ssh-gpg-agent

• Changelog

ssh-gpg-agent is an alternative SSH agent implementation that transparently supports PGP encrypted private SSH keys, instead of relying on password protection. Unlike SSH agents as provided by GnuPG or OpenSSH, there is no need to "add" them to the agent -- key discovery is automatic in a given directory.

The Context

For a given user, there can be merit in having a single SSH key per host instead of a shared one for all hosts a given user wants to connect to. Benefits include but are not necessary limited to:

• Compromise of a key only compromises a single service
• Simplified tracing of where a given key was used: the file name can identify the service it is used for while the public key's comment (as visible in the service's authorized_keys file) acts as a back link to the holder of the private key
• Potential for plausible deniability not present when a single key is used for all services -- same key means same user

At the same time it is advisable to not store private keys in unencrypted form on any system. Password protection comes to the rescue.

While a password provides security and ease of mind, it is often times inconvenient: repeated entry of passwords is necessary each and every time an SSH connection is established.

This is where SSH agents help out, which, upon asking for the key's password once, will cache the sensitive key material for a specific time or until the system is rebooted, depending on user preference.

The Problem

However, with a lot of systems to connect to, each using a separate key pair for authentication purposes, entering a password for each one of those -- even if that happens just once over the uptime of a system -- is still a significant burden on the user.

A Solution

In order to circumvent the repeated entry of passwords for encrypted keys, why not piggy back on a program available for encryption purposes on many systems anyway: GnuPG.

ssh-gpg-agent acts as an SSH agent that assumes PGP encrypted private SSH key files and can decrypt those transparently through GnuPG. Assuming an agent is used for GnuPG keys, decryption of SSH keys can be demoted to a background activity not requiring any user interaction whatsoever.

Additionally, if a smart card is used for decryption, there is the added benefit that simply removing it from the system will also prevent the automated usage of SSH keys.

Setup

As a first step to using the agent, existing password protected SSH keys should be re-encrypted with GnuPG.

The following function can be used to simplify the task:

function encrypt() {
  if [ $# -le 1 ]; then
    echo "Usage: encrypt <ssh-key-file> <gpg-identity-to-encrypt-to>"
    return
  fi

  mkfifo -m 0600 _fifo && \
  (cat "${1}" > _fifo && \
   cat "${1}" > _fifo && \
   gpg --encrypt --recipient="${2}" --yes --output="${1}".gpg < _fifo &) && \
  ssh-keygen -p -N "" -f _fifo && \
  rm _fifo
}

This function, applied to a single key, will ask for the key's current password, decrypt it, and encrypt it to the given GnuPG identity or recipient.

For example:

encrypt '~/.ssh/d-e-s-o@github:access_2018-01-01' '[email protected]'

The result is a new file with the extension .gpg that contains the encrypted key. The original file is left untouched. It can be kept around as a fall back (e.g., in the case of smart card usage where the card is not available).

After installation of the agent (through cargo install ssh-gpg-agent, for example) it can be started directly. By default it will work on the user's ~/.ssh/ directory and it will be used to serve identities that have an associated .gpg file available.

The agent listens for requests in a Unix domain socket, located in the system's tmp directory with the name ssh-gpg-agent.sock.

The SSH_AUTH_SOCK environment variable should be pointed to this path to instruct ssh to use ssh-gpg-agent if system-wide usage is desired.

Alternatively, if the agent is to be used only for a subset of hosts, usage of the agent can be configured to the hosts in question in ~/.ssh/config:

Host github
  Hostname github.com
  User git
  IdentityFile ~/.ssh/d-e-s-o@github:access_2018-01-01
  # Use ssh-gpg-agent for this host:
  IdentityAgent /tmp/ssh-gpg-agent.sock

After this setup, PGP encrypted SSH keys can be transparently decrypted and used for authentication with a given host.

More Advantages

Usage of ssh-gpg-agent provides a couple more advantages over typical work flows:

• It is no longer necessary to ssh-add keys to an agent, which is just an opaque second-tier key management mechanism that often is a source of confusion and/or its involvement simply forgotten
• The agent is stateless: public keys are loaded on demand; private keys are never cached and instead decrypted for every authentication request (note that a gpg-agent being used will still be stateful, but this agent does not introduce additional state to manage)
• When used in conjunction with a smart card that stores the GnuPG identity's key, physically removing the card is enough to prevent further usage SSH keys managed through the agent
• It effectively provides a way for multi-factor authentication for the usage of SSH keys when used in conjunction with a smart card (physical card & PIN in addition to the key itself), which is considered more secure than a mere password
• Existing password protected key material can stay as-is and be used as a fall back/backup

Alternative Approaches

ssh-gpg-agent is only one solution to the stated problem of repeated password entry when using per-host SSH keys. Alternative ones include:

• Pre-setting of passphrases using the PRESET_PASSPHRASE gpg-agent command (which just adds yet another indirection to the system)
• GnuPG conceptually supports a protected-shared-secret key format which may or may not be usable to have a shared password among keys and perhaps there would be a way to hook this up for SSH keys, but the format is not implemented in current versions of GnuPG (i.e., up to and including version 2.2)