ssh-pass

I've been using pass for a while now and I really like it. But I don't like putting passwords or passphrases into my system clipboard if I can help it: writing code to sniff the clipboard is child's play, so I'd like to avoid that attack vector if possible.

One place that I thought would be simple would be the SSH utility to add keys to your SSH Agent: ssh-add. It should be possible to do something like this:

$ pass github/sinewalker|head -1|ssh-add github/sinewalker

Unfortunately this isn't so: ssh-add doesn't accept your passphrase from STDIN when piped like this.

But, there is a way to do it.


Here's a script that does the job:

#!/bin/bash

#Add specified SSH key to the SSH Agent, using SSH_ASKPASS to retrieve
#the key's passphrase from the Unix password store (pass).
#This relies upon the keys having the same name in both your key directory
#and your password store.

if [[ -z ${1} ]]; then
  echo "$(basename ${0}): no SSH key specified" 1>&2
  exit 1;
fi
KEY=${1}
KEY_DIR=${HOME}/key

export DISPLAY=dummy
export SSH_ASKPASS=$(mktemp -t ssh-askpass)
cat > ${SSH_ASKPASS} << EOF
#!/bin/sh
pass ${KEY}|head -1
EOF
chmod +x ${SSH_ASKPASS}
ssh-add ${KEY_DIR}/${KEY} < /dev/null
rm ${SSH_ASKPASS}

Gist

The script is based upon this answer to the SO question "How to make ssh receive the password from stdin" which is for ssh but the same applies to ssh-add.

While this script is pretty simple, it deserves breaking down a little. There are 3 ingredients to it:

  1. Uses ssh-add's SSH_ASKPASS environment variable to retrieve the passphrase for the SSH key
  2. Dynamically creates a throw-away script for ssh-add to use
  3. Uses pass to retrieve the passphrase from the password store

There are some gymnastics required to convince ssh-add to actually use the Askpass script:

  • Must set the DISPLAY environment variable, even in non-X11 environments (doesn't hurt in X11)
  • Must run ssh-add dettached from a terminal. I do this by redirecting STDIN from /dev/null
  • Must set SSH_ASKPASS to the path of a script that retrieves the passphrase and writes it to SDTOUT, as the manual describes.

Finally, one little caveat to the way I've written the script: the name of the passphrase in my password-store must be the same as the path to the key file in my keys directory.

As an example, I have an SSH key for my github:

[[email protected]:~]
[10:21]β cd key
/Users/mjl/key
[[email protected]:~/key]
[10:21]β tree
.
├── bitbucket
├── celo
│   ├── Celo-OVPN-Bundled-Configs
...
├── github
│   └── sinewalker
...

I've stored the keyfile sinewalker under the directory github, so that it matches the path to the passphrase in my password-store:

[[email protected]:~/key]
[10:21]β pass
Password Store
...
├── github
│   ├── milohax
│   └── sinewalker
...

Then I can run my ssh-pass script like so:

[[email protected]:~/key]
[10:28]β ssh-pass github/sinewalker
Identity added: /Users/mjl/key/github/sinewalker (/Users/mjl/key/github/sinewalker)
[[email protected]:~/key]
[10:28]β

The pinentry prompts for my password-store master password (if necessary), but I don't have to copy/paste with the clipboard. As shown above, if you run ssh-pass from the directory that contains your SSH keys, then you can also use the shell's TAB-completion.