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# Monkeysphere Documentation #
+[[!toc]]
## Dependencies ##
- * Monkeysphere relies on [GnuTLS](http://gnutls.org/) version 2.4.0 or later.
+Monkeysphere relies on:
+
+ * [GnuTLS](http://gnutls.org/) version 2.4.0 or later
+ * [OpenSSH](http://openssh.com/)
+ * [GnuPG](http://gnupg.org/)
+
+## Getting started ##
+
+ * Getting started as a [user](/getting-started-user)
+ * Getting started as a [server admin](/getting-started-admin)
## References ##
* [Initial specifications at CMRG](http://cmrg.fifthhorseman.net/wiki/OpenPGPandSSH)
* [OpenPGP (RFC 4880)](http://tools.ietf.org/html/rfc4880)
* [Secure Shell Authentication Protocol (RFC 4252)](http://tools.ietf.org/html/rfc4252)
-* [URI scheme for SSH, RFC draft](http://tools.ietf.org/wg/secsh/draft-ietf-secsh-scp-sftp-ssh-uri/)
+ * [URI scheme for SSH, RFC draft](http://tools.ietf.org/wg/secsh/draft-ietf-secsh-scp-sftp-ssh-uri/)
+
+## 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.
+
+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).
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