Ciphers
diff --git a/docs/ciphers.rst b/docs/ciphers.rst
new file mode 100644
index 00000000000..12c403d09a8
--- /dev/null
+++ b/docs/ciphers.rst
@@ -0,0 +1,211 @@
+============
+Ciphersuites
+============
+
+.. contents:: Table of Contents
+   :local:
+
+
+.. _ciphersuites:
+
+Introduction
+============
+
+Autoupdates
+-----------
+
+Within certain limits, TLS server software can choose what kind of
+cryptography to use when a client connects. These choices can affect
+security, compatibility, and performance in complex ways. Most of
+these options are independent of a particular certificate. The Let's
+Encrypt client tries to provide defaults that we think are most useful
+to our users.
+
+As described below, the Let's Encrypt client will default to modifying
+server software's cryptographic settings to keep these up-to-date with
+what we think are appropriate defaults when new versions of the Let's
+Encrypt client are installed (for example, by an operating system package
+manager).
+
+When this feature is implemented, this document will be updated
+to describe how to disable these automatic changes.
+
+
+Cryptographic choices
+---------------------
+
+Software that uses cryptography must inevitably make choices about what
+kind of cryptography to use and how. These choices entail assumptions
+about how well particular cryptographic mechanisms resist attack, and what
+trade-offs are available and appropriate. The choices are constrained
+by compatibility issues (in order to interoperate with other software,
+an implementation must agree to use cryptographic mechanisms that the
+other side also supports) and protocol issues (cryptographic mechanisms
+must be specified in protocols and there must be a way to agree to use
+them in a particular context).
+
+The best choices for a particular application may change over time in
+response to new research, new standardization events, changes in computer
+hardware, and changes in the prevalence of legacy software. Much important
+research on cryptanalysis and cryptographic vulnerabilities is unpublished
+because many researchers have been working in the interest of improving
+some entities' communications security while weakening, or failing to
+improve, others' security. But important information that improves our
+understanding of the state of the art is published regularly.
+
+When enabling TLS support in a compatible web server (which is a separate
+step from obtaining a certificate), Let's Encrypt has the ability to
+update that web server's TLS configuration. Again, this is *different
+from the cryptographic particulars of the certificate itself*; the
+certificate as of the initial release will be RSA-signed using one of
+Let's Encrypt's 2048-bit RSA keys, and will describe the subscriber's
+RSA public key ("subject public key") of at least 2048 bits, which is
+used for key establishment.
+
+Note that the subscriber's RSA public key can be used in a wide variety
+of key establishment methods, most of which do not use RSA directly
+for key exchange, but only for authenticating the server!  For example,
+in DHE and ECDHE key exchanges, the subject public key is just used to
+sign other parameters for authentication. You do not have to "use RSA"
+for other purposes just because you're using an RSA key for authentication.
+
+The certificate doesn't specify other cryptographic or ciphersuite
+particulars; for example, it doesn't say whether or not parties should
+use a particular symmetric algorithm like 3DES, or what cipher modes
+they should use. All of these details are negotiated between client
+and server independent of the content of the ciphersuite. The
+Let's Encrypt project hopes to provide useful defaults that reflect
+good security choices with respect to the publicly-known state of the
+art. However, the Let's Encrypt certificate authority does *not*
+dictate end-users' security policy, and any site is welcome to change
+its preferences in accordance with its own policy or its administrators'
+preferences, and use different cryptographic mechanisms or parameters,
+or a different priority order, than the defaults provided by the Let's
+Encrypt client.
+
+If you don't use the Let's Encrypt client to configure your server
+directly, because the client doesn't integrate with your server software
+or because you chose not to use this integration, then the cryptographic
+defaults haven't been modified, and the cryptography chosen by the server
+will still be whatever the default for your software was.  For example,
+if you obtain a certificate using *standalone* mode and then manually
+install it in an IMAP or LDAP server, your cryptographic settings will
+not be modified by the client in any way.
+
+
+Sources of defaults
+-------------------
+
+Initially, the Let's Encrypt client will configure users' servers to
+use the cryptographic defaults recommended by the Mozilla project.
