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Given a big number $x$(1024 bits) and a large prime $N$($N>x$), I want to encrypt it with a smaller number $y$(128 bits).

$$Enc(x) = xy \mod{N}.$$

Is it safe? If not, how do I evaluate its leakage?

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The most basic security is "Indistinguishability in the presence of an eavesdropper". This is based on a simple experiment. An adversary $A$ sends two messages $m_0$, $m_1$ to a challenger $C$. The challenger encrypts one of the messages (randomly chosen, say $C$ chooses $b \in \{0,1\}$ uniform random and encrypts $c_b = Enc(m_b)$). The ciphertext is then send to $A$. The adversary now has to find out, which message was encrypted and sends $b'=0$ or $b'=1$ to $C$. If $b = b'$ we say $A$ wins.

Now we look at your Encryption: $A$ knows $Enc(x)$ and $x$. With the extended euclidean algorithm $A$ can compute $y$ and therefore can distinguish $c_b$. So $A$ wins the experiment and the Encryption system does not even provide the most basic concept of security.

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  • $\begingroup$ $A$ knows $\operatorname{Enc}(x)$, but no justification is given that $A$ knows $x$. $A$ knows $x\in\{m_0,m_1\}$, but it is not told how it is determined whether $x=m_0$ or $x=m_1$. In some cases, there is no possible advantage, e.g. with $m_0=1$, $m_1=2$, and it happens that the key $y=3-x$: for both $b$, $\operatorname{Enc}(x)=2$. $\endgroup$ – fgrieu Apr 13 at 9:16
  • $\begingroup$ When $Enc(x)$ and $x$ are given, $y$ can be computed. However, gettng $Enc(m_b)$, using $m_0$ and $m_1$ respectively, you can compute different $y_1$ and $y_2$. How do you know which one is right? $\endgroup$ – Wanyu Apr 13 at 9:22
  • $\begingroup$ @Wanyu: that's where the "128-bit" part of the problem statement comes into play. $\endgroup$ – fgrieu Apr 13 at 9:24
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    $\begingroup$ I think what @fgrieu is getting at is if any one of them requires y to be something larger than 128 bits then, we can tell which one was encrypted. If you get x and y to be of equal size, it becomes same as one time pad, hence indistinguishability is inevitable but it makes the encryption scheme practically useless. $\endgroup$ – Manish Adhikari Apr 13 at 10:24
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    $\begingroup$ But remember that even in this case, indistinguishability only exists upto a point of one round of game. It does not allow key reuse and since encryption oracle cannot be applied here as a single round reveals the key entirely. Thus the scheme is still not CPA-secure $\endgroup$ – Manish Adhikari Apr 13 at 10:33

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