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zkSNARK-toy

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.

howto

./get-libsnark && make && ./main

details

see here for details.

performance

  • 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 zcash/zips#43

代码详解

gadget 是构建 R1CS 实例的基础组件，gadget 是个基类，任何针对具体功能的 gadget 都继承自 gadget，比如说针对 sha256 功能的 sha256_compression_function_gadget、针对 Merkle 树的 merkle_tree_check_read_gadget 等。组合使用这些基本的 gadget，我们构建出复杂的 R1CS 实例。

代码结构

  • src/gadget.hpp : 提供了 toy_gadget 类，使用其它基础 gadget 构建 zkSNARK-toy 相关的 constraint 与 witness

  • src/snark.hpp : 提供对密钥生成、零知识证明生成以及验证零知识证明的简单封装，在 main.cpp 中使用

使用到的关键 gadget

  • sha256_compression_function_gadget : 进行 sha256 哈希运算时用到的 gadget

  • merkle_authentication_path_variable : 保存了 merkle 分支

  • merkle_tree_check_read_gadget : 进行 merkle 分支验证的 gadget