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# Monkeysphere Documentation #


## Dependencies ##

 * Monkeysphere relies on [GnuTLS](http://gnutls.org/) version 2.4.0 or later.

## 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/)

## 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.  A PKI becomes more useful the more people participate in
it, so widespread adoption is important.

### `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 significantly old; it doesn't appear to have been
   maintained beyond OpenSSH 3.6p1.  As of this writing, OpenSSH is on
   version 5.1p1.

 * 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 on the host keys.

 * It requires patching OpenSSH.


### 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 new 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 is quite useful, but doesn't take the system as far as it could
go, and doesn't tie into the 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 requires patching OpenSSH

### 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 before that).

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.

 * It requires patching OpenSSH.