Hybrid Sequence:
The starting point is $\lib{schnorr-real}$.
We can rearrange things so that $c$ is chosen first.
Now $r$ is essentially a OTP encryption of plaintext $ca$ under key $y$, using a variant of OTP with addition mod $\nmod$. We achieve the same distribution by choosing $r$ uniformly and solving for $y$:
Now $Y$ is computed as $Y = g^y = g^{r-ca} = g^r (g^{ac})^{-1} = g^r (A^c)^{-1}$. By rewriting $Y$ in this way, the private exponent $a$ is no longer needed. The result is $\lib{schnorr-fake}$, completing the proof.
$\lib{schnorr-real}$
$\schnorrtrans$($a$):
$A := g^a$
$y \gets \Z_\nmod$
$Y := g^y$
$c \gets \Z_\nmod$
$r $
${}:= (y+ca) \pct \nmod$
${}\gets \Z_\nmod$
return $(A,Y,c,r)$