Our intern Erick has been doing some great work reviving, polishing and finalizing the patches that will allow us to start signing OS X builds (more to come on that in his blog!). When we do start signing them we're planning to use our existing set of code signing certificates for them rather than buy new ones. I thought it would be a simple task to convert them so I set off to convert our internal, self-generated ones. After hours and hours of head scratching and frustration I learned that some versions of Microsoft's "makecert" tool are broken, and generate invalid PKCS7 certs that openssl can't cope with properly. From the OpenSSL PKCS#12 FAQ: Q. What are SPC files? A. They are simply DER encoded PKCS#7 files containing the certificates. Well they are in the newer versions of the tools. The older versions used an invalid PKCS#7 format. The end result of all my attempts ended up being a PKCS#12 certificate that Apple's codesign tool claimed couldn't be used to do code signing. After finding that FAQ, I decided to try to convert our Nightly code signing certificate instead. Following the great instructions found on Marc Liyanage's blog I managed to successfully convert the certificate, import it into a Keychain, and successfully sign something! Here's the shortened version of what I did. Note that it requires the PVK tool found here: ~/pvk.exe -in Nightly.pvk -out Nightly.key.pem openssl pkcs7 -inform der -print_certs Nightly.cert.pem openssl pkcs12 -export -inkey Nightly.key.pem -in Nightly.cert.pem -out Nightly.p12 I hope this helps someone else avoid the same frustration!