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Intro to darksidewalletd

Darksidewalletd is a feature included in lightwalletd, enabled by the --darkside-very-insecure flag, which can serve arbitrary blocks to a Zcash light client wallet. This is useful for security and reorg testing. It includes a minimally-functional mock zcashd which comes with a gRPC API for controlling which blocks it will serve.

This means that you can use darksidewalletd to control the blocks and transactions that are exposed to any light wallets that connect, to see how they behave under different circumstances. Multiple wallets can connect to the same darksidewalletd at the same time. Darksidewalletd should only be used for testing, and therefore is hard-coded to shut down after 30 minutes of operation to prevent accidental deployment as a server.

Security warning

Leaving darksidewalletd running puts your machine at greater risk because (a) it may be possible to use file: paths with StageBlocks to read arbitrary files on your system, and (b) also using StageBlocks, someone can force your system to make a web request to an arbitrary URL (which could have your system download questionable material, perform attacks on other systems, etc.). The maximum 30-minute run time limit built into darksidewalletd mitigates these risks, but users should still be cautious.

Dependencies

Lightwalletd and most dependencies of lightwalletd, including Go version 1.11 or later, but not zcashd. Since Darksidewalletd mocks zcashd, it can run standalone and does use zcashd to get blocks or send and receive transactions.

For the tutorial the grpcurl tool is needed to call the darksidewalletd gRPC API.

Overview

How Darksidewalletd Works

Lightwalletd and the wallets themselves don’t actually perform any validation of the blocks (beyond checking the blocks’ prevhashes, which is used to detect reorgs). That means the blocks we give darksidewalletd don’t need to be fully valid, see table:

Block component Must be valid Must be partially valid Not checked for validity
nVersion x
hashPrevBlock x
hashMerkleRoot x
hashFinalSaplingRoot x
nTime x
nBits x
nNonce x
Equihash solution x
Transaction Data* x

*Transactions in blocks must conform to the transaction format, but not need valid zero-knowledge proofs etc.

For more information about block headers, see the Zcash protocol specification.

Lightwalletd provides us with a gRPC API for generating these minimally-acceptable fake blocks. The API allows us to "stage" blocks and transactions and later "apply" the staged objects so that they become visible to lightwalletd and the wallets. How this is done is illustrated in the tutorial below, but first we must start darksidewalletd.

Running darksidewalletd

To start darksidewalletd, you run lightwalletd with the --darkside-very-insecure flag:

./lightwalletd --darkside-very-insecure --no-tls-very-insecure --data-dir . --log-file /dev/stdout

To prevent accidental deployment in production, it will automatically shut off after 30 minutes.

Now that darksidewalletd is running, you can control it by calling various gRPCs to reset its state, stage blocks, stage transactions, and apply the staged objects so that they become visible to the wallet. Examples of using these gRPCs are given in the following tutorial.

Tutorial

This tutorial is intended to illustrate basic control of darksidewalletd using the grpcurl tool. You can use any gRPC library of your choice in order to implement similar tests in your apps' test suite.

Simulating a reorg that moves a transaction

In this example, we will simulate a reorg that moves a transaction from one block height to another. This happens in two parts, first we create and apply the "before reorg" state. Then we create the "after reorg" stage and apply it, which makes the reorg happen.

Creating the Before-Reorg State

If you haven't already started darksidewalletd, please start it:

./lightwalletd --darkside-very-insecure --no-tls-very-insecure --data-dir . --log-file /dev/stdout

First, we need to reset darksidewalletd, specifying the sapling activation height, branch ID, and chain name that will be told to wallets when they ask:

grpcurl -plaintext -d '{"saplingActivation": 663150,"branchID": "bad", "chainName":"x"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/Reset

Next, we will stage the real mainnet block 663150. In ECC's example wallets, this block is used as a checkpoint so we need to use the real block to pass that check.

grpcurl -plaintext -d '{"url": "https://raw.githubusercontent.com/zcash-hackworks/darksidewalletd-test-data/master/basic-reorg/663150.txt"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageBlocks

This has put block 663150 into darksidewalletd's staging area. The block has not yet been exposed to the internal block-processing mechanism in lightwalletd, and thus any wallets connected will have no idea it exists yet.

Next, we will use the StageBlocksCreate gRPC to generate 100 fake blocks on top of 663150 in darksidewalletd's staging area:

grpcurl -plaintext -d '{"height":663151,"count":100}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageBlocksCreate

Still, everything is in darksidewalletd's staging area, nothing has been shown to any connected wallets yet. The staging area now contains the real mainnet block 663150 and 100 fake blocks from 663151 to 663250.

