openpgp: allow NewEntity to specify the default cipher

The openpgp package promotes bad defaults by not setting the
preferred cipher and hash of new entities created by
`openpgp.NewEntity`.

The preferred hash can be set by passing a `packet.Config`
with a `DefaultHash` set, but the same cannot be done for
the preferred cipher.

This change copies the DefaultCipher into the self-signature, similar to
DefaultHash.

Change-Id: I80e1289d67b7cd4079be8c1d5ba603a555dbe5c1
Reviewed-on: https://go-review.googlesource.com/66430
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

openpgp/keys.go

@@ -507,6 +507,11 @@ func NewEntity(name, comment, email string, config *packet.Config) (*Entity, err
 		e.Identities[uid.Id].SelfSignature.PreferredHash = []uint8{hashToHashId(config.DefaultHash)}
 	}
 
+	// Likewise for DefaultCipher.
+	if config != nil && config.DefaultCipher != 0 {
+		e.Identities[uid.Id].SelfSignature.PreferredSymmetric = []uint8{uint8(config.DefaultCipher)}
+	}
+
 	e.Subkeys = make([]Subkey, 1)
 	e.Subkeys[0] = Subkey{
 		PublicKey:  packet.NewRSAPublicKey(currentTime, &encryptingPriv.PublicKey),