I've never been ambitious enough to try building an extension. That said, have you gone through the entire Packaging and Signing Technical Note? It's a 6 page PDF with links to other resources.
http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/creativesuite/pdfs/SigningTechNote_ CC.pdf
An excerpt from page 5 of that TechNote:
"Certificates used to cryptographically sign documents or software commonly have expiration duration between one and four years. If the certificate has no valid timestamp, and the certificate used to sign the extension has expired, the extension cannot be installed or loaded."
Hope it helps!
Nancy O.