diff --git a/Cargo.lock b/Cargo.lock
index a754c35..3870758 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -230,6 +230,7 @@ dependencies = [
  "pairing 0.14.2",
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "sapling-crypto 0.0.1",
+ "zcash_primitives 0.0.0",
  "zcash_proofs 0.0.0",
  "zip32 0.0.0",
 ]
diff --git a/librustzcash/Cargo.toml b/librustzcash/Cargo.toml
index c2ab7ff..e633152 100644
--- a/librustzcash/Cargo.toml
+++ b/librustzcash/Cargo.toml
@@ -22,6 +22,7 @@ lazy_static = "1"
 byteorder = "1"
 rand = "0.4"
 sapling-crypto = { path = "../sapling-crypto" }
+zcash_primitives = { path = "../zcash_primitives" }
 zcash_proofs = { path = "../zcash_proofs" }
 zip32 = { path = "../zip32" }
 
diff --git a/librustzcash/src/rustzcash.rs b/librustzcash/src/rustzcash.rs
index f264690..29df19f 100644
--- a/librustzcash/src/rustzcash.rs
+++ b/librustzcash/src/rustzcash.rs
@@ -6,6 +6,7 @@ extern crate libc;
 extern crate pairing;
 extern crate rand;
 extern crate sapling_crypto;
+extern crate zcash_primitives;
 extern crate zcash_proofs;
 extern crate zip32;
 
@@ -61,6 +62,7 @@ use std::ffi::OsString;
 use std::os::windows::ffi::OsStringExt;
 
 use sapling_crypto::primitives::{ProofGenerationKey, ViewingKey};
+use zcash_primitives::sapling::spend_sig;
 use zcash_proofs::sapling::{
     CommitmentTreeWitness, SaplingProvingContext, SaplingVerificationContext,
 };
@@ -1069,27 +1071,8 @@ pub extern "system" fn librustzcash_sapling_spend_sig(
         Err(_) => return false,
     };
 
-    // We compute `rsk`...
-    let rsk = ask.randomize(ar);
-
-    // We compute `rk` from there (needed for key prefixing)
-    let rk =
-        redjubjub::PublicKey::from_private(&rsk, FixedGenerators::SpendingKeyGenerator, &JUBJUB);
-
-    // Compute the signature's message for rk/spend_auth_sig
-    let mut data_to_be_signed = [0u8; 64];
-    rk.0.write(&mut data_to_be_signed[0..32])
-        .expect("message buffer should be 32 bytes");
-    (&mut data_to_be_signed[32..64]).copy_from_slice(&(unsafe { &*sighash })[..]);
-
     // Do the signing
-    let mut rng = OsRng::new().expect("should be able to construct RNG");
-    let sig = rsk.sign(
-        &data_to_be_signed,
-        &mut rng,
-        FixedGenerators::SpendingKeyGenerator,
-        &JUBJUB,
-    );
+    let sig = spend_sig(ask, ar, unsafe { &*sighash }, &JUBJUB);
 
     // Write out the signature
     sig.write(&mut (unsafe { &mut *result })[..])
diff --git a/zcash_primitives/src/lib.rs b/zcash_primitives/src/lib.rs
index 6a84cc3..e053e50 100644
--- a/zcash_primitives/src/lib.rs
+++ b/zcash_primitives/src/lib.rs
@@ -10,6 +10,7 @@ extern crate sapling_crypto;
 
 use sapling_crypto::jubjub::JubjubBls12;
 
+pub mod sapling;
 mod serialize;
 pub mod transaction;
 
diff --git a/zcash_primitives/src/sapling.rs b/zcash_primitives/src/sapling.rs
new file mode 100644
index 0000000..b59dc2b
--- /dev/null
+++ b/zcash_primitives/src/sapling.rs
@@ -0,0 +1,37 @@
+use pairing::bls12_381::Bls12;
+use rand::OsRng;
+use sapling_crypto::{
+    jubjub::{fs::Fs, FixedGenerators, JubjubBls12},
+    redjubjub::{PrivateKey, PublicKey, Signature},
+};
+
+/// Create the spendAuthSig for a Sapling SpendDescription.
+pub fn spend_sig(
+    ask: PrivateKey<Bls12>,
+    ar: Fs,
+    sighash: &[u8; 32],
+    params: &JubjubBls12,
+) -> Signature {
+    // Initialize secure RNG
+    let mut rng = OsRng::new().expect("should be able to construct RNG");
+
+    // We compute `rsk`...
+    let rsk = ask.randomize(ar);
+
+    // We compute `rk` from there (needed for key prefixing)
+    let rk = PublicKey::from_private(&rsk, FixedGenerators::SpendingKeyGenerator, params);
+
+    // Compute the signature's message for rk/spend_auth_sig
+    let mut data_to_be_signed = [0u8; 64];
+    rk.0.write(&mut data_to_be_signed[0..32])
+        .expect("message buffer should be 32 bytes");
+    (&mut data_to_be_signed[32..64]).copy_from_slice(&sighash[..]);
+
+    // Do the signing
+    rsk.sign(
+        &data_to_be_signed,
+        &mut rng,
+        FixedGenerators::SpendingKeyGenerator,
+        params,
+    )
+}