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+% Single-Use SSH Keys
+%
+% 23 Aug 2015
+
+**NOTE:** This is a "retroposted" article -- I originally created and
+wrote this in the summer of 2015, but now (30 Nov 2019) I'm revising
+it and merging it into my new blog system. There was some discussion
+about this on [Hacker
+News](https://news.ycombinator.com/item?id=10105661) at the time. The
+concept here is a bit useless now (everyone's got an SSH client on
+their phone, right?), but I think it's a neat thing to have, just in
+case.
+
+This article outlines a system of "single-use SSH keys" -- SSH keys
+which, when used to log in, automatically delete themselves from the
+user's `authorized_keys` file.
+
+## Motivation
+
+Say you're stranded without a laptop, but you need to SSH into a
+remote box for some urgent maintenance. You could carry a flash drive
+around with a long-term SSH key, but would you trust that to a public
+computer?
+
+This issue could be partially resolved with a "disposable" SSH key --
+a key that can only be used to log in once (ideally you'd never have
+to do this -- but the world is non-ideal^\[[citation
+needed](https://xkcd.com/285/)\]^). The idea is that you'd generate one
+or two keys in advance and use them as needed in situations like the
+one above.
+
+## How It Works
+
+Each key in a user\'s `.ssh/authorized_keys` file can be modified to run
+a command when the key is used for authentication. This mechanism can be
+(ab)used to delete the key from the list after it is used to log in:
+
+~~~ {bash}
+command="sed -i \"/MYMH_user_DONOTMODIFYTHISCOMMENT_onetime0^/d\" $HOME/.ssh/authorized_keys ; $SHELL" ssh-rsa AAAA.... MYMH_user_DONOTMODIFYTHISCOMMENT_onetime0
+~~~
+
+## Threat Model
+
+This system is far from perfect. It does *not* offer any protection
+against the following:
+
+- Theft of unused, unencrypted keys.
+- Injection of commands by an SSH client.
+
+It *does*, however, protect against a long-term key from being stolen
+and being used by an attacker to authenticate later, because a key is
+rendered worthless after being used.
+
+## Script Download
+
+To automate the process, I\'ve written a simple shell script that
+automatically generates and sets up some single-use keys.
+
+The script can be downloaded from [here](/pub/onetime_ssh.sh).\
+
+::: {.fine}
+SHA1: 5a68f99d933003dc4aac17134af5186c65d50efa\
+MD5: c1e4b1d03d516711f006d96e974ce9e9
+:::