I'm reading Proof Systems for General Statements about Discrete Logarithms, and I think I'll have a better understanding of the process if I can understand where the terms come from. They give a basic example which I'll just post a snapshot of below:

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Can someone explain the logic behind the naming of these terms?


1 Answer 1


This is because:

  1. For the commitment $t$, it comes first and uses the difficulty of the discrete logarithm problem to protect the value $v$. Once you've published the value $t=g^v \bmod q$, if someones knows the value $v$, then they can easily verify that the value $t$ corresponds to the value $g^v \bmod q$, but on the other hand it is really difficult to find the value $v$ knowing only the value $t$. Hence, you commit to the knowledge of the value $v$ by giving out the value $t$.
  2. For the challenge $c$, you can see this value as being a "challenge": anybody able to give you a value $r$ such that $g^ry^c=g^v=t \bmod q$ must be knowing the value $x$, otherwise they wouldn't have been able to compute $r$ such that the $y^c=g^{xc}$ cancels out with the $g^r$ value. So you challenge them to provide you with a correct $r$ value.
  3. And for the response $r$, it is an answer to the question of finding a value $r$ such that $g^ry^c=g^v$. If you know $x$ and $v$ then you can compute the value $r$ in such a way that the challenge would not be hard, but if you do not know those values, your chances to find a correct value $r$ are negligible. So you're responding to the challenge using that value $r$.

This is possible in the end because $g^ry^c=g^rg^{xc}=g^{v-xc}g^{xc}=g^v=t$ by definition of $y$ and $r$.

I'm speaking $\bmod q$ for all of it to hold, obviously.


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