Make serialization less wasteful.

This removes the unneeded length from key and signature representation,
removing 8 bytes from each. Also adds `from_bytes` and `to_bytes`
methods to convert keys and signatures.

src/error.rs

@@ -28,3 +28,13 @@ mod tests {
         is_send_and_sync(Error::NotEnoughShares);
     }
 }
+
+/// An error reading a structure from an array of bytes.
+#[derive(Clone, Eq, PartialEq, Debug, Fail)]
+pub enum FromBytesError {
+    #[fail(display = "Invalid representation.")]
+    Invalid,
+}
+
+/// The result of attempting to read a structure from an array of bytes.
+pub type FromBytesResult<T> = ::std::result::Result<T, FromBytesError>;

src/lib.rs

@@ -44,29 +44,37 @@ use byteorder::{BigEndian, ByteOrder};
 use hex_fmt::HexFmt;
 use init_with::InitWith;
 use log::debug;
-use pairing::{CurveAffine, CurveProjective, Engine, Field};
+use pairing::{CurveAffine, CurveProjective, EncodedPoint, Engine, Field};
 use rand::{ChaChaRng, OsRng, Rand, Rng, SeedableRng};
 use rand_derive::Rand;
 use tiny_keccak::sha3_256;
 
-use error::{Error, Result};
+use error::{Error, FromBytesError, FromBytesResult, Result};
 use into_fr::IntoFr;
 use poly::{Commitment, Poly};
 use secret::{clear_fr, ContainsSecret, MemRange, FR_SIZE};
 use serde_derive::{Deserialize, Serialize};
 
 #[cfg(not(feature = "use-insecure-test-only-mock-crypto"))]
-pub use pairing::bls12_381::{Bls12 as PEngine, Fr, G1Affine, G2Affine, G1, G2};
+pub use pairing::bls12_381::{Bls12 as PEngine, Fr, FrRepr, G1Affine, G2Affine, G1, G2};
 
 #[cfg(feature = "use-insecure-test-only-mock-crypto")]
 mod mock;
 
 #[cfg(feature = "use-insecure-test-only-mock-crypto")]
 pub use mock::{
-    Mersenne8 as Fr, Mocktography as PEngine, Ms8Affine as G1Affine, Ms8Affine as G2Affine,
-    Ms8Projective as G1, Ms8Projective as G2,
+    Mersenne8 as Fr, Mersenne8 as FrRepr, Mocktography as PEngine, Ms8Affine as G1Affine,
+    Ms8Affine as G2Affine, Ms8Projective as G1, Ms8Projective as G2, PK_SIZE, SIG_SIZE,
 };
 
+/// The size of a key's representation in bytes.
+#[cfg(not(feature = "use-insecure-test-only-mock-crypto"))]
+pub const PK_SIZE: usize = 48;
+
+/// The size of a signature's representation in bytes.
+#[cfg(not(feature = "use-insecure-test-only-mock-crypto"))]
+pub const SIG_SIZE: usize = 96;
+
 /// The number of words (`u32`) in a ChaCha RNG seed.
 const CHACHA_RNG_SEED_SIZE: usize = 8;
 
@@ -122,9 +130,20 @@ impl PublicKey {
         Ciphertext(u, v, w)
     }
 
+    /// Returns the key with the given representation, if valid.
+    pub fn from_bytes<B: Borrow<[u8; PK_SIZE]>>(bytes: B) -> FromBytesResult<Self> {
+        let mut compressed: <G1Affine as CurveAffine>::Compressed = EncodedPoint::empty();
+        compressed.as_mut().copy_from_slice(bytes.borrow());
+        let opt_affine = compressed.into_affine().ok();
+        let projective = opt_affine.ok_or(FromBytesError::Invalid)?.into_projective();
+        Ok(PublicKey(projective))
+    }
+
     /// Returns a byte string representation of the public key.
-    pub fn to_bytes(&self) -> Vec<u8> {
-        self.0.into_affine().into_compressed().as_ref().to_vec()
+    pub fn to_bytes(&self) -> [u8; PK_SIZE] {
+        let mut bytes = [0u8; PK_SIZE];
+        bytes.copy_from_slice(self.0.into_affine().into_compressed().as_ref());
+        bytes
     }
 }
 
@@ -159,8 +178,13 @@ impl PublicKeyShare {
         PEngine::pairing(share.0, hash) == PEngine::pairing((self.0).0, *w)
     }
 
