p11: Add cast methods to Object

Expecting the user to manually cast Objects to Keys is counterintuitive,
and also blocks converting the Object and Key structs into interfaces
(since the cast targets will not be exported).  Adding an explicit
method that does the casting allows us to abstract this from the user.

Refs miekg#144

p11/crypto.go

@@ -2,16 +2,16 @@ package p11
 
 import "github.com/miekg/pkcs11"
 
-// PublicKey is an Object representing a public key. Since any object can be cast to a
-// PublicKey, it is the user's responsibility to ensure that the object is
-// actually a public key. For instance, if you use a FindObjects template that
-// includes CKA_CLASS: CKO_PUBLIC_KEY, you can be confident the resulting object
-// is a public key.
+// PublicKey is an Object representing a public key. Since the PublicKey method
+// can be called on any object, it is the user's responsibility to ensure that
+// the object is actually a public key. For instance, if you use a FindObjects
+// template that includes CKA_CLASS: CKO_PUBLIC_KEY, you can be confident the
+// resulting object is a public key.
 type PublicKey Object
 
-// PrivateKey is an Object representing a private key. Since any object can be cast to a
-// PrivateKey, it is the user's responsibility to ensure that the object is
-// actually a private key.
+// PrivateKey is an Object representing a private key. Since the PrivateKey
+// method can be called on any object, it is the user's responsibility to
+// ensure that the object is actually a private key.
 type PrivateKey Object
 
 // Decrypt decrypts the input with a given mechanism.

p11/module.go

@@ -51,7 +51,7 @@
 //   if err != nil {
 //     return err
 //   }
-//   privateKey := p11.PrivateKey(privateKeyObject)
+//   privateKey := privateKeyObject.PrivateKey()
 //   signature, err := privateKey.Sign(..., []byte{"hello"})
 //   if err != nil {
 //     return err

p11/object.go

@@ -100,3 +100,18 @@ func (o Object) Destroy() error {
 	defer s.Unlock()
 	return s.ctx.DestroyObject(s.handle, o.objectHandle)
 }
+
+// PrivateKey returns this object as a PrivateKey.
+func (o Object) PrivateKey() PrivateKey {
+	return PrivateKey(o)
+}
+
+// PublicKey returns this object as a PublicKey.
+func (o Object) PublicKey() PublicKey {
+	return PublicKey(o)
+}
+
+// SecretKey returns this object as a SecretKey.
+func (o Object) SecretKey() SecretKey {
+	return SecretKey(o)
+}

p11/secret_key.go

@@ -2,11 +2,11 @@ package p11
 
 import "github.com/miekg/pkcs11"
 
-// SecretKey is an Object representing a secret (symmetric) key. Since any object can be cast to a
-// SecretKey, it is the user's responsibility to ensure that the object is
-// actually a secret key. For instance, if you use a FindObjects template that
-// includes CKA_CLASS: CKO_SECRET_KEY, you can be confident the resulting object
-// is a secret key.
+// SecretKey is an Object representing a secret (symmetric) key. Since the
+// SecretKey method can be called on any object, it is the user's
+// responsibility to ensure that the object is actually a secret key. For
+// instance, if you use a FindObjects template that includes CKA_CLASS:
+// CKO_SECRET_KEY, you can be confident the resulting object is a secret key.
 type SecretKey Object
 
 // Encrypt encrypts a plaintext with a given mechanism.

p11/session.go

@@ -65,25 +65,25 @@ type sessionImpl struct {
 func (s *sessionImpl) FindPrivateKey(label string) (PrivateKey, error) {
 	obj, err := s.findObjectWithClassAndLabel(pkcs11.CKO_PRIVATE_KEY, label)
 	if err != nil {
-		return PrivateKey(obj), err
+		return obj.PrivateKey(), err
 	}
-	return PrivateKey(obj), nil
+	return obj.PrivateKey(), nil
 }
 
 func (s *sessionImpl) FindPublicKey(label string) (PublicKey, error) {
 	obj, err := s.findObjectWithClassAndLabel(pkcs11.CKO_PUBLIC_KEY, label)
 	if err != nil {
-		return PublicKey(obj), err
+		return obj.PublicKey(), err
 	}
-	return PublicKey(obj), nil
+	return obj.PublicKey(), nil
 }
 
 func (s *sessionImpl) FindSecretKey(label string) (SecretKey, error) {
 	obj, err := s.findObjectWithClassAndLabel(pkcs11.CKO_SECRET_KEY, label)
 	if err != nil {
-		return SecretKey(obj), err
+		return obj.SecretKey(), err
 	}
-	return SecretKey(obj), nil
+	return obj.SecretKey(), nil
 }
 
 func (s *sessionImpl) findObjectWithClassAndLabel(class uint, label string) (Object, error) {
@@ -219,14 +219,17 @@ func (s *sessionImpl) GenerateKeyPair(request GenerateKeyPairRequest) (*KeyPair,
 	if err != nil {
 		return nil, err
 	}
+
+	pubObj := Object{
+		session:      s,
+		objectHandle: pubHandle,
+	}
+	privObj := Object{
+		session:      s,
+		objectHandle: privHandle,
+	}
 	return &KeyPair{
-		Public: PublicKey(Object{
-			session:      s,
-			objectHandle: pubHandle,
-		}),
-		Private: PrivateKey(Object{
-			session:      s,
-			objectHandle: privHandle,
-		}),
+		Public: pubObj.PublicKey(),
+		Private: privObj.PrivateKey(),
 	}, nil
 }