General case
Question 1
Instead of 365365365
 days we have d⇒1−(d−1d)nd \Rightarrow \boxed{1 - \left( \dfrac {d-1} {d}\right)^n}d⇒1−(dd−1​)n​
​
Question 2
Instead of 365365365
 days we have d⇒1−∏i=1n−1(1−id)d \Rightarrow \boxed{1 - \prod_{i=1}^{n-1} \left(1 - \dfrac i {d}\right)}d⇒1−i=1∏n−1​(1−di​)​
​
Code examples
1
def my_birthday(n, d):
2
    return 1 - pow((d-1)/d , n)
3
​
4
def same_birthday(n, d):
5
    p = 1
6
    for i in range(1, n): #1 -> n-1
7
        p*=(1-i/d)
8
    return 1 - p
9
​
10
print(same_birthday(23, 365), same_birthday(32, 365), same_birthday(100, 365))
11
# (0.5072972343239854, 0.7533475278503207, 0.9999996927510721)
Copied!
 An useful approximation
From the Taylor approximation we know ex=1+x+x22!+⋯=>ex≈1+xe^x = 1 + x + \dfrac {x^2} {2!} + \dots => e_x\approx 1 + xex=1+x+2!x2​+⋯=>ex​≈1+x
 for x≪1x \ll 1x≪1

Apply for each event: ⇒x=−ad⇒e−a/d≈1−ad⇒Pr(A)=1−∏i=1n−1e−i/d=1−e−n(n−1)/2d≈1−e−n2/2d\Rightarrow x = -\dfrac a d \Rightarrow  e^{ -a /d} \approx 1- \dfrac a d \Rightarrow  Pr(A) = 1 - \prod_{i=1}^{n-1}e^{-i/d} = 1-e^{-n(n-1) /{2d}} \approx 1-\boxed{e^{-{n^2} / {2d}}}⇒x=−da​⇒e−a/d≈1−da​⇒Pr(A)=1−∏i=1n−1​e−i/d=1−e−n(n−1)/2d≈1−e−n2/2d​
​
If we want to solve for nnn
 knowing Pr(A)Pr(A)Pr(A)
 we take the ln⁡\lnln
 => n≈2dln⁡(11−Pr(A))\boxed{n \approx \sqrt{2d \ln \left(\dfrac 1 {1-Pr(A)}\right)}}n≈2dln(1−Pr(A)1​)​​
​
1
def approx_same_birthday(n, d):
2
    return 1 - pow(e, -pow(n, 2) / (2*d)).n()
3
    
4
print(approx_same_birthday(23, 365)) 
5
print(approx_same_birthday(32, 365))
6
print(approx_same_birthday(100, 365))
7
​
8
# 0.515509538061517
9
# 0.754077719532824
10
# 0.999998876014983
Copied!
Finding a collision
Let H:M⟶TH:\mathcal{M} \longrightarrow \mathcal{T}H:M⟶T
 be a hash function with ∣M∣>>∣T∣|\mathcal{M}| >> |T|∣M∣>>∣T∣
​
Let's denote N=∣T∣N = |\mathcal{T}|N=∣T∣
​
Algorithm 
1. Choose s≈Ns \approx \sqrt{N}s≈N​
 random distinct messages in M\mathcal{M}M
 
2. Compute ti=H(mi)t_i = H(m_i)ti​=H(mi​)
 for 1≤i≤N1\leq i \leq \sqrt{N}1≤i≤N​
   
3. Look for (ti=tj)→(t_i = t_j) \to(ti​=tj​)→
  If not found go to step 1
Example: 
Consider the following hash function: 
1
import hashlib
2
import random
3
from Crypto.Util.number import long_to_bytes, bytes_to_long
4
​
5
def small_hash(m, hash_bits):
6
    '''
7
    Arguments
8
        m: bytes -- input
9
        hash_bits: int -- number of bits of the hash
10
    Returns:
11
        {bytes} - hash of the message of dimension `hash_bits`
12
        '''
13
    assert hash_bits > 0, "no negative number of bits"
14
    