Next we'll stage a transaction to go into block 663190. 663190 is within the range of blocks we've staged; when we "apply" the staging area later on darksidewalletd will merge this transaction into the fake 663190 block.

grpcurl -plaintext -d '{"height":663190,"url":"https://raw.githubusercontent.com/zcash-hackworks/darksidewalletd-test-data/master/transactions/recv/0821a89be7f2fc1311792c3fa1dd2171a8cdfb2effd98590cbd5ebcdcfcf491f.txt"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageTransactions

We have now finished filling darksidewalletd's staging area with the "before reorg" state blocks. In darksidewalletd's staging area, we have blocks from 663150 to 663250, with a transaction staged to go in block 663190. All that's left to do is "apply" the staging area, which will reveal the blocks to lightwalletd's internal block processor and then on to any wallets that are connected. We will apply the staged blocks up to height 663210 (any higher staged blocks will remain in the staging area):

grpcurl -plaintext -d '{"height":663210}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/ApplyStaged

Note that we could have done this in the opposite order, it would have been okay to stage the transaction first, and then stage the blocks later. All that matters is that the transactions we stage get staged into block heights that will have blocks staged for them before we "apply".

Now we can check that the transaction is in block 663190:

$ grpcurl -plaintext -d '{"height":663190}' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetBlock
{
  "height": "663190",
  "hash": "Ax/AHLeTfnDuXWX3ZiYo+nWvh24lyMjvR0e2CAfqEok=",
  "prevHash": "m5/epQ9d3wl4Z8bctOB/ZCuSl8Uko4DeIpKtKZayK4U=",
  "time": 1,
  "vtx": [
    {
      "index": "1",
      "hash": "H0nPz83r1cuQhdn/LvvNqHEh3aE/LHkRE/zy55uoIQg=",
      "spends": [
        {
          "nf": "xrZLCu+Kbv6PXo8cqM+f25Hp55L2cm95bM68JwUnDHg="
        }
      ],
      "outputs": [
        {
          "cmu": "pe/G9q13FyE6vAhrTPzIGpU5Dht5DvJTuc9zmTEx0gU=",
          "epk": "qw5MPsRoe8aOnvZ/VB3r1Ja/WkHb52TVU1vyHjGEOqc=",
          "ciphertext": "R2uN3CHagj7Oo+6O9VeBrE6x4dQ07Jl18rVM27vGhl1Io75lFYCHA1SrV72Zu+bgwMilTA=="
        },
        {
          "cmu": "3rQ9DMmk7RaWGf9q0uOYQ7FieHL/TE8Z+QCcS/IJfkA=",
          "epk": "U1NCOlTzIF1qlprAjuGUUj591GpO5Vs5WTsmCW35Pio=",
          "ciphertext": "2MbBHjPbkDT/GVsXgDHhihFQizxvizHINXKVbXKnv3Ih1P4c1f3By+TLH2g1yAG3lSARuQ=="
        }
      ]
    }
  ]
}
$

Creating the After-Reorg State

Now, we can stage that same transaction into a different height, and force a reorg.

First, stage 100 fake blocks starting at height 663180. This stages empty blocks for heights 663180 through 663279. These are the blocks that will change after the reorg.

grpcurl -plaintext -d '{"height":663180,"count":100}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageBlocksCreate

Now, stage that same transaction as before, but this time to height 663195 (previously we had put it in 663190):

grpcurl -plaintext -d '{"height":663195,"url":"https://raw.githubusercontent.com/zcash-hackworks/darksidewalletd-test-data/master/transactions/recv/0821a89be7f2fc1311792c3fa1dd2171a8cdfb2effd98590cbd5ebcdcfcf491f.txt"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageTransactions

Finally, we can apply the staged blocks and transactions to trigger a reorg:

grpcurl -plaintext -d '{"height":663210}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/ApplyStaged

This will simulate a reorg back to 663180 (new versions of 663180 and beyond, same 663179), and the transaction will now be included in 663195 and will not be in 663190.

After a moment you should see some "reorg" messages in the lightwalletd log output to indicate that lightwalletd's internal block processor detected and handled the reorg. If a wallet were connected to the lightwalletd instance, it should also detect a reorg too.

