crypto/openpgp: make it possible to set the key length

Fixes golang/go#6693.

Change-Id: I7322e107bd5f7ad07062dcaadeaa3e85a101015a
Reviewed-on: https://go-review.googlesource.com/12473
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

openpgp/keys.go

@@ -464,15 +464,20 @@ const defaultRSAKeyBits = 2048
 func NewEntity(name, comment, email string, config *packet.Config) (*Entity, error) {
 	currentTime := config.Now()
 
+	bits := defaultRSAKeyBits
+	if config != nil && config.RSABits != 0 {
+		bits = config.RSABits
+	}
+
 	uid := packet.NewUserId(name, comment, email)
 	if uid == nil {
 		return nil, errors.InvalidArgumentError("user id field contained invalid characters")
 	}
-	signingPriv, err := rsa.GenerateKey(config.Random(), defaultRSAKeyBits)
+	signingPriv, err := rsa.GenerateKey(config.Random(), bits)
 	if err != nil {
 		return nil, err
 	}
-	encryptingPriv, err := rsa.GenerateKey(config.Random(), defaultRSAKeyBits)
+	encryptingPriv, err := rsa.GenerateKey(config.Random(), bits)
 	if err != nil {
 		return nil, err
 	}