Re:Upgrade Your SSH Key to Ed25519 (2018)
by tialaramex on Tuesday June 22, 2021 @08:41AM
(#27590910)
[Edited: This used to begin "However SSH keys enjoy forward secrecy" but on more consideration that's the wrong way to look at it anyway]
The only role your SSH key plays (which is why you can use Ed25519) is to make signatures. So, if the government discovers next Thursday how to break my archaic RSA key, they don't magically get to read transcripts of SSH sessions they recorded, they only get some sort of attack that could help them to impersonate me on fresh connections after Thursday.
Essentially the authentication step goes like this:
You: I know the private key corresponding to this public key 123456, does that help?
Server: (Examines authorized_keys list) That would work, prove you know that key
You: <Signature of SSH setup transcript using private key>
Server: Welcome southerntofu
All this is happening inside an encrypted session. Your client (and remote servers) automatically negotiate the safest mutually intelligible session key agreement and encryption to set that up, independent from stuff like your personal keys.
The only role your SSH key plays (which is why you can use Ed25519) is to make signatures. So, if the government discovers next Thursday how to break my archaic RSA key, they don't magically get to read transcripts of SSH sessions they recorded, they only get some sort of attack that could help them to impersonate me on fresh connections after Thursday.
Essentially the authentication step goes like this:
You: I know the private key corresponding to this public key 123456, does that help?
Server: (Examines authorized_keys list) That would work, prove you know that key
You: <Signature of SSH setup transcript using private key>
Server: Welcome southerntofu
All this is happening inside an encrypted session. Your client (and remote servers) automatically negotiate the safest mutually intelligible session key agreement and encryption to set that up, independent from stuff like your personal keys.