I have been told that one of the keys in an affine cipher is 33. The Mod is 37 - the set being the alphabet + numbers + space. So the affine function is
$ax + b$ $(mod$ $37)$ with $a$ and $b$ being the keys. I cannot figure out if $33$ is $a$ or $b$.
Is there a definitive way to figure that out? I thought there might be a relationship between $a$ or $b$ and the $mod$, but all I can recall is that $gcd(a,mod)=1$. This is satisfied by $33$ and $37$, but I do not confirm this to mean $a$ is $33$.
Any help is appreciated.