Fix failing tests on Rubinius

lib/cryptography/serializable.rb

@@ -57,6 +57,15 @@ def from_hash(hash)
 
     def from_sodium_buffer(buffer)
       self.from_protocol_buffer self.serializer.parse(buffer.to_s)
+    rescue
+      # ProtocolBuffers::Message#parse doesn't gracefully handle
+      # errors. It just kind of throws whatever exception seemed
+      # convenient at the time. So we just always convert it to an
+      # ArgumentError (the string was an Argument, and it was
+      # errorful, so... it's not any worse than what it was).
+      raise ArgumentError.new($!.message).tap {|e|
+        e.set_backtrace($!.backtrace)
+      }
     end
 
     def from_protocol_buffer(buffer)

test/cryptography/symmetric/authenticated_message_test.rb

@@ -4,7 +4,10 @@
   subject         { self.klass.new(self.key, self.plaintext) }
   let(:klass)     { Cryptography::Symmetric::AuthenticatedMessage }
   let(:key)       { self.klass.key }
-  let(:plaintext) { Sodium::Buffer.new('message') }
+
+  def plaintext
+    Sodium::Buffer.new 'message'.tr('', '')
+  end
 
   def self.protocol_buffer_authenticated_message_test_vector(plaintext, key, bytes)
     it "must never break backwards compatibility with the serialization format" do
@@ -65,8 +68,9 @@ def self.protocol_buffer_authenticated_message_test_vector(plaintext, key, bytes
   it '#contents must not reveal the message if it has been altered' do
     # we have to jump through hoops here because it *shouldn't* be
     # easy to modify these strings!
-    tampered = self.subject.to_s.to_str.dup.tap {|s| s[-1] = s[-1].succ }
-    hmac     = self.klass.from_s(tampered)
+    plaintext = self.plaintext.to_s.to_str
+    tampered  = self.subject.to_s.to_str.dup.tap {|s| s[plaintext] = plaintext.succ }
+    hmac      = self.klass.from_s(tampered)
 
     lambda do
       hmac.contents(self.key)