Using recover_key with eosjs

Question

I want to validate user signatures in my smart contract. As I understood, I am supposed to do that with recover_key function from crypto.h

I am using JavaScript with eosjs and eosjs-ecc for integration. There i sign some data and pass it to my contract action:

    let data = "Some Data"
    let sig = ecc.sign(data, keyProvider)

    let result = await eos.transaction('contract', contract => {
        contract.testrecover(data, sig, pubkey, tester_options)
    })

In my contract I want to recover and verify a public key from this signature. Because recover_key function requires a char*, and ABI doesn't support passing char* to actions, I use following code:

  void testrecover( std::string data, std::string sig, std::string pub ) {
      char* datachar = new char[data.size() + 1];
      std::copy(data.begin(), data.end(), datachar);
      datachar[data.size()] = '\0';

      char* sigchar = new char[sig.size() + 1];
      std::copy(sig.begin(), sig.end(), sigchar);
      sigchar[sig.size()] = '\0';

      char* pubchar = new char[pub.size() + 1];
      std::copy(pub.begin(), pub.end(), pubchar);
      pubchar[pub.size()] = '\0';

      checksum256* digest;
      sha256(datachar, data.size(), digest);
      assert_recover_key(digest, sigchar, sig.size(), pubchar, pub.size());
  }

But when running, contract action execution fails with the following error:

2018-07-31T15:49:33.376 thread-0 http_plugin.cpp:475
handle_exception ] FC Exception encountered while processing chain.push_transaction 2018-07-31T15:49:33.376 thread-0
http_plugin.cpp:476 handle_exception ] Exception Details: 3070002 wasm_execution_error: Runtime Error Processing WASM access violation {} thread-0 binaryen.hpp:81 assert_memory_is_accessible

I have no idea why it is appearing. Hoping for your help.