Here’s the next problem:

The public key is $N=209$. What is the Rabin decryption of $80$?

• What is Rabin decryption?
wikipedia: The Rabin cryptosystem is an asymmetric cryptographic technique, whose security, like that of RSA, is related to the difficulty of integer factorization. However the Rabin cryptosystem has the advantage that it has been mathematically proven to be computationally secure against a chosen-plaintext attack as long as the attacker cannot efficiently factor integers, while there is no such proof known for RSA.

• What is a chosen plaintext attack?
wikipedia: A chosen-plaintext attack (CPA) is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts. The goal of the attack is to gain information that reduces the security of the encryption scheme.

• How does the Rabin Encryption scheme work?
wikipedia: The keys for the Rabin cryptosystem are generated as follows:
1. Choose two large distinct prime numbers $p$ and $q$ such that $p\equiv 3\bmod 4$ and $q\equiv 3\bmod 4$.
2. Compute $n=pq$. Then $n$ is the public key and the pair $(p,q)$ is the private key.
• Decryption
Since 209 is not a large number, we can easily factorise it through trial and error:
i = 2
while 209 % i != 0: i+=1
print(i, 209/i)


11 19.0

Let $p=11$ and $q=19$. Now, following the procedure from wikipedia,

The message $m$ can be recovered from the ciphertext $80$ by taking its square root modulo $209$ as follows.

1. Compute the square root of $c$ modulo $p$ and $q$:

$m_p = 80^{\frac{1}{4}(11+1)} \bmod{11} = 5 \\ m_q = 80^{\frac{1}{4}(19+1)} \bmod{19} = 17$
2. Using the extended Euclidean algorithm to find $y_p$ and $y_q$ such that

$y_p \cdot p + y_q \cdot q = 1$
 def ExtEuclid (a,b):
# returns a triple (d,s,t) such that d = gcd(a,b) and
# d == a*s + b*t
if b == 0: return (a,1,0)
(d1, s1, t1) = ExtEuclid(b,a%b)
d = d1
s = t1
t = s1 - int(a / b) * t1
return (d,s,t)

print('{} = 19*({}) + 11*({})'.format(*ExtEuclid(19,11)))


1 = 19(-4) + 11(7)

So we have $y_p = 7$ and $y_q = -4$.
These two articles (this one and this one) were useful in understanding the algorithm.

3. Using the Chinese remainder theorem to find the four square roots of $c$ modulo $n$: $$r_1 = (7 \cdot 11 \cdot 17 + (-4) \cdot 19 \cdot 5) \bmod 209 = 93\\ r_2 = 209 - 93 = 116\\ r_3 = (7 \cdot 11 \cdot 17 - (-4) \cdot 19 \cdot 5) \bmod 209 = 17\\ r_4 = 209 - 17 = 192$$

One of these four values is the original plaintext $m$, although which of the four is the correct one cannot be determined without additional information.
Of these four, $93$ is the only value that matches an option in my assignment, which means that it is the original plaintext in this case.

Here’s the next one:

The public key is $N = 713$. What is the Rabin Digital Signature of $473$?

• What is the Rabin Signature Scheme?
From my class notes,
Signature: $s = m^{1/2} \bmod n$ where $s$ is the signature
Verification: $m = s^2 \bmod n$

Now I can think of two ways to solve this:

• the one where we use pen and paper

$x^2 \equiv 473 \bmod 713$
$\implies x^2 \equiv 473 \bmod 23$ and $x^2 \equiv 473 \bmod 31$
$\implies x^2 \equiv 13 \bmod 23$ and $x^2 \equiv 8 \bmod 31$
$\implies x^2 \equiv 36 \bmod 23$ and $x^2 \equiv 225 \bmod 31$
$\implies x \equiv \pm 6 \bmod 23$ and $x \equiv \pm 15 \bmod 31$

We can then solve the four sets of equations using the Chinese Remainder theorem. These two videos (this one and this one) helped me in this approach.

• the one where we use a brute force approach

  i = j = 1
while j <= 4:
while i**2 % 713 != 473: i+=1
print(i)
i+=1
j+=1


Either way, the four roots we get are: $109$, $201$, $512$ and $604$.

Of these four, $201$ is the only one present in the given options in my assignment, so that is the solution to this problem.