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+[[!template id="nav"]]
+[[meta title="Similar Projects"]]
+
+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.
+
+[[toc ]]
+
+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).