  A Verifiable Delay Function (VDF) is a function that takes a specified
sequential time $T$ to be evaluated, but can be verified in
$\Omega(\log{T})$-time. For meaningful security, $T$ can be at most
subexponential in the security parameter $\lambda$ but has no lower bound. VDFs
are useful in several applications ranging from randomness beacons to
sustainable blockchains but are really rare in practice. The verification in
all of these VDFs requires $\Omega(\lambda, \log{T})$ sequential time. This
paper derives a verifiable delay function that is verifiable in
$O(1)$-sequential time. The key observation is that the prior works use
subexponentially-hard algebraic assumptions for their sequentiality. On the
contrary, we derive our VDF from a polynomially-hard sequential assumption
namely the time-lock puzzle over the group of known order. In particular, we
show that time-lock puzzle can be sequentially-hard even when the order of the
group is known but the delay parameter is polynomially-bounded i.e., $T \le
\mathtt{poly}(\lambda)$. As an add-on advantage, our VDF requires only one
sequential squaring to verify. Thus, in our VDF, the sequential effort required
for verification is fixed and independent of the delay parameter $T$.
