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The monkeysphere isn't the only project intending to implement a PKI
for OpenSSH.  We provide links to these other projects because they're
interesting, though we have concerns with their approaches.

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All of the other projects we've found so far require a patched version
of OpenSSH, which makes adoption more difficult.  Most people don't
build their own software, and simply overlaying a patched binary is
associated with significant maintenance (and therefore security)
problems.  

While ultimately contributing a patch to
[OpenSSH](http://openssh.com/) (or any
[free](http://www.chiark.greenend.org.uk/~sgtatham/putty/)
[SSH](http://www.lysator.liu.se/~nisse/lsh/)
[implementation](http://matt.ucc.asn.au/dropbear/dropbear.html)) is
not a bad thing, we hope to be able to better establish the use of a
PKI without resorting to source modification.

## openssh-gpg ##

[openssh-gpg](http://www.red-bean.com/~nemo/openssh-gpg/) is a patch
against OpenSSH to support OpenPGP certificates.  According to its
documentation, it is intended to support [`pgp-sign-rsa` and
`pgp-sign-dss` public key algorithms for hosts, as specified by the
IETF](http://tools.ietf.org/html/rfc4253#section-6.6).

Some concerns with `openssh-gpg`:

 * This patch is old; it doesn't appear to have been maintained beyond
   OpenSSH 3.6p1.  As of this writing, OpenSSH 5.1p1 is current.

 * It only provides infrastructure in one direction: the user
   authenticating the host by name.  There doesn't seem to be a
   mechanism for dealing with identifying users by name, or allowing
   users to globally revoke or update keys.

 * The choice of User ID (`anything goes here (and here!)
   <ssh@foo.example.net>`) for host keys overlaps with the current use
   of the User ID space.  While it's unlikely that someone actually
   uses this e-mail address in the web of trust, it would be a nasty
   collision, as the holder of that key could impersonate the server
   in question.  The monkeysphere uses [User IDs of the form
   `ssh://foo.example.net`](http://tools.ietf.org/wg/secsh/draft-ietf-secsh-scp-sftp-ssh-uri/)
   to avoid collisions with existing use.

 * It's not clear that `openssh-gpg` acknowledges or respects the
   [usage flags](http://tools.ietf.org/html/rfc4880#section-5.2.3.21)
   on the host keys.  This means that it could accept a "sign-only"
   key as suitable for authenticating a host, despite the
   clearly-marked intentions of the key-holder.

## Perspectives OpenSSH client ##

[The Perspectives project](http://www.cs.cmu.edu/~perspectives/) at
CMU has released an [openssh client that uses network
notaries](http://www.cs.cmu.edu/~perspectives/openssh.html) to bolster
your confidence in newly-seen keys.  This offers a defense against a
narrow MITM attack (e.g. by someone who controls your local gateway)
by simply verifying that other machines from around the network see
the same keys for the remote host that you're seeing.

This tactic is quite useful, but doesn't take the system as far as it
could go, and doesn't tie into any existing web of trust.

Some concerns with the Perspectives OpenSSH client:

 * This client won't help if you are connecting to machines behind
   firewalls, on NAT'ed LANs, with source IP filtering, or otherwise
   in a restricted network state.

 * There is still a question of why you should trust these particular
   notaries during your verification.  Who are the notaries?  How
   could they be compromised?

 * It only provides infrastructure in one direction: the user
   authenticating the host by name.  There is no mechanism for dealing
   with identifying users by name, or allowing users to globally
   revoke or change keys.

 * It doesn't provide any mechanism for key rotation or revocation:
   Perspectives won't help you if you need to re-key your machine.

## OpenSSH with X.509v3 certificates ##

Roumen Petrov [maintains a patch to OpenSSH that works with the X.509
PKI model](http://www.roumenpetrov.info/openssh/).  This is the
certificate hierarchy commonly used by TLS (and SSL).

Some concerns about OpenSSH with X.509v3:

 * the X.509 certificate specification itself [encourages corporate
   consolidation and centralized global "trust" because of its
   single-issuer architectural
   limitation](http://lair.fifthhorseman.net/~dkg/tls-centralization/).
   This results in an expensive and cumbersome system for smaller
   players, and it also doesn't correspond to the true distributed
   nature of human-to-human trust.  Furthermore, centralized global
   "trusted authorities" create a tempting target for attack, and a
   single-point-of-failure if an attack is successful.

   Depending on how you declare your trust relationships, OpenPGP is
   capable of providing the same hierarchical structure as X.509, but
   it is not limited to such a structure.  The OpenPGP Web of Trust
   model is more flexible and more adaptable to represent real-world
   trust than X.509's rigid hierarchy.

 * X.509 certificates can identify hosts by name, but not by
   individual service.  This means that a compromised web or e-mail
   server with access to the X.509 key for that service could re-use
   its certificate as an SSH server, and it would be able to
   masquerade successfully.

   The monkeysphere uses [User IDs of the form
   `ssh://foo.example.net`](http://tools.ietf.org/wg/secsh/draft-ietf-secsh-scp-sftp-ssh-uri/),
   so they are not by-default shared across services on the same host
   (you can still share a key across services on the same host if you
   like, but the service User IDs can be certified independently of
   one another).