More information about why proof of freshness is needed for canaries:
https://groups.google.com/forum/#!topic/qubes-devel/MERCI9lEyb8 "Why proof of freshness?"

examples of proofs generators:

from QubesOS (news & bitcoin blockchain based):
https://github.com/QubesOS/qubes-secpack/blob/master/utils/proof_of_freshness_generator.sh

froom autocanary (news based):
https://github.com/firstlookmedia/autocanary/blob/master/autocanary/headlines.py

fix:
add proof of freshness to new canaries.