diff --git a/src/bls12_381/ec.rs b/src/bls12_381/ec.rs
index a4eaa1a..16deca5 100644
--- a/src/bls12_381/ec.rs
+++ b/src/bls12_381/ec.rs
@@ -108,6 +108,7 @@ macro_rules! curve_impl {
         }
 
         impl CurveAffine for $affine {
+            type Engine = Bls12;
             type Scalar = $scalarfield;
             type Base = $basefield;
             type Prepared = $prepared;
@@ -174,6 +175,7 @@ macro_rules! curve_impl {
         }
 
         impl CurveProjective for $projective {
+            type Engine = Bls12;
             type Scalar = $scalarfield;
             type Base = $basefield;
             type Affine = $affine;
@@ -582,7 +584,7 @@ macro_rules! curve_impl {
 pub mod g1 {
     use rand::{Rand, Rng};
     use super::g2::G2Affine;
-    use super::super::{Fq, Fr, FrRepr, FqRepr, Fq12};
+    use super::super::{Bls12, Fq, Fr, FrRepr, FqRepr, Fq12};
     use ::{CurveProjective, CurveAffine, PrimeField, SqrtField, PrimeFieldRepr, Field, BitIterator, EncodedPoint, GroupDecodingError, Engine};
 
     curve_impl!("G1", G1, G1Affine, G1Prepared, Fq, Fr, G1Uncompressed, G1Compressed, G2Affine);
@@ -1134,7 +1136,7 @@ pub mod g1 {
 
 pub mod g2 {
     use rand::{Rand, Rng};
-    use super::super::{Fq2, Fr, Fq, FrRepr, FqRepr, Fq12};
+    use super::super::{Bls12, Fq2, Fr, Fq, FrRepr, FqRepr, Fq12};
     use super::g1::G1Affine;
     use ::{CurveProjective, CurveAffine, PrimeField, SqrtField, PrimeFieldRepr, Field, BitIterator, EncodedPoint, GroupDecodingError, Engine};
 
diff --git a/src/lib.rs b/src/lib.rs
index 96ea04b..55b0c19 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,21 +34,21 @@ use std::io::{self, Read, Write};
 /// An "engine" is a collection of types (fields, elliptic curve groups, etc.)
 /// with well-defined relationships. In particular, the G1/G2 curve groups are
 /// of prime order `r`, and are equipped with a bilinear pairing function.
-pub trait Engine {
+pub trait Engine: Sized {
     /// This is the scalar field of the G1/G2 groups.
     type Fr: PrimeField;
 
     /// The projective representation of an element in G1.
-    type G1: CurveProjective<Base=Self::Fq, Scalar=Self::Fr, Affine=Self::G1Affine> + From<Self::G1Affine>;
+    type G1: CurveProjective<Engine=Self, Base=Self::Fq, Scalar=Self::Fr, Affine=Self::G1Affine> + From<Self::G1Affine>;
 
     /// The affine representation of an element in G1.
-    type G1Affine: CurveAffine<Base=Self::Fq, Scalar=Self::Fr, Projective=Self::G1, Pair=Self::G2Affine, PairingResult=Self::Fqk> + From<Self::G1>;
+    type G1Affine: CurveAffine<Engine=Self, Base=Self::Fq, Scalar=Self::Fr, Projective=Self::G1, Pair=Self::G2Affine, PairingResult=Self::Fqk> + From<Self::G1>;
 
     /// The projective representation of an element in G2.
-    type G2: CurveProjective<Base=Self::Fqe, Scalar=Self::Fr, Affine=Self::G2Affine> + From<Self::G2Affine>;
+    type G2: CurveProjective<Engine=Self, Base=Self::Fqe, Scalar=Self::Fr, Affine=Self::G2Affine> + From<Self::G2Affine>;
 
     /// The affine representation of an element in G2.
-    type G2Affine: CurveAffine<Base=Self::Fqe, Scalar=Self::Fr, Projective=Self::G2, Pair=Self::G1Affine, PairingResult=Self::Fqk> + From<Self::G2>;
+    type G2Affine: CurveAffine<Engine=Self, Base=Self::Fqe, Scalar=Self::Fr, Projective=Self::G2, Pair=Self::G1Affine, PairingResult=Self::Fqk> + From<Self::G2>;
 
     /// The base field that hosts G1.
     type Fq: PrimeField + SqrtField;
@@ -97,6 +97,7 @@ pub trait CurveProjective: PartialEq +
                            rand::Rand +
                            'static
 {
+    type Engine: Engine;
     type Scalar: PrimeField;
     type Base: SqrtField;
     type Affine: CurveAffine<Projective=Self, Scalar=Self::Scalar>;
@@ -166,6 +167,7 @@ pub trait CurveAffine: Copy +
                        Eq +
                        'static
 {
+    type Engine: Engine;
     type Scalar: PrimeField;
     type Base: SqrtField;
     type Projective: CurveProjective<Affine=Self, Scalar=Self::Scalar>;