Add ECC curve selection order config in TLS server

As per RFC 4492 Sec 5.1, the preferred order of selection of named
curves is based on client preferences.

Currently, the SSL application only picks entries according to the
absolute order of entries as tracked in a hardcoded list in code.

This patch changes things so that the client-specified order is
preferred. It also allows a mode where the server can be configured to
override the client's preferred order with its own, although the chosen
ECC must still be within both lists.

The configuration is done through the following options:

- `eccs`, shared by clients and servers alike, allows the specification
  of the supported named curves, in their preferred order, and may
  eventually support more values for explicit primes and so on.
- `honor_ecc_order`, a server-only option, is similar to
  `honor_cipher_order` and will, by default let the server pick the
  client-preferred ECC, and otherwise pick the server-preferred one.

The default value for `eccs` is the same as before, although the
server-chosen ECC now defaults to the client rather than previous
choice.

A function `ssl:eccs()` has been added that returns the highest
supported ECCs for the library.

1 files changed, 30 insertions, 1 deletions

diff --git a/lib/ssl/doc/src/ssl.xml b/lib/ssl/doc/src/ssl.xml
index 68f2f97b6e..edc7e0d8b2 100644
--- a/lib/ssl/doc/src/ssl.xml
+++ b/lib/ssl/doc/src/ssl.xml
@@ -170,6 +170,14 @@
       <tag><c>SNIfun::fun()</c></tag>
       <item><p><c>= fun(ServerName :: string()) -> [ssl_option()]</c></p></item>
 
+      <tag><c>named_curve() =</c></tag>
+      <item><p><c>sect571r1 | sect571k1 | secp521r1 | brainpoolP512r1
+       | sect409k1 | sect409r1 | brainpoolP384r1 | secp384r1
+       | sect283k1 | sect283r1 | brainpoolP256r1 | secp256k1 | secp256r1
+       | sect239k1 | sect233k1 | sect233r1 | secp224k1 | secp224r1
+       | sect193r1 | sect193r2 | secp192k1 | secp192r1 | sect163k1
+       | sect163r1 | sect163r2 | secp160k1 | secp160r1 | secp160r2</c></p></item>
+
     </taglist>
   </section>
 
@@ -217,6 +225,11 @@
       Anonymous cipher suites are supported for testing purposes
       only and are not be used when security matters.</p></item>
 
+      <tag><c>{eccs, [named_curve()]}</c></tag>
+      <item><p> Allows to specify the order of preference for named curves
+      and to restrict their usage when using a cipher suite supporting them.
+      </p></item>
+
       <tag><c>{secure_renegotiate, boolean()}</c></tag>
       <item><p>Specifies if to reject renegotiation attempt that does
       not live up to 
@@ -751,6 +764,11 @@ fun(srp, Username :: string(), UserState :: term()) ->
       (the default), use the client's preference.
       </item>
       
+      <tag><c>{honor_ecc_order, boolean()}</c></tag>
+      <item>If true, use the server's preference for ECC curve selection. If false
+      (the default), use the client's preference.
+      </item>
+
       <tag><c>{signature_algs, [{hash(), ecdsa | rsa | dsa}]}</c></tag>
       <item><p> The algorithms specified by
       this option will be the ones accepted by the server in a signature algorithm
@@ -804,6 +822,17 @@ fun(srp, Username :: string(), UserState :: term()) ->
     </func>
 
     <func>
+      <name>eccs() -></name>
+      <name>eccs(protocol()) -> [named_curve()]</name>
+      <fsummary>Returns a list of supported ECCs.</fsummary>
+
+      <desc><p>Returns a list of supported ECCs. <c>eccs()</c>
+      is equivalent to calling <c>eccs(Protocol)</c> with all
+      supported protocols and then deduplicating the output.</p>
+      </desc>
+    </func>
+
+    <func>
       <name>clear_pem_cache() -> ok </name>
       <fsummary> Clears the pem cache</fsummary>
 
@@ -898,7 +927,7 @@ fun(srp, Username :: string(), UserState :: term()) ->
       <fsummary>Returns all the connection information.
       </fsummary>
       <type>
-        <v>Item = protocol | cipher_suite | sni_hostname | atom()</v>
+        <v>Item = protocol | cipher_suite | sni_hostname | ecc | atom()</v>
 	<d>Meaningful atoms, not specified above, are the ssl option names.</d>
 	<v>Result = [{Item::atom(), Value::term()}]</v>
         <v>Reason = term()</v>