How do I get a PEM file into the javascript object required for node-bignumber?

I want to configure my Node app to decode public key encrypted messages POST'ed to it using its private key. The private key is in PEM format.

I wanted to use a javascript only solution on the server side for maximum portability (hence ursa is out), this appear to leave node-bignumber ( on the phone app side, I will be using C# and probably something like Scrypt). The single example given for node-bignumber seems to work well so long as yo use the Key method to generate the certificate, however I already have certificates (the public one installed in my phone app and the private one used by my server) - how do I get this into a format recognised by the the library?

I admit I am totally new to NodeJS and javascript and would greatly appreciate any more general advice you may have in providing backends for phone apps ...