
Ciphersuites
************


Table of Contents
^^^^^^^^^^^^^^^^^

* Introduction

  * Autoupdates

  * Cryptographic choices

  * Sources of defaults

  * Resources for recommendations

  * Changing your settings

  * TODO


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. Certbot
tries to provide defaults that we think are most useful to our users.

As described below, Certbot 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 Certbot 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vulnerabilities is unpublished because many researchers have been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), Certbot 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 Certbot.

If you don't use Certbot 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, Certbot 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 Certbot will be the version that was
most current as of the release date of each client version. Mozilla
offers three separate sets of cryptographic options, which trade off
security and compatibility differently. These are referred to 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 Certbot 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 Certbot 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, Certbot 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 Certbot developers to change
Certbot'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 Encrypt 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, Certbot 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/certbot/certbot/wiki/Ciphersuite-guidance

Certbot 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

   certbot --cipher-recommendations mozilla-secure
   certbot --cipher-recommendations mozilla-intermediate
   certbot --cipher-recommendations mozilla-old

to track Mozilla's *Secure*, *Intermediate*, or *Old* recommendations,
and

   certbot --update-ciphers on

to enable updating ciphers with each new Certbot release, or

   certbot --update-ciphers off

to disable automatic configuration updates. These features have not
yet been implemented and this syntax may change when they are
implemented.


TODO
----

The status of this feature is tracked as part of issue #1123 in our
bug tracker.

https://github.com/certbot/certbot/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.
