From raywenderlich.com

cert2pem

# Convert the .cer file into a .pem file:
$ openssl x509 -in aps_development.cer -inform der -out PushChatCert.pem

# Convert the private key’s .p12 file into a .pem file:
$ openssl pkcs12 -nocerts -in PushChatKey.p12 -out PushChatKey.pem

# Finally, combine the certificate and key into a single .pem file
$ cat PushChatCert.pem PushChatKey.pem > ck.pem

# At this point it’s a good idea to test whether the certificate works.
$ telnet gateway.sandbox.push.apple.com 2195

# Let’s try connecting again, this time using our SSL certificate and private key to set up a secure connection.
$ openssl s_client -connect gateway.sandbox.push.apple.com:2195 -cert PushChatCert.pem -key PushChatKey.pem