*MAC USER* CWM: Root E:signature verification failed: HELP?

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http://androidforums.com – Firstly, I am a Mac user so I do not have access to Odin. I have followed the step by step instructions to root my Galaxy S3 i9300 which is currently running the Official Samsung firmware XXLLA 4.1.2 Jelly Bean. I got as far as installing from the external SD Card via CWM when I got a message displaying 'E:Signature Verification Failed'. I am stuck now because I can see that people have solved the issue via Odin. The Mac alternative is Heimdall. IS ANYONE HERE A MAC USER THAT CAN HELP ME OUT? Thank you. (General)