+    /// Returns the key share with the given representation, if valid.
+    pub fn from_bytes<B: Borrow<[u8; PK_SIZE]>>(bytes: B) -> FromBytesResult<Self> {
+        Ok(PublicKeyShare(PublicKey::from_bytes(bytes)?))
+    }
+
     /// Returns a byte string representation of the public key share.
-    pub fn to_bytes(&self) -> Vec<u8> {
+    pub fn to_bytes(&self) -> [u8; PK_SIZE] {
         self.0.to_bytes()
     }
 }
@@ -191,6 +215,22 @@ impl Signature {
         debug!("Signature: {:0.10}, parity: {}", HexFmt(uncomp), parity);
         parity
     }
+
+    /// Returns the signature with the given representation, if valid.
+    pub fn from_bytes<B: Borrow<[u8; SIG_SIZE]>>(bytes: B) -> FromBytesResult<Self> {
+        let mut compressed: <G2Affine as CurveAffine>::Compressed = EncodedPoint::empty();
+        compressed.as_mut().copy_from_slice(bytes.borrow());
+        let opt_affine = compressed.into_affine().ok();
+        let projective = opt_affine.ok_or(FromBytesError::Invalid)?.into_projective();
+        Ok(Signature(projective))
+    }
+
+    /// Returns a byte string representation of the signature.
+    pub fn to_bytes(&self) -> [u8; SIG_SIZE] {
+        let mut bytes = [0u8; SIG_SIZE];
+        bytes.copy_from_slice(self.0.into_affine().into_compressed().as_ref());
+        bytes
+    }
 }
 
 /// A signature share.
@@ -205,6 +245,18 @@ impl fmt::Debug for SignatureShare {
     }
 }
 
+impl SignatureShare {
+    /// Returns the signature share with the given representation, if valid.
+    pub fn from_bytes<B: Borrow<[u8; SIG_SIZE]>>(bytes: B) -> FromBytesResult<Self> {
+        Ok(SignatureShare(Signature::from_bytes(bytes)?))
+    }
+
+    /// Returns a byte string representation of the signature share.
+    pub fn to_bytes(&self) -> [u8; SIG_SIZE] {
+        self.0.to_bytes()
+    }
+}
+
 /// A secret key; wraps a single prime field element. The field element is
 /// heap allocated to avoid any stack copying that result when passing
 /// `SecretKey`s between stack frames.
@@ -827,6 +879,17 @@ mod tests {
         assert_eq!(20, xwh(g0, &[0; 20]).len());
     }
 
+    #[test]
+    fn test_from_to_bytes() {
+        let sk: SecretKey = random();
+        let sig = sk.sign("Please sign here: ______");
+        let pk = sk.public_key();
+        let pk2 = PublicKey::from_bytes(pk.to_bytes()).expect("invalid pk representation");
+        assert_eq!(pk, pk2);
+        let sig2 = Signature::from_bytes(sig.to_bytes()).expect("invalid sig representation");
+        assert_eq!(sig, sig2);
+    }
+
     #[test]
     fn test_serde() {
         use bincode;
@@ -836,9 +899,17 @@ mod tests {
         let pk = sk.public_key();
         let ser_pk = bincode::serialize(&pk).expect("serialize public key");
         let deser_pk = bincode::deserialize(&ser_pk).expect("deserialize public key");
+        assert_eq!(ser_pk.len(), PK_SIZE);
         assert_eq!(pk, deser_pk);
         let ser_sig = bincode::serialize(&sig).expect("serialize signature");
         let deser_sig = bincode::deserialize(&ser_sig).expect("deserialize signature");
+        assert_eq!(ser_sig.len(), SIG_SIZE);
         assert_eq!(sig, deser_sig);
     }
+
+    #[test]
+    fn test_size() {
+        assert_eq!(<G1Affine as CurveAffine>::Compressed::size(), PK_SIZE);
+        assert_eq!(<G2Affine as CurveAffine>::Compressed::size(), SIG_SIZE);
+    }
 }

src/mock/mod.rs

@@ -21,6 +21,11 @@ use super::{CurveAffine, CurveProjective, Engine};
 
 pub use self::ms8::Mersenne8;
 
+/// The size of a key's representation in bytes.
+pub const PK_SIZE: usize = 4;
+/// The size of a signature's representation in bytes.
+pub const SIG_SIZE: usize = 4;
+
 /// A `pairing` Engine based on `Mersenne8` prime fields.
 #[derive(Clone, Debug)]
 pub struct Mocktography;
@@ -292,7 +297,7 @@ mod test {
     // There are copy & pasted results of calculations from external programs in these tests.
     #![cfg_attr(feature = "cargo-clippy", allow(unreadable_literal))]
 
