.CER -> .PEM
A good snippit to have is the code to convert Apple push certificates to the pem file format for your server.
In KeyChain Access, select the certificate and choose File > Export Items. Select the Personal Information Exchange (.p12) option, select a save location, and click Save.
Launch the Terminal application and enter the following command after the prompt:
openssl pkcs12 -in CertificateName.p12 -out CertificateName.pem -nodes