I am trying to argue about the hardness of LWE, but in a setting that is different from the standard one.

Consider the task of learning the LWE secret $s$ from noisy samples. The specifications of the problem are an $m \times n$ matrix $A$, a $n \times 1$ vector $s$, and a Gaussian noise vector $e$. By a noisy sample, I mean a vector that evaluates to $As + e$. The Gaussian noise is sampled independently at random from an appropriately chosen Gaussian distribution every time a noisy sample is asked for. $m$ is chosen to be sufficiently large.

The process works as follows:

  1. The learner receives a noisy sample.
  2. The learner processes the sample and guesses what the secret is.
  3. There is a ”judge” who tells the learner how close or far they are, in $L_1$ norm, from the actual secret.
  4. The process repeats.

Is it known that the learner will not converge well to the secret in polynomial time?


1 Answer 1


This would be very bad. Assuming that you're calculating $\mod q$ for some $q$, the learner can recover $s$ with at most $3n$ queries even while ignoring the samples.

I'll assume that $q$ is even, but the odd case is not much harder.

For a first guess the learner could try $s=(0,0,\ldots,0)^T$ and for a second try $(q/2,0,\ldots,0)^T$. Let $a$ be the $L_1$ distance of the first component from 0 and $b$ be the $L_1$ distance of the first component from $q/2$.

I know that $a+b=q/2$ and I known that $a-b$ is the difference of the two pieces of advice which I received in Step 3. I can now compute $a$ and $b$ and narrow the first component down to 2 possible values. A third query will disambiguate.

I can obviously repeat the process for other components.


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