-    use super::{Mersenne8, Mocktography, Ms8Affine};
+    use super::{EncodedPoint, Mersenne8, Mocktography, Ms8Affine, PK_SIZE, SIG_SIZE};
     use Engine;
 
     #[test]
@@ -319,4 +324,10 @@ mod test {
             assert_eq!(Mocktography::pairing(q, p), Mersenne8::new(res));
         }
     }
+
+    #[test]
+    fn size() {
+        assert_eq!(<Ms8Affine as EncodedPoint>::size(), PK_SIZE);
+        assert_eq!(<Ms8Affine as EncodedPoint>::size(), SIG_SIZE);
+    }
 }

src/serde_impl.rs

@@ -1,9 +1,9 @@
 use std::borrow::Cow;
 
-use super::G1;
 use serde::de::Error as DeserializeError;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use serde_derive::{Deserialize, Serialize};
+use G1;
 
 use poly::{coeff_pos, BivarCommitment};
 
@@ -43,11 +43,13 @@ impl<'de> Deserialize<'de> for BivarCommitment {
 
 /// Serialization and deserialization of a group element's compressed representation.
 pub mod projective {
+    use std::fmt;
+    use std::marker::PhantomData;
+
     use pairing::{CurveAffine, CurveProjective, EncodedPoint};
-    use serde::de::Error as DeserializeError;
-    use serde::{Deserialize, Deserializer, Serialize, Serializer};
+    use serde::de::{Error as DeserializeError, SeqAccess, Visitor};
+    use serde::{ser::SerializeTuple, Deserializer, Serializer};
 
-    const ERR_LEN: &str = "wrong length of deserialized group element";
     const ERR_CODE: &str = "deserialized bytes don't encode a group element";
 
     pub fn serialize<S, C>(c: &C, s: S) -> Result<S::Ok, S::Error>
@@ -55,7 +57,12 @@ pub mod projective {
         S: Serializer,
         C: CurveProjective,
     {
-        c.into_affine().into_compressed().as_ref().serialize(s)
+        let len = <C::Affine as CurveAffine>::Compressed::size();
+        let mut tup = s.serialize_tuple(len)?;
+        for byte in c.into_affine().into_compressed().as_ref() {
+            tup.serialize_element(byte)?;
+        }
+        tup.end()
     }
 
     pub fn deserialize<'de, D, C>(d: D) -> Result<C, D::Error>
@@ -63,14 +70,32 @@ pub mod projective {
         D: Deserializer<'de>,
         C: CurveProjective,
     {
-        let bytes = <Vec<u8>>::deserialize(d)?;
-        if bytes.len() != <C::Affine as CurveAffine>::Compressed::size() {
-            return Err(D::Error::custom(ERR_LEN));
+        struct TupleVisitor<C> {
+            _ph: PhantomData<C>,
         }
-        let mut compressed = <C::Affine as CurveAffine>::Compressed::empty();
-        compressed.as_mut().copy_from_slice(&bytes);
-        let to_err = |_| D::Error::custom(ERR_CODE);
-        Ok(compressed.into_affine().map_err(to_err)?.into_projective())
+
+        impl<'de, C: CurveProjective> Visitor<'de> for TupleVisitor<C> {
+            type Value = C;
+
+            fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                let len = <C::Affine as CurveAffine>::Compressed::size();
+                write!(f, "a tuple of size {}", len)
+            }
+
+            #[inline]
+            fn visit_seq<A: SeqAccess<'de>>(self, mut seq: A) -> Result<C, A::Error> {
+                let mut compressed = <C::Affine as CurveAffine>::Compressed::empty();
+                for (i, byte) in compressed.as_mut().iter_mut().enumerate() {
+                    let len_err = || DeserializeError::invalid_length(i, &self);
+                    *byte = seq.next_element()?.ok_or_else(len_err)?;
+                }
+                let to_err = |_| DeserializeError::custom(ERR_CODE);
+                Ok(compressed.into_affine().map_err(to_err)?.into_projective())
+            }
+        }
+
+        let len = <C::Affine as CurveAffine>::Compressed::size();
+        d.deserialize_tuple(len, TupleVisitor { _ph: PhantomData })
     }
 }
 
@@ -130,90 +155,66 @@ pub mod projective_vec {
 /// Serialization and deserialization of vectors of field elements.
 pub mod field_vec {
     use std::borrow::Borrow;
-    use std::marker::PhantomData;
 
-    use pairing::{PrimeField, PrimeFieldRepr};
+    use pairing::PrimeField;
     use serde::de::Error as DeserializeError;
-    use serde::ser::Error as SerializeError;
     use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
-    /// A wrapper type to facilitate serialization and deserialization of field elements.
-    pub struct FieldWrap<F, B>(B, PhantomData<F>);
+    use {Fr, FrRepr};
 
