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This answer talks about how non-prime numbers would make the algorithm easier to break, not that the algorithm doesn't work full-stop: Why does RSA need p and q to be prime numbers?

Question 1: this implementation uses the number 57, which is not prime as it divides by 3.

  1. Choose p = 13, q = 57, e = 5 and message 'm' = 4
  2. Therefore n = pq = 741 and (p - 1)(q - 1) = 672
  3. encrypt message m as m^e mod n = 4^5 mod 741 = 283
  4. calculate decryption key d as e^-1 mod (p–1)(q–1) = 5^-1 mod 812 = 269
  5. decrypt '283' by 283^d mod n = 283^269 mod 741 = 199 ≠ 4

How does using a non-prime number cause the result to be wrong? I suspect the answer has something to do with the way that an inverse mod calculation looks for prime factors using the Euclidean algorithm, and if n is not the product of two prime numbers then it will give a different answer. But how is (p-1)(n-1) affected by p or q not being prime?

Question 2: p and q are the same prime number:

  1. Choose p = 11, q = 11, e = 3 and message 'm' = 2
  2. Therefore n = pq = 121 and (p - 1)(q - 1) = 100
  3. encrypt message m as m^e mod n = 2^2 mod 121 = 8
  4. calculate decryption key d as e^-1 mod (p–1)(q–1) = 5^-1 mod 100 = 67
  5. decrypt '8' by 8^d mod n = 8^67 mod 121 = 24 ≠ 4

How does using two (prime) identical numbers cause the result to be wrong? For this one I don't know where to start.

Burrough Clarke
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2 Answers2

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A first step is to use the correct value for the totient function $\phi(n),$ which is $432$ and $110$ in your examples.

Even better use the Carmichael function $\lambda(n)$ as described in RSA key generation, which is $36$ and $110.$

With either of these numbers, your encrypted messages are decrypted correctly.

gammatester
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Wanted to add something to @gammatester's answer.
He is correct in saying that your decryption results are wrong because you are using incorrect values for the totient function $\phi(n)$. If you look at the Euler's theorem, you will see that taking a wrong value for $\phi(n)$, will result in a residue ≠ 1 mod $n$ when you do decryption. In this case you will get residue as $m^{k}$ mod $n$, where $k = e.d (= a.\phi(n) + k)$ mod $\phi(n)$. Here $\phi(n)$ is the correct value of the totient function, which you are taking as incorrect in the example above.
It is also not at all secure to use either $p = q$ or one (or both) of $p$ or (and) $q$ to be composite. If you take $p = q$, then your implementation is not backed up by the hardness of the integer factorization problem, no matter how big you take you primes $p$ and $q$ to be. On the other hand, if you take composites instead of primes $p$ or (and) $q$ then you are just making it easier for someone to factorize the modulus value $n = p.q$.
You should not even try to go with these options given security falls apart then.

Mayank
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