15
    mask = (1 << hash_bits) - 1 # Mask of bits
16
    t = hashlib.sha256(m).digest() # the hash in bytes
17
    t = bytes_to_long(t)
18
    t = t & mask # get the last 12 bits
19
    return long_to_bytes(t)
Copied!
We make the following function to find the hashes:
1
​
2
def small_hash_colision(M_bits, T_bits):
3
    '''
4
    Arguments
5
        M_bits: int -- dimension of the message space
6
        T_bits: int -- dimension of the hash space
7
    Returns:
8
        {(bytes, bytes, bytes)} -- message1, message2, hash
9
        or
10
        {(b'', b'', b'')} -- if no collision found
11
    '''
12
    N = 1<<T_bits 
13
    print('Hash size: ', N)
14
    # This is `s`
15
    num_samples = 1 * isqrt(N)
16
    num_samples += num_samples//5 + 1 # num_samples = 1.2 * isqrt(N) + 1
17
    print(f'Making a list of {num_samples} hashes')
18
    print(f'Probability of finding a collision is {same_birthday(num_samples, N)}')
19
    
20
    m_list = []
21
    t_list = []
22
    
23
    # Make a list of hashes
24
    for i in range(num_samples):
25
        m = random.getrandbits(M_bits) # Get random message
26
        #m = random.randint(0, M_bits-1) 
27
        m = long_to_bytes(m)
28
        
29
        t = small_hash(m, T_bits) # Hash it
30
        if m not in m_list:
31
            m_list.append(m)
32
            t_list.append(t)
33
            
34
    # Check for collisionn
35
    for i in range(len(t_list)):
36
        for j in range(i+1, len(t_list)):
37
            if t_list[i] == t_list[j]:
38
                print('Collision found!')
39
                return m_list[i], m_list[j], t_list[i]
40
    else:
41
        print('Collision not found :(')
42
        return b"", b"", b""
Copied!
We use it as follows:
1
M_bits = 30
2
T_bits = 20
3
​
4
m1, m2, t = small_hash_colision(M_bits, T_bits)
5
print(m1, m2, t)
6
print(small_hash(m1, T_bits) == small_hash(m2, T_bits))
7
​
8
# Hash size:  1048576
9
# Making a list of 1229 hashes
10
# Probability of finding a collision is 0.513213460854798
11
# Collision found!
12
# b'!\xafB\xc5' b'4F\xb6w' b'\x01y\xf7'
13
# True
14
​
Copied!
Eliminating Storage Requirements with Pollard's Rho
While the above algorithm works to find a hash collision in time O(N)O(\sqrt N )O(N​)
, it also requires storing O(N)O(\sqrt N )O(N​)
hash values. As such, it represents a classic time-space tradeoff over the naive approach, which involves randomly selecting pairs of inputs until they hash to the same value. While the naive approach does not require any additional storage, it does have runtime O(N)O(N )O(N)
.
However, there is a better approach combining the best of both worlds: constant storage requirements and O(N)O(\sqrt N)O(N​)
runtime. This approach is based on Pollard's Rho algorithm, which is better-known for its application to solving discrete logarithms. The core insight behind the algorithm is that by the Birthday Paradox, we expect to encounter a hash collision after trying O(N)O(\sqrt N)O(N​)
random inputs. However, it is possible to detect whether a collision has occurred without needing to store all of the inputs and hashes if the inputs are chosen in a clever way.
This is done by choosing the next input based on the hash of the previous input, according to the following sequence:
x0=g(seed)x1=g(H(x0))x2=g(H(x1))…xn+1=g(H(xn))x_0 =  g(seed) \\
x_1 = g(H(x_0)) \\
x_2 = g(H(x_1)) \\
\dots \\
x_{n+1} = g(H(x_n)) \\x0​=g(seed)x1​=g(H(x0​))x2​=g(H(x1​))…xn+1​=g(H(xn​))
Where H:M⟶TH:\mathcal{M} \longrightarrow \mathcal{T}H:M⟶T
 is our hash function and g:T⟶Mg: \mathcal{T} \longrightarrow \mathcal{M} g:T⟶M
 is a "sufficiently random" function which takes a hash value and produces a new. We define the composition of the functions to bef=H∘g:T⟶Tf = H \circ g : \mathcal{T} \longrightarrow \mathcal{T} f=H∘g:T⟶T
. 
1
# TODO:
2
# have the two hash functions used in this chapter be the same
3
​
4
from Crypto.Hash import SHA256
5
from Crypto.Util.number import bytes_to_long, long_to_bytes
6
​
7
from tqdm.autonotebook import tqdm
8
​
9
# bits in hash output
10
n = 30
11
​
12
# H
13
def my_hash(x):
14
    x = bytes(x, 'ascii')
15
    h = SHA256.new()
16
    h.update(x)
17
    y = h.digest()
18
    y = bytes_to_long(y)
19
    y = y % (1<<n)
20
    y = int(y)
21
    return y
22
​
23
# g
24
def get_message(r):
25
    x = "Crypto{" + str(r) + "}"
26
    return x
27
​
28
# f
29
def f(r):
30
    return my_hash(get_message(r))
31
​
Copied!
By the Birthday Paradox, we expect that the sequence will have a collision (where xi=xjx_i = x_jxi​=xj​
 for two distinct values i,ji,ji,j
) after O(N)O(\sqrt N)O(N​)
values. But once this occurs, then the sequence will begin to cycle, because xi+1=g(H(xi))=g(H(xj))=xj+1x_{i+1} = g(H(x_i)) = g(H(x_j)) = x_{j+1}xi+1​=g(H(xi​))=g(H(xj​))=xj+1​
.
Therefore, we can detect that a collision has occurred by using standard cycle-detection algorithms, such as Floyd's tortoise and hare!
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.
For Floyd's tortoise and hare, this is done by noting that when we found our collision after nnn
 iterations, we were comparing H(xn)=H(x2n)H(x_{n}) = H(x_{2 n})H(xn​)=H(x2n​)
. And because the sequence is cyclic, if the first colliding input is xix_ixi​
, then it collides withH(xi)=H(xi+n)H(x_{i}) = H(x_{i+n})H(xi​)=H(xi+n​)
. So we define the new sequence yi=xn+iy_i = x_{n+i}yi​=xn+i​
, and step through the xix_ixi​
 and yiy_iyi​
 sequences together until we find our collision!
This is implemented in the following code snippet.
1
"""
2
initialization
3
​
4
This routine will find a hash collision in sqrt time with constant space.
5
"""
6
​
7
seed = 0
8
​
9
x0 = get_message(seed)
10
x = x0
11
y = f(x)
12
​
13
"""
14
detect collision
15
using Floyd / tortoise and hare cycle finding algorithm
16
​
17
expected number of iterations is ~ sqrt(pi/2) * 2^(n/2),
18
we run for up to 4 * 2^(n/2) iterations
19
"""
20
for i in tqdm(range(4 << (n//2))):
21
        
