How can a tor relay op display a given signing key's expiry date?
I don't think there is an option for this.
filed a ticket for it: https://trac.torproject.org/projects/tor/ticket/17639
Is there a custom openssl command to display the expiry date until this gets implemented in tor?
No. The on disk Ed25519 key format is custom to Tor, and the code doesn't use OpenSSL for any of the Ed25519 operations anyway.
Someone that wants to work on this should find the relevant data formats documented in prop 220.
The spec [1] does not mention the first 32 bytes (== ed25519v1-cert: type4 ==) but after them it is fine.
if anyone else needs a quick'n dirty solution: python import time f = open('ed25519_signing_cert','rb') x = f.read() time.ctime(int(x[35:38].encode('hex'),16)*3600) 'Sat Dec 19 02:00:00 2015'
[1] https://gitweb.torproject.org/torspec.git/tree/proposals/220-ecc-id-keys.txt...