-    impl<F, B> FieldWrap<F, B> {
-        pub fn new(f: B) -> Self {
-            FieldWrap(f, PhantomData)
-        }
-    }
+    /// A wrapper type to facilitate serialization and deserialization of field elements.
+    pub struct FieldWrap<B>(B);
 
-    impl<F> FieldWrap<F, F> {
-        pub fn into_inner(self) -> F {
+    impl FieldWrap<Fr> {
+        pub fn into_inner(self) -> Fr {
             self.0
         }
     }
 
-    impl<F: PrimeField, B: Borrow<F>> Serialize for FieldWrap<F, B> {
+    impl<B: Borrow<Fr>> Serialize for FieldWrap<B> {
         fn serialize<S: Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
-            let mut bytes = Vec::new();
-            self.0
-                .borrow()
-                .into_repr()
-                .write_be(&mut bytes)
-                .map_err(|_| S::Error::custom("failed to write bytes"))?;
-            bytes.serialize(s)
+            self.0.borrow().into_repr().0.serialize(s)
         }
     }
 
-    impl<'de, F: PrimeField> Deserialize<'de> for FieldWrap<F, F> {
+    impl<'de> Deserialize<'de> for FieldWrap<Fr> {
         fn deserialize<D: Deserializer<'de>>(d: D) -> Result<Self, D::Error> {
-            let bytes: Vec<u8> = Deserialize::deserialize(d)?;
-            let mut repr = F::zero().into_repr();
-            repr.read_be(&bytes[..])
-                .map_err(|_| D::Error::custom("failed to write bytes"))?;
-            Ok(FieldWrap::new(F::from_repr(repr).map_err(|_| {
+            let repr = FrRepr(Deserialize::deserialize(d)?);
+            Ok(FieldWrap(Fr::from_repr(repr).map_err(|_| {
                 D::Error::custom("invalid field element representation")
             })?))
         }
     }
 
-    pub fn serialize<S, F>(vec: &[F], s: S) -> Result<S::Ok, S::Error>
-    where
-        S: Serializer,
-        F: PrimeField,
-    {
-        let wrap_vec: Vec<FieldWrap<F, &F>> = vec.iter().map(FieldWrap::new).collect();
+    pub fn serialize<S: Serializer>(vec: &[Fr], s: S) -> Result<S::Ok, S::Error> {
+        let wrap_vec: Vec<FieldWrap<&Fr>> = vec.iter().map(FieldWrap).collect();
         wrap_vec.serialize(s)
     }
 
-    pub fn deserialize<'de, D, F>(d: D) -> Result<Vec<F>, D::Error>
-    where
-        D: Deserializer<'de>,
-        F: PrimeField,
-    {
-        let wrap_vec = <Vec<FieldWrap<F, F>>>::deserialize(d)?;
-        Ok(wrap_vec.into_iter().map(|FieldWrap(f, _)| f).collect())
+    pub fn deserialize<'de, D: Deserializer<'de>>(d: D) -> Result<Vec<Fr>, D::Error> {
+        let wrap_vec = <Vec<FieldWrap<Fr>>>::deserialize(d)?;
+        Ok(wrap_vec.into_iter().map(FieldWrap::into_inner).collect())
     }
 }
 
 #[cfg(test)]
 mod tests {
-    use super::super::PEngine;
     use bincode;
-    use pairing::Engine;
     use rand::{self, Rng};
     use serde_derive::{Deserialize, Serialize};
 
     use poly::BivarPoly;
+    use {Fr, G1};
 
     #[derive(Debug, Serialize, Deserialize)]
-    pub struct Vecs<E: Engine> {
+    pub struct Vecs {
         #[serde(with = "super::projective_vec")]
-        curve_points: Vec<E::G1>,
+        curve_points: Vec<G1>,
         #[serde(with = "super::field_vec")]
-        field_elements: Vec<E::Fr>,
+        field_elements: Vec<Fr>,
     }
 
-    impl<E: Engine> PartialEq for Vecs<E> {
+    impl PartialEq for Vecs {
         fn eq(&self, other: &Self) -> bool {
             self.curve_points == other.curve_points && self.field_elements == other.field_elements
         }
@@ -222,7 +223,7 @@ mod tests {
     #[test]
     fn vecs() {
         let mut rng = rand::thread_rng();
-        let vecs: Vecs<PEngine> = Vecs {
+        let vecs = Vecs {
             curve_points: rng.gen_iter().take(10).collect(),
             field_elements: rng.gen_iter().take(10).collect(),
         };