  Modern cryptographic protocols rely on sophisticated hash functions to
generate quasi-unique numbers that serve as signatures for user authentication
and other security verifications. The security could be compromised by finding
texts hash-mappable to identical numbers, forming so-called collision attack.
Seeding a disturbance vector in the hash mapping to obtain a successful
collision is that a major focus of cryptography study in the past two decades
to improve hash protocols. We propose an algorithm that takes advantage of
entangled quantum states for concurrent seeding of candidate disturbance
vectors, out of which the one entailing collision is selected through a
combination of quantum search, phase gating, diffusion gating, and information
feedbacks from classical computing machinery. The complexity reduction is shown
to be on the order of $\mathcal{O}(2^{n/2+1})$ where $n$ is the number of
qubits encoding addresses. We demonstrate the practicality of the proposed by
an implementation scheme based on degenerate optical parametric oscillators.
