in traditional IOS (not pix/asa), k9 is the encryption (3des/aes) image, while k8 doesn't support anything beyond des due to export controls.
In pix/asa for whatever reason, the image is the same no matter what, you just need the activation key to turn on those features - which doesn't change the image name. hence, the k8 image is just fine, as long as the 3des/aes license has been applied once.
