openpgp: sign Entity during instantiation in NewEntity

Previously if you created a new Entity then ran `Serialize` _before_ running `SerializePrivate`, the resulting armored public key was corrupted, giving the error of `unexpected EOF`. This fix signs the public key with the private key upon creation of a NewEntity. Since SerializePrivate only is applicable to entities created with NewEntity per the docs we can also safely remove the signing portion from that function.

Fixes #25463

Change-Id: I58b808987ee173079f33bce3d6c3527f9233b2cd
GitHub-Last-Rev: 2c4b8e4d630a06d782816f8fbd3d59f01ece2565
GitHub-Pull-Request: golang/crypto#47
Reviewed-on: https://go-review.googlesource.com/114001
Reviewed-by: Filippo Valsorda <filippo@golang.org>

openpgp/keys.go

@@ -500,6 +500,10 @@ func NewEntity(name, comment, email string, config *packet.Config) (*Entity, err
 			IssuerKeyId:  &e.PrimaryKey.KeyId,
 		},
 	}
+	err = e.Identities[uid.Id].SelfSignature.SignUserId(uid.Id, e.PrimaryKey, e.PrivateKey, config)
+	if err != nil {
+		return nil, err
+	}
 
 	// If the user passes in a DefaultHash via packet.Config,
 	// set the PreferredHash for the SelfSignature.
@@ -529,14 +533,17 @@ func NewEntity(name, comment, email string, config *packet.Config) (*Entity, err
 	}
 	e.Subkeys[0].PublicKey.IsSubkey = true
 	e.Subkeys[0].PrivateKey.IsSubkey = true
-
+	err = e.Subkeys[0].Sig.SignKey(e.Subkeys[0].PublicKey, e.PrivateKey, config)
+	if err != nil {
+		return nil, err
+	}
 	return e, nil
 }
 
 // SerializePrivate serializes an Entity, including private key material, to
 // the given Writer. For now, it must only be used on an Entity returned from
 // NewEntity.
-// If config is nil, sensible defaults will be used.
+// config is ignored
 func (e *Entity) SerializePrivate(w io.Writer, config *packet.Config) (err error) {
 	err = e.PrivateKey.Serialize(w)
 	if err != nil {
@@ -547,10 +554,6 @@ func (e *Entity) SerializePrivate(w io.Writer, config *packet.Config) (err error
 		if err != nil {
 			return
 		}
-		err = ident.SelfSignature.SignUserId(ident.UserId.Id, e.PrimaryKey, e.PrivateKey, config)
-		if err != nil {
-			return
-		}
 		err = ident.SelfSignature.Serialize(w)
 		if err != nil {
 			return
@@ -561,10 +564,6 @@ func (e *Entity) SerializePrivate(w io.Writer, config *packet.Config) (err error
 		if err != nil {
 			return
 		}
-		err = subkey.Sig.SignKey(subkey.PublicKey, e.PrivateKey, config)
-		if err != nil {
-			return
-		}
 		err = subkey.Sig.Serialize(w)
 		if err != nil {
 			return