added RSA

added RSA-SHA1 client support.
This code correct working with JIRA.
You still have the !Windows (i.e. secure!) version of getRandomBytes disabled (the Dart random function is not at all suitable for this work). Can you elaborate on what errors you're getting with the Linux code? It Works For Me (TM) in a production app running on Linux.

Being unable to read /dev/urandom seems to indicate something is pretty busted.