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+======================
+Third Party Identities
+======================
+
+A description of how email addresses, mobile phone numbers and other third
+party identifiers can be used to authenticate and discover users in Matrix.
+
+
+Overview
+========
+
+New users need to authenticate their account. An email or SMS text message can 
+be a convenient form of authentication. Users already have email addresses 
+and phone numbers for contacts in their address book. They want to communicate
+with those contacts in Matrix without manually exchanging a Matrix User ID with 
+them.
+
+Third Party IDs
+---------------
+
+[[TODO(markjh): Describe the format of a 3PID]]
+
+
+Third Party ID Associations
+---------------------------
+
+An Associaton is a binding between a Matrix User ID and a Third Party ID (3PID).
+Each 3PID can be associated with one Matrix User ID at a time.
+
+[[TODO(markjh): JSON format of the association.]]
+
+Verification 
+------------
+
+An Assocation must be verified by a trusted Verification Server. Email 
+addresses and phone numbers can be verified by sending a token to the address 
+which a client can supply to the verifier to confirm ownership.
+
+An email Verification Server may be capable of verifying all email 3PIDs or may
+be restricted to verifying addresses for a particular domain. A phone number
+Verification Server may be capable of verifying all phone numbers or may be
+restricted to verifying numbers for a given country or phone prefix.
+
+Verification Servers fulfil a similar role to Certificate Authorities in PKI so
+a similar level of vetting should be required before clients trust their
+signatures.
+
+A Verification Server may wish to check for existing Associations for a 3PID 
+before creating a new Association.
+
+Discovery
+---------
+
+Users can discover Associations using a trusted Identity Server. Each 
+Association will be signed by the Identity Server. An Identity Server may store
+the entire space of Associations or may delegate to other Identity Servers when
+looking up Associations.
+
+Each Association returned from an Identity Server must be signed by a 
+Verification Server. Clients should check these signatures.
+
+Identity Servers fulfil a similar role to DNS servers.
+
+Privacy
+-------
+
+A User may publish the association between their phone number and Matrix User ID
+on the Identity Server without publishing the number in their Profile hosted on
+their Home Server.
+
+Identity Servers should refrain from publishing reverse mappings and should 
+take steps, such as rate limiting, to prevent attackers enumerating the space of
+mappings.
+
+Federation
+==========
+
+Delegation
+----------
+
+Verification Servers could delegate signing to another server by issuing 
+certificate to that server allowing it to verify and sign a subset of 3PID on 
+its behalf. It would be necessary to provide a language for describing which
+subset of 3PIDs that server had authority to validate. Alternatively it could 
+delegate the verification step to another server but sign the resulting
+association itself.
+
+The 3PID space will have a heirachical structure like DNS so Identity Servers
+can delegate lookups to other servers. An Identity Server should be prepared 
+to host or delegate any valid association within the subset of the 3PIDs it is 
+resonsible for.
+
+Multiple Root Verification Servers
+----------------------------------
+
+There can be multiple root Verification Servers and an Association could be
+signed by multiple servers if different clients trust different subsets of
+the verification servers.
+
+Multiple Root Identity Servers
+------------------------------
+
+There can be be multiple root Identity Servers. Clients will add each
+Association to all root Identity Servers.
+
+[[TODO(markjh): Describe how clients find the list of root Identity Servers]]
+
+