zcash_client_backend: Add a Tor client using the Arti library

Cargo.toml

@@ -125,6 +125,13 @@ proptest = "1"
 rand_chacha = "0.3"
 rand_xorshift = "0.3"
 
+# Tor
+# - `arti-client` depends on `rusqlite`, and a version mismatch there causes a compilation
+#   failure due to incompatible `libsqlite3-sys` versions.
+arti-client = { version = "0.11", default-features = false, features = ["compression", "rustls", "tokio"] }
+tokio = "1"
+tor-rtcompat = "0.9"
+
 # ZIP 32
 aes = "0.8"
 fpe = "0.6"

deny.toml

@@ -31,16 +31,27 @@ allow = [
 ]
 exceptions = [
     { name = "arrayref", allow = ["BSD-2-Clause"] },
+    { name = "async_executors", allow = ["Unlicense"] },
+    { name = "bounded-vec-deque", allow = ["BSD-3-Clause"] },
+    { name = "coarsetime", allow = ["ISC"] },
+    { name = "curve25519-dalek", allow = ["BSD-3-Clause"] },
+    { name = "ed25519-dalek", allow = ["BSD-3-Clause"] },
     { name = "matchit", allow = ["BSD-3-Clause"] },
     { name = "minreq", allow = ["ISC"] },
+    { name = "option-ext", allow = ["MPL-2.0"] },
+    { name = "priority-queue", allow = ["MPL-2.0"] },
     { name = "ring", allow = ["LicenseRef-ring"] },
     { name = "rustls-webpki", allow = ["ISC"] },
     { name = "secp256k1", allow = ["CC0-1.0"] },
     { name = "secp256k1-sys", allow = ["CC0-1.0"] },
+    { name = "simple_asn1", allow = ["ISC"] },
+    { name = "slotmap", allow = ["Zlib"] },
     { name = "subtle", allow = ["BSD-3-Clause"] },
+    { name = "tinystr", allow = ["Unicode-3.0"] },
     { name = "unicode-ident", allow = ["Unicode-DFS-2016"] },
     { name = "untrusted", allow = ["ISC"] },
     { name = "webpki-roots", allow = ["MPL-2.0"] },
+    { name = "x25519-dalek", allow = ["BSD-3-Clause"] },
 ]
 
 [[licenses.clarify]]

zcash_client_backend/CHANGELOG.md

@@ -38,6 +38,7 @@ funds to those addresses. See [ZIP 320](https://zips.z.cash/zip-0320) for detail
 - `zcash_client_backend::scanning`:
   - `testing` module
 - `zcash_client_backend::sync` module, behind the `sync` feature flag.
+- `zcash_client_backend::tor` module, behind the `tor` feature flag.
 - `zcash_client_backend::wallet::Recipient::map_ephemeral_transparent_outpoint`
 
 ### Changed

zcash_client_backend/Cargo.toml

@@ -27,6 +27,7 @@ features = [
     "lightwalletd-tonic",
     "transparent-inputs",
     "test-dependencies",
+    "tor",
     "unstable",
     "unstable-serialization",
     "unstable-spanning-tree",
@@ -94,6 +95,12 @@ jubjub = { workspace = true, optional = true }
 # - ZIP 321
 nom = "7"
 
+# - Tor
+# -- Exposed error types: `arti_client::Error`, `arti_client::config::ConfigBuildError`,
+#    `hyper::Error`, `hyper::http::Error`, `serde_json::Error`. We could avoid this with
+#    changes to error handling.
+arti-client = { workspace = true, optional = true }
+
 # Dependencies used internally:
 # (Breaking upgrades to these are usually backwards-compatible, but check MSRVs.)
 # - Documentation
@@ -107,6 +114,10 @@ percent-encoding.workspace = true
 crossbeam-channel.workspace = true
 rayon.workspace = true
 
+# - Tor
+tokio = { workspace = true, optional = true, features = ["fs"] }
+tor-rtcompat = { workspace = true, optional = true }
+
 [build-dependencies]
 tonic-build = { workspace = true, features = ["prost"] }
 which = "4"
@@ -148,6 +159,14 @@ sync = [
     "dep:futures-util",
 ]
 
+## Exposes a Tor client for hiding a wallet's IP address while performing certain wallet
+## operations.
+tor = [
+    "dep:arti-client",
+    "dep:tokio",
+    "dep:tor-rtcompat",
+]
+
 ## Exposes APIs that are useful for testing, such as `proptest` strategies.
 test-dependencies = [
     "dep:proptest",

zcash_client_backend/src/lib.rs

@@ -80,6 +80,9 @@ pub mod sync;
 #[cfg(feature = "unstable-serialization")]
 pub mod serialization;
 
+#[cfg(feature = "tor")]
+pub mod tor;
+
 pub use decrypt::{decrypt_transaction, DecryptedOutput, TransferType};
 pub use zcash_protocol::{PoolType, ShieldedProtocol};
 

zcash_client_backend/src/tor.rs

@@ -0,0 +1,87 @@
+//! Tor support for Zcash wallets.
+
+use std::{fmt, io, path::Path};
+
+use arti_client::{config::TorClientConfigBuilder, TorClient};
+use tor_rtcompat::PreferredRuntime;
+use tracing::debug;
+
+/// A Tor client that exposes capabilities designed for Zcash wallets.
+pub struct Client {
+    inner: TorClient<PreferredRuntime>,
+}
+
+impl Client {
+    /// Creates and bootstraps a Tor client.
+    ///
+    /// The client's persistent data and cache are both stored in the given directory.
+    /// Preserving the contents of this directory will speed up subsequent calls to
+    /// `Client::create`.
+    ///
+    /// Returns an error if `tor_dir` does not exist, or if bootstrapping fails.
+    pub async fn create(tor_dir: &Path) -> Result<Self, Error> {
+        let runtime = PreferredRuntime::current()?;
+
+        if !tokio::fs::try_exists(tor_dir).await? {
+            return Err(Error::MissingTorDirectory);
+        }
+
+        let config = TorClientConfigBuilder::from_directories(
+            tor_dir.join("arti-data"),
+            tor_dir.join("arti-cache"),
+        )
+        .build()
+        .expect("all required fields initialized");
+
+        let client_builder = TorClient::with_runtime(runtime).config(config);
+
+        debug!("Bootstrapping Tor");
+        let inner = client_builder.create_bootstrapped().await?;
+        debug!("Tor bootstrapped");
+
+        Ok(Self { inner })
+    }
+}
+
+/// Errors that can occur while creating or using a Tor [`Client`].
+#[derive(Debug)]
+pub enum Error {
+    /// The directory passed to [`Client::create`] does not exist.
+    MissingTorDirectory,
+    /// An IO error occurred while interacting with the filesystem.
+    Io(io::Error),
+    /// A Tor-specific error.
+    Tor(arti_client::Error),
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::MissingTorDirectory => write!(f, "Tor directory is missing"),
+            Error::Io(e) => write!(f, "IO error: {}", e),
+            Error::Tor(e) => write!(f, "Tor error: {}", e),
+        }
+    }
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::MissingTorDirectory => None,
+            Error::Io(e) => Some(e),
+            Error::Tor(e) => Some(e),
+        }
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(e: io::Error) -> Self {
+        Error::Io(e)
+    }
+}
+
+impl From<arti_client::Error> for Error {
+    fn from(e: arti_client::Error) -> Self {
+        Error::Tor(e)
+    }
+}