There now exists a library module 'oe.gpg_sign' for performing signing functions. In order to eliminate code duplication, sstate.bbclass should be converted to use that module for gpg signing and signature verification.
Patches sent for review: http://lists.openembedded.org/pipermail/openembedded-core/2016-February/117322.html
Merged into master in http://git.openembedded.org/openembedded-core/commit/?id=a3916b29c8c3d0b1570c40c0ed3a1a15c43925bb
Verified on master efa08813d749c61303ebbadcf1a9f99affbdaed9