# Laplace mechanism in Differential Privacy

From The Algorithmic Foundations of Diﬀerential Privacy It wrote that :

But from this pdf

I am confused which one is right, or I misunderstand.

In second method, after I compute Pr[v], and then what to do?

I am not sure how to implement the Laplace mechanism.

These two formulas are the same thing. The second formula is the probability density function of the Laplace distribution centered on 0 ($$\mu=0$$) — although rather than $$Pr[v]$$, the second PDF should probably have used a notation like $$f(v)$$ to make it clear that this is a probability density function and not the probability of returning exactly $$v$$.

If the question about "how to implement the Laplace mechanism" was referring to something you want to do in practice, then you probably don't want to do it yourself; but you want to use an implementation safe against floating-point attacks. Here's an example of open-source software you can use (disclaimer: I'm part of the team behind that library).

If I would see both I would have the same misunderstanding you had. I got a little late but in order to connect ideas we have $$v = x$$ where $$v \in Y_i$$ and $$\mathcal{M}_L = f(x)+v$$. What I see here is a bad notation on the last one, since $$v$$ is a point, not a distribution as $$Y_i$$. This should be something like $$V_i \sim Lap(\Delta f / \epsilon)$$ where you want to know $$Pr[V=v]$$.

Then I always going to prefer the full notation of Laplace and function which is $$Lap(\mu,b)$$ which has a mean of $$\mu$$ and a variance of $$2b$$. Now the probability of $$x$$ in Laplace is $$Lap(x \mid (\mu,b))$$.

$$Lap(x \mid (\mu,b))=\frac{1}{2b}e^{\frac{-|x-\mu|}{b}}$$

What you want to compute is the $$f(x)$$ with the noise. If you only are going to compute the noise then you should use only $$Pr[v]=Pr[Y=x]$$ (Remember $$x=v$$). What you want to compute is $$\mathcal{M}_L(x)$$. Which by definition (now this is why is useful the mean notation) is :

$$\mathcal{M}_L(x \mid (\mu,b))=\frac{1}{2b}e^{\frac{-\varepsilon|x-f(x)|}{\Delta f}}$$

Because you only move the distribution to the left by $$f(x)$$. Then you have :

If you want to implement the mechanism you can obtain the query $$f(x)$$ from a Database and using a library that has Laplace distribution do $$\mathcal{M}_L(x) = f(x)+ Y_i$$ where $$Y_i \sim Lap(\Delta f / \epsilon)$$. Remember $$x$$ is the database and $$f(x) \in \mathbb{R}^k$$