btcec: add deep copy method for public keys

btcec/field.go

@@ -1,6 +1,8 @@
 package btcec
 
-import secp "github.com/decred/dcrd/dcrec/secp256k1/v4"
+import (
+	secp "github.com/decred/dcrd/dcrec/secp256k1/v4"
+)
 
 // FieldVal implements optimized fixed-precision arithmetic over the secp256k1
 // finite field. This means all arithmetic is performed modulo

btcec/pubkey.go

@@ -52,6 +52,15 @@ func NewPublicKey(x, y *FieldVal) *PublicKey {
 	return secp.NewPublicKey(x, y)
 }
 
+// CopyPublicKey returns a deep copy of the public key.
+func CopyPublicKey(pk *PublicKey) *PublicKey {
+	var result secp.JacobianPoint
+	pk.AsJacobian(&result)
+	result.ToAffine()
+
+	return NewPublicKey(&result.X, &result.Y)
+}
+
 // SerializedKey is a type for representing a public key in its compressed
 // serialized form.
 //

btcec/pubkey_test.go

@@ -6,7 +6,9 @@ package btcec
 
 import (
 	"bytes"
+	"math/rand"
 	"testing"
+	"testing/quick"
 
 	"github.com/davecgh/go-spew/spew"
 )
@@ -292,3 +294,61 @@ func TestIsCompressed(t *testing.T) {
 		}
 	}
 }
+
+// TestCopyPublicKeyProperties tests that the CopyPublicKey function deep
+// copies the original public key and that the copy is equal to the original.
+func TestCopyPublicKeyProperties(t *testing.T) {
+	f := func(x, y [32]byte) bool {
+		// Convert byte slices to FieldVals.
+		xf := new(FieldVal)
+		yf := new(FieldVal)
+		if overflow := xf.SetByteSlice(x[:]); overflow {
+			// Only check the function for valid coordinates.
+			return true
+		}
+
+		if overflow := xf.SetByteSlice(y[:]); overflow {
+			// Only check the function for valid coordinates.
+			return true
+		}
+
+		// Create an original public key.
+		original := NewPublicKey(xf, yf)
+		if original == nil {
+			// Skip invalid inputs.
+			return true
+		}
+
+		// Make a copy of the original public key.
+		copied := CopyPublicKey(original)
+		if copied == nil {
+			// copy should succeed if original was valid.
+			return false
+		}
+
+		// The copy should not be the same instance.
+		if original == copied {
+			return false
+		}
+
+		// The copy should be equal to the original
+		// First check the serialized forms.
+		originalBytes := original.SerializeCompressed()
+		copiedBytes := copied.SerializeCompressed()
+		if !bytes.Equal(originalBytes, copiedBytes) {
+			return false
+		}
+
+		return original.IsEqual(copied)
+	}
+
+	// Create a deterministic random source using a fixed seed.
+	rng := rand.New(rand.NewSource(42))
+	config := &quick.Config{
+		Rand: rng,
+	}
+
+	if err := quick.Check(f, config); err != nil {
+		t.Error(err)
+	}
+}