converting certbot's private key from JSON to PEM; how to find them,

on OSX/Linux: sudo find /etc/letsencrypt/accounts/ -name private_key.json

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./certbot-jsonkey-convert.py private_key.json > private-key.asn1
openssl asn1parse -genconf private-key.asn1 -noout -out private-key.der
openssl rsa -inform DER -in private-key.der -outform PEM -out cert-private-key.pem

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to have tarsnap backup working, create an account at https://www.tarsnap.com/

then save (all) your tarsnap private server keys in this /files folder as:

{put-server's-FQDN-here}-tarsnap.key