feat: RistrettoSecretKey inversion (#173)

Adds multiplicative inversion to `RistrettoSecretKey`. Adds a few unit
tests.

Note that the underlying `invert` functionality from `curve25519-dalek`
does not assert that the `Scalar` in question is nonzero, but requires
the caller to check this; inverting zero is undefined. This PR
specifically checks this, and as a result, the inversion returns an
`Option`. This is probably annoying for the caller, but might stop bad
things from happening.

src/ristretto/ristretto_keys.rs

@@ -138,6 +138,16 @@ impl RistrettoSecretKey {
     pub fn reveal(&self) -> RevealedSecretKey<'_> {
         RevealedSecretKey { secret: self }
     }
+
+    /// Get the multiplicative inverse of a nonzero secret key
+    /// If zero is passed, returns `None`; annoying, but a useful guardrail
+    pub fn invert(&self) -> Option<Self> {
+        if self.0 == Scalar::zero() {
+            None
+        } else {
+            Some(RistrettoSecretKey(self.0.invert()))
+        }
+    }
 }
 
 impl<'a> fmt::Display for RevealedSecretKey<'a> {
@@ -991,6 +1001,29 @@ mod test {
         assert_eq!(p.compressed.get().unwrap().as_bytes(), &zeros); // check directly for good measure
     }
 
+    #[test]
+    fn test_inverse() {
+        // 0^{-1} is undefined
+        assert!(RistrettoSecretKey::from(0u64).invert().is_none());
+
+        // 1^{-1} == 1
+        assert_eq!(
+            RistrettoSecretKey::from(1u64).invert(),
+            Some(RistrettoSecretKey::from(1u64))
+        );
+
+        // 2^{-1}*2 == 1
+        assert!(RistrettoSecretKey::from(2u64).invert().is_some());
+        assert_ne!(
+            RistrettoSecretKey::from(2u64).invert(),
+            Some(RistrettoSecretKey::from(2u64))
+        );
+        assert_eq!(
+            RistrettoSecretKey::from(2u64).invert().unwrap() * RistrettoSecretKey::from(2u64),
+            RistrettoSecretKey::from(1u64)
+        );
+    }
+
     #[cfg(feature = "borsh")]
     mod borsh {
         use borsh::{BorshDeserialize, BorshSerialize};