Implement Hash for "public" cryptographic types.

This adds a `Hash` implementation for public keys, commitments,
ciphertexts and signatures — types that might make sense to be included
in special transactions. The `DynamicHoneyBadger` implementation will
require some of them.

mod.rs

@@ -5,6 +5,7 @@ pub mod protobuf_impl;
 mod serde_impl;
 
 use std::fmt;
+use std::hash::{Hash, Hasher};
 
 use byteorder::{BigEndian, ByteOrder};
 use init_with::InitWith;
@@ -31,6 +32,12 @@ impl<E: Engine> PartialEq for PublicKey<E> {
     }
 }
 
+impl<E: Engine> Hash for PublicKey<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.into_affine().into_compressed().as_ref().hash(state);
+    }
+}
+
 impl<E: Engine> PublicKey<E> {
     /// Returns `true` if the signature matches the element of `E::G2`.
     pub fn verify_g2<H: Into<E::G2Affine>>(&self, sig: &Signature<E>, hash: H) -> bool {
@@ -85,6 +92,12 @@ impl<E: Engine> PartialEq for Signature<E> {
     }
 }
 
+impl<E: Engine> Hash for Signature<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.into_affine().into_compressed().as_ref().hash(state);
+    }
+}
+
 impl<E: Engine> Signature<E> {
     pub fn parity(&self) -> bool {
         let uncomp = self.0.into_affine().into_uncompressed();
@@ -160,6 +173,14 @@ impl<E: Engine> PartialEq for Ciphertext<E> {
     }
 }
 
+impl<E: Engine> Hash for Ciphertext<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.into_affine().into_compressed().as_ref().hash(state);
+        self.1.hash(state);
+        self.2.into_affine().into_compressed().as_ref().hash(state);
+    }
+}
+
 impl<E: Engine> Ciphertext<E> {
     /// Returns `true` if this is a valid ciphertext. This check is necessary to prevent
     /// chosen-ciphertext attacks.
@@ -180,8 +201,14 @@ impl<E: Engine> PartialEq for DecryptionShare<E> {
     }
 }
 
+impl<E: Engine> Hash for DecryptionShare<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.into_affine().into_compressed().as_ref().hash(state);
+    }
+}
+
 /// A public key and an associated set of public key shares.
-#[derive(Serialize, Deserialize, Clone, Debug)]
+#[derive(Serialize, Deserialize, Clone, Debug, Hash)]
 pub struct PublicKeySet<E: Engine> {
     /// The coefficients of a polynomial whose value at `0` is the "master key", and value at
     /// `i + 1` is key share number `i`.

poly.rs

@@ -20,6 +20,7 @@
 // TODO: Expand this explanation and add examples, once the API is complete and stable.
 
 use std::borrow::Borrow;
+use std::hash::{Hash, Hasher};
 use std::{cmp, iter, ops};
 
 use pairing::{CurveAffine, CurveProjective, Engine, Field, PrimeField};
@@ -261,6 +262,15 @@ impl<E: Engine> PartialEq for Commitment<E> {
     }
 }
 
+impl<E: Engine> Hash for Commitment<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.coeff.len().hash(state);
+        for c in &self.coeff {
+            c.into_affine().into_compressed().as_ref().hash(state);
+        }
+    }
+}
+
 impl<B: Borrow<Commitment<E>>, E: Engine> ops::AddAssign<B> for Commitment<E> {
     fn add_assign(&mut self, rhs: B) {
         let len = cmp::max(self.coeff.len(), rhs.borrow().coeff.len());
@@ -403,6 +413,15 @@ pub struct BivarCommitment<E: Engine> {
     coeff: Vec<E::G1>,
 }
 
+impl<E: Engine> Hash for BivarCommitment<E> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.degree.hash(state);
+        for c in &self.coeff {
+            c.into_affine().into_compressed().as_ref().hash(state);
+        }
+    }
+}
+
 impl<E: Engine> BivarCommitment<E> {
     /// Returns the polynomial's degree: It is the same in both variables.
     pub fn degree(&self) -> usize {