22
    if f(x) == f(y):
23
        break
24
        
25
    x = f(x)
26
    
27
    y = f(y)
28
    y = f(y)
29
    
30
​
31
"""
32
locate collision
33
"""
34
x = x0
35
y = f(y)
36
​
37
​
38
for j in tqdm(range(i)):
39
    if f(x) == f(y):
40
        break
41
    
42
    x = f(x)
43
    
44
    y = f(y)
45
    
46
​
47
m1 = get_message(x)
48
m2 = get_message(y)
49
​
50
assert my_hash(m1) == f(x)
51
assert my_hash(m2) == f(y)
52
​
53
print("[+] seeds for messages: {}, {}".format(x, y))
54
print("[+] messages: {}, {}".format(m1, m2))
55
print("[+] collided hashes of messages: {}, {}".format(my_hash(m1), my_hash(m2)))
56
​
57
​
58
# 31%    40391/131072 [00:03<00:08, 10666.20it/s]
59
# 30%    12032/40391 [00:00<00:02, 13112.15it/s]
60
# 
61
# [+] seeds for messages: 404842900, 254017312
62
# [+] messages: Crypto{404842900}, Crypto{254017312}
63
# [+] collided hashes of messages: 1022927209, 1022927209
Copied!
Finally, there is a third algorithm for finding hash collisions in O(N)O(\sqrt N)O(N​)
time, namely the van Oorschot-Wiener algorithm based on Distinguished Points. While it does have additional storage requirements over Pollard's Rho, the main advantage of this algorithm is that it parallelizes extremely well, achieving O(Nm)O(\frac{\sqrt N}{m})O(mN​​)
runtime when run on mmm
 processors.
Resources
​https://en.wikipedia.org/wiki/Birthday_problem - wiki entry
​https://en.wikipedia.org/wiki/Birthday_attack - wiki entry
​https://www.youtube.com/watch?v=ofTb57aZHZs - vsauce2 video
​van Oorschot-Wiener Parallel Collision Search with Cryptanalytic Applications​
​http://www.cs.umd.edu/~jkatz/imc/hash-erratum.pdf​
​https://crypto.stackexchange.com/questions/3295/how-does-a-birthday-attack-on-a-hashing-algorithm-work?rq=1​
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Eliminating Storage Requirements with Pollard's Rho
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