zcash_proofs: Add API for downloading the Sapling parameters

Includes an example that exposes the API as a binary.

zcash_proofs/Cargo.toml

@@ -17,6 +17,7 @@ blake2b_simd = "0.5"
 byteorder = "1"
 directories = { version = "1", optional = true }
 ff = { version = "0.6", path = "../ff" }
+minreq = { version = "2", features = ["https"], optional = true }
 pairing = { version = "0.16", path = "../pairing" }
 rand_core = "0.5.1"
 zcash_primitives = { version = "0.2", path = "../zcash_primitives" }
@@ -26,8 +27,13 @@ rand_xorshift = "0.2"
 
 [features]
 default = ["local-prover", "multicore"]
+download-params = ["minreq"]
 local-prover = ["directories"]
 multicore = ["bellman/multicore"]
 
+[[example]]
+name = "download-params"
+required-features = ["download-params"]
+
 [badges]
 maintenance = { status = "actively-developed" }

zcash_proofs/examples/download-params.rs

@@ -0,0 +1,3 @@
+fn main() -> Result<(), minreq::Error> {
+    zcash_proofs::download_parameters()
+}

zcash_proofs/src/lib.rs

@@ -29,6 +29,14 @@ pub mod prover;
 const SAPLING_SPEND_NAME: &str = "sapling-spend.params";
 const SAPLING_OUTPUT_NAME: &str = "sapling-output.params";
 
+// Circuit hashes
+const SAPLING_SPEND_HASH: &str = "8270785a1a0d0bc77196f000ee6d221c9c9894f55307bd9357c3f0105d31ca63991ab91324160d8f53e2bbd3c2633a6eb8bdf5205d822e7f3f73edac51b2b70c";
+const SAPLING_OUTPUT_HASH: &str = "657e3d38dbb5cb5e7dd2970e8b03d69b4787dd907285b5a7f0790dcc8072f60bf593b32cc2d1c030e00ff5ae64bf84c5c3beb84ddc841d48264b4a171744d028";
+const SPROUT_HASH: &str = "e9b238411bd6c0ec4791e9d04245ec350c9c5744f5610dfcce4365d5ca49dfefd5054e371842b3f88fa1b9d7e8e075249b3ebabd167fa8b0f3161292d36c180a";
+
+#[cfg(feature = "download-params")]
+const DOWNLOAD_URL: &str = "https://download.z.cash/downloads";
+
 /// Returns the default folder that the Zcash proving parameters are located in.
 #[cfg(feature = "directories")]
 fn default_params_folder() -> Option<PathBuf> {
@@ -41,6 +49,58 @@ fn default_params_folder() -> Option<PathBuf> {
     })
 }
 
+/// Download the Zcash Sapling parameters, storing them in the default location.
+///
+/// This mirrors the behaviour of the `fetch-params.sh` script from `zcashd`.
+#[cfg(feature = "download-params")]
+pub fn download_parameters() -> Result<(), minreq::Error> {
+    // Ensure that the default Zcash parameters location exists.
+    let params_dir = default_params_folder().ok_or(io::Error::new(
+        io::ErrorKind::Other,
+        "Could not load default params folder",
+    ))?;
+    std::fs::create_dir_all(&params_dir)?;
+
+    let fetch_params = |name: &str, expected_hash: &str| -> Result<(), minreq::Error> {
+        use std::io::Write;
+
+        // Download the parts directly (Sapling parameters are small enough for this).
+        let part_1 = minreq::get(format!("{}/{}.part.1", DOWNLOAD_URL, name)).send()?;
+        let part_2 = minreq::get(format!("{}/{}.part.2", DOWNLOAD_URL, name)).send()?;
+
+        // Verify parameter file hash.
+        let hash = blake2b_simd::State::new()
+            .update(part_1.as_bytes())
+            .update(part_2.as_bytes())
+            .finalize()
+            .to_hex();
+        if &hash != expected_hash {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                format!(
+                    "{} failed validation (expected: {}, actual: {}, fetched {} bytes)",
+                    name,
+                    expected_hash,
+                    hash,
+                    part_1.as_bytes().len() + part_2.as_bytes().len()
+                ),
+            )
+            .into());
+        }
+
+        // Write parameter file.
+        let mut f = File::create(params_dir.join(name))?;
+        f.write_all(part_1.as_bytes())?;
+        f.write_all(part_2.as_bytes())?;
+        Ok(())
+    };
+
+    fetch_params(SAPLING_SPEND_NAME, SAPLING_SPEND_HASH)?;
+    fetch_params(SAPLING_OUTPUT_NAME, SAPLING_OUTPUT_HASH)?;
+
+    Ok(())
+}
+
 pub fn load_parameters(
     spend_path: &Path,
     output_path: &Path,
@@ -52,11 +112,6 @@ pub fn load_parameters(
     PreparedVerifyingKey<Bls12>,
     Option<PreparedVerifyingKey<Bls12>>,
 ) {
-    // Sapling circuit hashes
-    const SAPLING_SPEND_HASH: &str = "8270785a1a0d0bc77196f000ee6d221c9c9894f55307bd9357c3f0105d31ca63991ab91324160d8f53e2bbd3c2633a6eb8bdf5205d822e7f3f73edac51b2b70c";
-    const SAPLING_OUTPUT_HASH: &str = "657e3d38dbb5cb5e7dd2970e8b03d69b4787dd907285b5a7f0790dcc8072f60bf593b32cc2d1c030e00ff5ae64bf84c5c3beb84ddc841d48264b4a171744d028";
-    const SPROUT_HASH: &str = "e9b238411bd6c0ec4791e9d04245ec350c9c5744f5610dfcce4365d5ca49dfefd5054e371842b3f88fa1b9d7e8e075249b3ebabd167fa8b0f3161292d36c180a";
-
     // Load from each of the paths
     let spend_fs = File::open(spend_path).expect("couldn't load Sapling spend parameters file");
     let output_fs = File::open(output_path).expect("couldn't load Sapling output parameters file");