Merge pull request #404 from uhoreg/patch-2

add information about Perspectives

supporting-docs/guides/2015-08-19-faq.md

@@ -492,12 +492,16 @@ Yes. Matrix is just a spec, so implementations of the spec are very welcome! It
 
 ##### How secure is this?
 
-Server-server traffic is mandatorily TLS from the outset.  Server-client traffic mandates transport layer encryption other than for tinkering.  Servers maintain a public/private key pair, and sign the integrity of all messages in the context of the historical conversation, preventing tampering.  Server keys are distributed using a PERSPECTIVES-style system.
+Server-server traffic is mandatorily TLS from the outset.  Server-client traffic mandates transport layer encryption other than for tinkering.  Servers maintain a public/private key pair, and sign the integrity of all messages in the context of the historical conversation, preventing tampering.  Server keys are distributed using a [Perspectives](https://perspectives-project.org/)-style system.
 
 End-to-end encryption is coming shortly to clients for both 1:1 and group chats to protect user data stored on servers, using the [Olm](https://matrix.org/git/olm) cryptographic ratchet implementation.  As of October 2015 this is blocked on implementing the necessary key distribution and fingerprint management.
 
 Privacy of metadata is not currently protected from server administrators - a malicious homeserver administrator can see who is talking to who and when, but not what is being said (once E2E encryption is enabled).  See [this presentation from Jardin Entropique](http://matrix.org/~matthew/2015-06-26%20Matrix%20Jardin%20Entropique.pdf) for a more comprehensive discussion of privacy in Matrix.
 
+##### What is Perspectives?
+
+Rather than relying on Certificate Authorities (CAs) as in traditional SSL, a [Perspectives](https://perspectives-project.org/)-style system uses a more decentralized model for verifying keys.  Perspectives uses notary servers to verify that the same key is seen across the network, making a man-in-the-middle attack much harder since an attacker must insert itself into multiple places.  For federation in Matrix, each Home Server acts as a notary.  When one Home Server connects to another Home Server that uses a key that it doesn't recognize, it contacts other Home Servers to ensure that they all see the same key from that Home Server.
+
 ##### Why HTTP? Doesn't HTTP suck?
 
 HTTP is indeed not the most efficient transport, but it is ubiquitous, very well understood and has numerous implementations on almost every platform and language. It also has a simple upgrade path to HTTP/2, which is relatively bandwidth and round-trip efficient.