I have two plain texts files cipher with RSA (with the same public and private key) and I have their cipher texts too. Would it be possible know the public key from that information?

My messages have 10 characters and I cannot calculate a huge exponential operation. No padding is used on the messages before encryption.

  • $\begingroup$ What are the ciphertext sizes in bits? What is the source of this question? $\endgroup$
    – kelalaka
    Apr 29, 2020 at 12:50
  • $\begingroup$ Cipher text have a size of 128 bytes. The messages are "First test" and "Second test" $\endgroup$
    – root2d3877
    Apr 29, 2020 at 13:05
  • $\begingroup$ Suggestion: maybe you converted your plaintexts into integers incorrectly. Did you try little endian or big endian? $\endgroup$
    – user69015
    Apr 30, 2020 at 12:37

1 Answer 1


From a couple of plaintext/ciphertext $(m_1, c_1)$ and $(m_2, c_2)$, then the modulus $n$ is a factor of $$ \gcd(m_1^e-c_1, m_2^e-c_2). $$ The explanation is that $$ c_i = m_i^e - \lambda_i n $$ for some unknown $\lambda_i$, so $n$ both divides $m_1^e-c_1$ and $m_2^e-c_2$.

Of course, this solution works if you know $e$, but it can be tried with some classically used values ($e=3$ for instance).

  • $\begingroup$ Thank you so much for your answer. That it is what I tried and I have not any result yet. I tried with the first 8000 numbers and the first 4 fermat numbers. I hope that I come up with the e value or with another way to solve this problem $\endgroup$
    – root2d3877
    Apr 29, 2020 at 14:08
  • 1
    $\begingroup$ The most used value for $e$ is $65537$, so if you can manage exponentiation with it, you may find something. $\endgroup$
    – user69015
    Apr 29, 2020 at 15:27
  • $\begingroup$ I know, 65537 is the 4th fermat number and I tried with it. Thank you anyway. $\endgroup$
    – root2d3877
    Apr 29, 2020 at 15:38
  • $\begingroup$ @root2d3877 Are you sure that the plaintext values are not padded somehow? $\endgroup$
    – Maarten Bodewes
    Apr 29, 2020 at 22:32
  • $\begingroup$ @MaartenBodewes Yes, I am. The problems says that there is no padding $\endgroup$
    – root2d3877
    Apr 30, 2020 at 8:07

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