Now we can check that the transaction is no longer in 663190:

$ grpcurl -plaintext -d '{"height":663190}' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetBlock
{
  "height": "663190",
  "hash": "btosPfiJBX9m3nNSCP+vjAxWpEDS7Kfut9H7FY+mSYo=",
  "prevHash": "m5/epQ9d3wl4Z8bctOB/ZCuSl8Uko4DeIpKtKZayK4U=",
  "time": 1
}
$

Instead, it has "moved" to 663195:

$ grpcurl -plaintext -d '{"height":663195}' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetBlock
{
  "height": "663195",
  "hash": "CmcEQ/NZ9nSk+VdNfCEHvKu9MTNeWKoF1dZ7cWUTnCc=",
  "prevHash": "04i1neRIgx7vgtDkrydYJu3KWjbY5g7QvUygNBfu6ug=",
  "time": 1,
  "vtx": [
    {
      "index": "1",
      "hash": "H0nPz83r1cuQhdn/LvvNqHEh3aE/LHkRE/zy55uoIQg=",
      "spends": [
        {
          "nf": "xrZLCu+Kbv6PXo8cqM+f25Hp55L2cm95bM68JwUnDHg="
        }
      ],
      "outputs": [
        {
          "cmu": "pe/G9q13FyE6vAhrTPzIGpU5Dht5DvJTuc9zmTEx0gU=",
          "epk": "qw5MPsRoe8aOnvZ/VB3r1Ja/WkHb52TVU1vyHjGEOqc=",
          "ciphertext": "R2uN3CHagj7Oo+6O9VeBrE6x4dQ07Jl18rVM27vGhl1Io75lFYCHA1SrV72Zu+bgwMilTA=="
        },
        {
          "cmu": "3rQ9DMmk7RaWGf9q0uOYQ7FieHL/TE8Z+QCcS/IJfkA=",
          "epk": "U1NCOlTzIF1qlprAjuGUUj591GpO5Vs5WTsmCW35Pio=",
          "ciphertext": "2MbBHjPbkDT/GVsXgDHhihFQizxvizHINXKVbXKnv3Ih1P4c1f3By+TLH2g1yAG3lSARuQ=="
        }
      ]
    }
  ]
}
$

Just to illustrate a little more about how ApplyStaged works, we can check that the current height is 663210 just like we specified in our last call to ApplyStaged:

$ grpcurl -plaintext -d '' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetLatestBlock
{
  "height": "663210"
}

Then apply 10 more of the blocks that are still in the staging area:

grpcurl -plaintext -d '{"height":663220}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/ApplyStaged

And confirm that the current height has increased:

$ grpcurl -plaintext -d '' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetLatestBlock
{
  "height": "663220"
}

That concludes the tutorial. You should now know how to stage blocks from a URL using StageBlocks, stage synthetic empty blocks using StageBlocksCreate, stage transactions from a URL to go into particular blocks using StageTransactions, and then make the staged blocks and transactions live using ApplyStaged.

On top of what we covered in the tutorial, you can also...

  • Stage blocks and transactions directly (without them having to be accessible at a URL) using StageBlocksStream and StageTransactionsStream.
  • Get all of the transactions sent by connected wallets using GetIncomingTransactions (and clear the buffer that holds them using ClearIncomingTransactions).

See darkside.proto for a complete definition of all the gRPCs that darksidewalletd supports.

Generating Fake Block Sets

There’s a tool to help with generating these fake just-barely-valid-enough blocks, it’s called genblocks. To use it you create a directory of text files, one file per block, and each line in the file is a hex-encoded transaction that should go into that block:

mkdir blocksA
touch blocksA/{1000,1001,1002,1003,1004,1005}.txt
echo “some hex-encoded transaction you want to put in block 1003” > blocksA/1003.txt

This will output the blocks, one hex-encoded block per line. This is the format that will be accepted by StageBlocks.

Tip: Because nothing is checking the full validity of transactions, you can get any hex-encoded transaction you want from a block explorer and put those in the block files. The sochain block explorer makes it easy to obtain the raw transaction hex, by viewing the transaction (example), clicking “Raw Data”, then copying the “tx_hex” field.

Simulating the mempool

The GetMempoolTx gRPC will return staged transactions that are either within staged blocks or that have been staged separately. Here is an example:

grpcurl -plaintext -d '{"saplingActivation": 663150,"branchID": "bad", "chainName":"x"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/Reset
grpcurl -plaintext -d '{"url": "https://raw.githubusercontent.com/zcash-hackworks/darksidewalletd-test-data/master/tx-incoming/blocks.txt"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/StageBlocks
grpcurl -plaintext -d '{"txid":["qg=="]}' localhost:9067 cash.z.wallet.sdk.rpc.CompactTxStreamer/GetMempoolTx

Use cases

Check out some of the potential security test cases here: wallet <-> lightwalletd integration tests