+These settings are well-reasoned recommendations that carefully
+consider client software compatibility. They are described at
+
+https://wiki.mozilla.org/Security/Server_Side_TLS
+
+and the version implemented by the Let's Encrypt client will be the
+version that was most current as of the release date of each client
+version. Mozilla offers three seperate sets of cryptographic options,
+which trade off security and compatibility differently. These are
+referred to as as the "Modern", "Intermediate", and "Old" configurations
+(in order from most secure to least secure, and least-backwards compatible
+to most-backwards compatible). The client will follow the Mozilla defaults
+for the *Intermediate* configuration by default, at least with regards to
+ciphersuites and TLS versions. Mozilla's web site describes which client
+software will be compatible with each configuration. You can also use
+the Qualys SSL Labs site, which the Let's Encrypt software will suggest
+when installing a certificate, to test your server and see whether it
+will be compatible with particular software versions.
+
+It will be possible to ask the Let's Encrypt client to instead apply
+(and track) Modern or Old configurations.
+
+The Let's Encrypt project expects to follow the Mozilla recommendations
+in the future as those recommendations are updated. (For example, some
+users have proposed prioritizing a new ciphersuite known as ``0xcc13``
+which uses the ChaCha and Poly1305 algorithms, and which is already
+implemented by the Chrome browser.  Mozilla has delayed recommending
+``0xcc13`` over compatibility and standardization concerns, but is likely
+to recommend it in the future once these concerns have been addressed. At
+that point, the Let's Encrypt client would likely follow the Mozilla
+recommendations and favor the use of this ciphersuite as well.)
+
+The Let's Encrypt project may deviate from the Mozilla recommendations
+in the future if good cause is shown and we believe our users'
+priorities would be well-served by doing so. In general, please address
+relevant proposals for changing priorities to the Mozilla security
+team first, before asking the Let's Encrypt project to change the
+client's priorities. The Mozilla security team is likely to have more
+resources and expertise to bring to bear on evaluating reasons why its
+recommendations should be updated.
+
+The Let's Encrpyt project will entertain proposals to create a *very*
+small number of alternative configurations (apart from Modern,
+Intermediate, and Old) that there's reason to believe would be widely
+used by sysadmins; this would usually be a preferable course to modifying
+an existing configuration. For example, if many sysadmins want their
+servers configured to track a different expert recommendation, Let's
+Encrypt could add an option to do so.
+
+
+Resources for recommendations
+-----------------------------
+
+In the course of considering how to handle this issue, we received
+recommendations with sources of expert guidance on ciphersuites and other
+cryptographic parameters. We're grateful to everyone who contributed
+suggestions. The recommendations we received are available at
+
+https://github.com/letsencrypt/letsencrypt/wiki/Ciphersuite-guidance
+
+Let's Encrypt client users are welcome to review these authorities to
+better inform their own cryptographic parameter choices. We also
+welcome suggestions of other resources to add to this list. Please keep
+in mind that different recommendations may reflect different priorities
+or evaluations of trade-offs, especially related to compatibility!
+
+
+Changing your settings
+----------------------
+
+This will probably look something like
+
+..code-block: shell
+
+  letsencrypt --cipher-recommendations mozilla-secure
+  letsencrypt --cipher-recommendations mozilla-intermediate
+  letsencrypt --cipher-recommendations mozilla-old
+
+to track Mozilla's *Secure*, *Intermediate*, or *Old* recommendations,
+and
+
+..code-block: shell
+
+  letsencrypt --update-ciphers on
+
+to enable updating ciphers with each new Let's Encrypt client release,
+or
+
+..code-block: shell
+
+  letsencrypt --update-ciphers off
+
+to disable automatic configuration updates. These features have not yet
+been implemented and this syntax may change then they are implemented.
+
+
+TODO
+----
+
+The status of this feature is tracked as part of issue #1123 in our
+bug tracker.
+
+https://github.com/letsencrypt/letsencrypt/issues/1123
+
+Prior to implementation of #1123, the client does not actually modify
+ciphersuites (this is intended to be implemented as a "configuration
+enhancement", but the only configuration enhancement implemented
+so far is redirecting HTTP requests to HTTPS in web servers, the
+"redirect" enhancement). The changes here would probably be either a new
+"ciphersuite" enhancement in each plugin that provides an installer,
+or a family of enhancements, one per selectable ciphersuite configuration.

Fixes #555 (in terms of providing and documenting a course of action, not implementing it!).

The theory is that we have decided to track Mozilla's ciphersuite recommendations, which Mozilla has extensively considered for web client compatibility issues and continues to update. We are also open to providing options to track other sources of recommendations, recognizing that different sites have different needs. This patch documents the issues involved in general terms and sets out what we have decided to do. As @pde described, we still need to actually implement these things, which is tracked at #1123.
