bpo-43998: Default to TLS 1.2 and increase cipher suite security (GH-25778)

The ssl module now has more secure default settings. Ciphers without forward
secrecy or SHA-1 MAC are disabled by default. Security level 2 prohibits
weak RSA, DH, and ECC keys with less than 112 bits of security.
:class:`~ssl.SSLContext` defaults to minimum protocol version TLS 1.2.
Settings are based on Hynek Schlawack's research.

```
$ openssl version
OpenSSL 1.1.1k  FIPS 25 Mar 2021
$ openssl ciphers -v '@SECLEVEL=2:ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES:DHE+AES:!aNULL:!eNULL:!aDSS:!SHA1:!AESCCM'
TLS_AES_256_GCM_SHA384  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(256) Mac=AEAD
TLS_CHACHA20_POLY1305_SHA256 TLSv1.3 Kx=any      Au=any  Enc=CHACHA20/POLY1305(256) Mac=AEAD
TLS_AES_128_GCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(128) Mac=AEAD
TLS_AES_128_CCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESCCM(128) Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256) Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(128) Mac=AEAD
ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
ECDHE-ECDSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-RSA-CHACHA20-POLY1305 TLSv1.2 Kx=ECDH     Au=RSA  Enc=CHACHA20/POLY1305(256) Mac=AEAD
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA256
ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA256
DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(128) Mac=AEAD
DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA256
DHE-RSA-AES128-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA256
```

Signed-off-by: Christian Heimes <christian@python.org>

1 files changed, 38 insertions, 5 deletions

diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 65370c5..9163927 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -152,15 +152,27 @@ extern const SSL_METHOD *TLSv1_2_method(void);
   #ifndef PY_SSL_DEFAULT_CIPHER_STRING
      #error "Py_SSL_DEFAULT_CIPHERS 0 needs Py_SSL_DEFAULT_CIPHER_STRING"
   #endif
+  #ifndef PY_SSL_MIN_PROTOCOL
+    #define PY_SSL_MIN_PROTOCOL TLS1_2_VERSION
+  #endif
 #elif PY_SSL_DEFAULT_CIPHERS == 1
 /* Python custom selection of sensible cipher suites
- * DEFAULT: OpenSSL's default cipher list. Since 1.0.2 the list is in sensible order.
+ * @SECLEVEL=2: security level 2 with 112 bits minimum security (e.g. 2048 bits RSA key)
+ * ECDH+*: enable ephemeral elliptic curve Diffie-Hellman
+ * DHE+*: fallback to ephemeral finite field Diffie-Hellman
+ * encryption order: AES AEAD (GCM), ChaCha AEAD, AES CBC
  * !aNULL:!eNULL: really no NULL ciphers
- * !MD5:!3DES:!DES:!RC4:!IDEA:!SEED: no weak or broken algorithms on old OpenSSL versions.
  * !aDSS: no authentication with discrete logarithm DSA algorithm
- * !SRP:!PSK: no secure remote password or pre-shared key authentication
+ * !SHA1: no weak SHA1 MAC
+ * !AESCCM: no CCM mode, it's uncommon and slow
+ *
+ * Based on Hynek's excellent blog post (update 2021-02-11)
+ * https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
  */
-  #define PY_SSL_DEFAULT_CIPHER_STRING "DEFAULT:!aNULL:!eNULL:!MD5:!3DES:!DES:!RC4:!IDEA:!SEED:!aDSS:!SRP:!PSK"
+  #define PY_SSL_DEFAULT_CIPHER_STRING "@SECLEVEL=2:ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES:DHE+AES:!aNULL:!eNULL:!aDSS:!SHA1:!AESCCM"
+  #ifndef PY_SSL_MIN_PROTOCOL
+    #define PY_SSL_MIN_PROTOCOL TLS1_2_VERSION
+  #endif
 #elif PY_SSL_DEFAULT_CIPHERS == 2
 /* Ignored in SSLContext constructor, only used to as _ssl.DEFAULT_CIPHER_STRING */
   #define PY_SSL_DEFAULT_CIPHER_STRING SSL_DEFAULT_CIPHER_LIST
@@ -3095,8 +3107,25 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         ERR_clear_error();
         PyErr_SetString(get_state_ctx(self)->PySSLErrorObject,
                         "No cipher can be selected.");
-        return NULL;
+        goto error;
     }
+#ifdef PY_SSL_MIN_PROTOCOL
+    switch(proto_version) {
+    case PY_SSL_VERSION_TLS:
+    case PY_SSL_VERSION_TLS_CLIENT:
+    case PY_SSL_VERSION_TLS_SERVER:
+        result = SSL_CTX_set_min_proto_version(ctx, PY_SSL_MIN_PROTOCOL);
+        if (result == 0) {
+            PyErr_Format(PyExc_ValueError,
+                         "Failed to set minimum protocol 0x%x",
+                          PY_SSL_MIN_PROTOCOL);
+            goto error;
+        }
+        break;
+    default:
+        break;
+    }
+#endif
 
     /* Set SSL_MODE_RELEASE_BUFFERS. This potentially greatly reduces memory
        usage for no cost at all. */
@@ -3119,6 +3148,10 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
 #endif
 
     return (PyObject *)self;
+  error:
+    Py_XDECREF(self);
+    ERR_clear_error();
+    return NULL;
 }
 
 static int