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+..
+  This file is part of Anastasis
+  Copyright (C) 2019-2021 Anastasis SARL
+
+  Anastasis is free software; you can redistribute it and/or modify it under the
+  terms of the GNU Affero General Public License as published by the Free Software
+  Foundation; either version 2.1, or (at your option) any later version.
+
+  Anastasis is distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more details.
+
+  You should have received a copy of the GNU Affero General Public License along with
+  Anastasis; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+
+  @author Christian Grothoff
+  @author Dominik Meister
+  @author Dennis Neufeld
+
+============
+Introduction
+============
+
+To understand how Anastasis works, you need to understand three key
+concepts: user identifiers, our adversary model and the role of the
+recovery document.
+
+
+User Identifiers
+----------------
+
+To uniquely identify users, an "unforgettable" **identifier** is used.  This
+identifier should be difficult to guess for anybody but the user. However, the
+**identifier** is not expected to have sufficient entropy or secrecy to be
+cryptographically secure. Examples for such identifier would be a
+concatenation of the full name of the user and their social security or
+passport number(s).  For Swiss citizens, the AHV number could also be used.
+
+
+Adversary models
+----------------
+
+The adversary model of Anastasis has two types of adversaries: weak
+adversaries which do not know the user's **identifier**, and strong
+adversaries which somehow do know a user's **identifier**.  For weak
+adversaries the system guarantees full confidentiality.  For strong
+adversaries, breaking confidentiality additionally requires that Anastasis
+escrow providers must have colluded.  The user is able to specify a set of
+**policies** which determine which Anastasis escrow providers would need to
+collude to break confidentiality. These policies also set the bar for the user
+to recover their core secret.
+
+
+The recovery document
+---------------------
+
+A **recovery document** includes all of the information a user needs to
+recover access to their core secret.  It specifies a set of **escrow
+methods**, which specify how the user should convince the Anastasis server
+that they are "real".  Escrow methods can for example include SMS-based
+verification, video identification or a security question.  For each escrow
+method, the Anastasis server is provided with **truth**, that is data the
+Anastasis operator may learn during the recovery process to authenticate the
+user.  Examples for truth would be a phone number (for SMS), a picture of the
+user (for video identification), or the (hash of) a security answer.  A strong
+adversary is assumed to be able to learn the truth, while weak adversaries
+must not.  In addition to a set of escrow methods and associated Anastasis
+server operators, the **recovery document** also specifies **policies**, which
+describe the combination(s) of the escrow methods that suffice to obtain
+access to the core secret.  For example, a **policy** could say that the
+escrow methods (A and B) suffice, and a second policy may permit (A and C).  A
+different user may choose to use the policy that (A and B and C) are all
+required.  Anastasis imposes no limit on the number of policies in a
+**recovery document**, or the set of providers or escrow methods involved in
+guarding a user's secret.  Weak adversaries must not be able to deduce
+information about a user's **recovery document** (except for its length, which
+may be exposed to an adversary which monitors the user's network traffic).