Issue #20995: Enhance default ciphers used by the ssl module

Closes #20995 by Enabling better security by prioritizing ciphers
such that:

* Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
* Prefer ECDHE over DHE for better performance
* Prefer any AES-GCM over any AES-CBC for better performance and security
* Then Use HIGH cipher suites as a fallback
* Then Use 3DES as fallback which is secure but slow
* Finally use RC4 as a fallback which is problematic but needed for
  compatibility some times.
* Disable NULL authentication, NULL encryption, and MD5 MACs for security
  reasons

3 files changed, 38 insertions, 19 deletions

diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index 0ed5fb2..3aa1e36 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -1665,17 +1665,10 @@ If you have advanced security requirements, fine-tuning of the ciphers
 enabled when negotiating a SSL session is possible through the
 :meth:`SSLContext.set_ciphers` method.  Starting from Python 3.2.3, the
 ssl module disables certain weak ciphers by default, but you may want
-to further restrict the cipher choice.  For example::
-
-   context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
-   context.set_ciphers('HIGH:!aNULL:!eNULL')
-
-The ``!aNULL:!eNULL`` part of the cipher spec is necessary to disable ciphers
-which don't provide both encryption and authentication.  Be sure to read
-OpenSSL's documentation about the `cipher list
-format <http://www.openssl.org/docs/apps/ciphers.html#CIPHER_LIST_FORMAT>`_.
-If you want to check which ciphers are enabled by a given cipher list,
-use the ``openssl ciphers`` command on your system.
+to further restrict the cipher choice. Be sure to read OpenSSL's documentation
+about the `cipher list format <http://www.openssl.org/docs/apps/ciphers.html#CIPHER_LIST_FORMAT>`_.
+If you want to check which ciphers are enabled by a given cipher list, use the
+``openssl ciphers`` command on your system.
 
 Multi-processing
 ^^^^^^^^^^^^^^^^
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 87b70fa..da80802 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -162,14 +162,37 @@ else:
 
 # Disable weak or insecure ciphers by default
 # (OpenSSL's default setting is 'DEFAULT:!aNULL:!eNULL')
-_DEFAULT_CIPHERS = 'DEFAULT:!aNULL:!eNULL:!LOW:!EXPORT:!SSLv2'
-
-# restricted and more secure ciphers
-# HIGH: high encryption cipher suites with key length >= 128 bits (no MD5)
-# !aNULL: only authenticated cipher suites (no anonymous DH)
-# !RC4: no RC4 streaming cipher, RC4 is broken
-# !DSS: RSA is preferred over DSA
-_RESTRICTED_CIPHERS = 'HIGH:!aNULL:!RC4:!DSS'
+# Enable a better set of ciphers by default
+# This list has been explicitly chosen to:
+#   * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
+#   * Prefer ECDHE over DHE for better performance
+#   * Prefer any AES-GCM over any AES-CBC for better performance and security
+#   * Then Use HIGH cipher suites as a fallback
+#   * Then Use 3DES as fallback which is secure but slow
+#   * Finally use RC4 as a fallback which is problematic but needed for
+#     compatibility some times.
+#   * Disable NULL authentication, NULL encryption, and MD5 MACs for security
+#     reasons
+_DEFAULT_CIPHERS = (
+    'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+HIGH:'
+    'DH+HIGH:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+HIGH:RSA+3DES:ECDH+RC4:'
+    'DH+RC4:RSA+RC4:!aNULL:!eNULL:!MD5'
+)
+
+# Restricted and more secure ciphers
+# This list has been explicitly chosen to:
+#   * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
+#   * Prefer ECDHE over DHE for better performance
+#   * Prefer any AES-GCM over any AES-CBC for better performance and security
+#   * Then Use HIGH cipher suites as a fallback
+#   * Then Use 3DES as fallback which is secure but slow
+#   * Disable NULL authentication, NULL encryption, MD5 MACs, DSS, and RC4 for
+#     security reasons
+_RESTRICTED_CIPHERS = (
+    'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+HIGH:'
+    'DH+HIGH:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+HIGH:RSA+3DES:!aNULL:'
+    '!eNULL:!MD5:!DSS:!RC4'
+)
 
 
 class CertificateError(ValueError):
diff --git a/Misc/NEWS b/Misc/NEWS
index ab25f1a..b5b719d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -21,6 +21,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20995: Enhance default ciphers used by the ssl module to enable
+  better security an prioritize perfect forward secrecy.
+
 - Issue #20884: Don't assume that __file__ is defined on importlib.__init__.
 
 - Issue #20879: Delay the initialization of encoding and decoding tables for