This is a toy exmaple about how to use libsnark. libsnark implements zkSNARK algorithm. This toy example do the following:
The prover know a merkle tree root rt, a leaf leaf, a valid merkle branch path from leaf to rt, and prev_leaf, the relation between prev_leaf and leaf is : prev_leaf = sha256(leaf).
The verifier is given rt and prev_leaf, can verify that prover know a valid preimage of prev_leaf, and a valid merkle brach path from leaf to root using zkSNARK algorithm.
./get-libsnark && make && ./main
see here for details.
proof generation time : 10.297805s
proof verification time : 0.041092s
proof size : 2294 bits == 7 G1 and 1 G2 element. sizeof(G1 element) = sizeof(x,y) = 512 can be compressed into x coordinate and flag(8 bits) which represent y coordinate even or odd，so sizeof(compressed G1) = 264; Similarly, sizeof(compressed G2 element) = sizeof (x coordinate of G2) + 8 bits = 520 bits, so proof size is 2368 bits. the output of libsnark is different, see https://github.com/zcash/zips/issues/43