Add TLS::Policy::server_uses_own_ciphersuite_preferences()

Previously the server always took its most-preferred cipher out of the
client's list, but this policy allows telling a server to follow the
client's preferences insetad.

4 files changed, 33 insertions, 5 deletions

diff --git a/src/tls/tls_policy.cpp b/src/tls/tls_policy.cpp
index 98e3c6bca..1048e0a62 100644
--- a/src/tls/tls_policy.cpp
+++ b/src/tls/tls_policy.cpp
@@ -131,6 +131,14 @@ u32bit Policy::session_ticket_lifetime() const
 bool Policy::acceptable_protocol_version(Protocol_Version version) const
    {
    return version.known_version(); // accept any version we know about
+
+   // maybe someday...
+   //return version >= Protocol_Version::TLS_V11;
+   }
+
+bool Policy::server_uses_own_ciphersuite_preferences() const
+   {
+   return true;
    }
 
 namespace {
diff --git a/src/tls/tls_policy.h b/src/tls/tls_policy.h
index 125faa665..7176f7fd5 100644
--- a/src/tls/tls_policy.h
+++ b/src/tls/tls_policy.h
@@ -126,6 +126,13 @@ class BOTAN_DLL Policy
       */
       virtual bool acceptable_protocol_version(Protocol_Version version) const;
 
+      /**
+      * @return true if servers should choose the ciphersuite matching
+      *         their highest preference, rather than the clients.
+      *         Has no effect on client side.
+      */
+      virtual bool server_uses_own_ciphersuite_preferences() const;
+
       virtual ~Policy() {}
    };
 
diff --git a/src/tls/tls_server.cpp b/src/tls/tls_server.cpp
index 2c393b32d..d8e827b39 100644
--- a/src/tls/tls_server.cpp
+++ b/src/tls/tls_server.cpp
@@ -112,6 +112,8 @@ u16bit choose_ciphersuite(
    const std::map<std::string, std::vector<X509_Certificate> >& cert_chains,
    const Client_Hello* client_hello)
    {
+   const bool our_choice = policy.server_uses_own_ciphersuite_preferences();
+
    const bool have_srp = creds.attempt_srp("tls-server",
                                            client_hello->sni_hostname());
 
@@ -128,12 +130,15 @@ u16bit choose_ciphersuite(
    const bool have_shared_ecc_curve =
       (policy.choose_curve(client_hello->supported_ecc_curves()) != "");
 
-   // Ordering by our preferences rather than by clients
-   for(size_t i = 0; i != server_suites.size(); ++i)
-      {
-      const u16bit suite_id = server_suites[i];
+   std::vector<u16bit> pref_list = server_suites;
+   std::vector<u16bit> other_list = client_suites;
 
-      if(!value_exists(client_suites, suite_id))
+   if(!our_choice)
+      std::swap(pref_list, other_list);
+
+   for(auto suite_id : pref_list)
+      {
+      if(!value_exists(other_list, suite_id))
          continue;
 
       Ciphersuite suite = Ciphersuite::by_id(suite_id);
diff --git a/src/tls/tls_version.h b/src/tls/tls_version.h
index 39712db27..2fb5365dc 100644
--- a/src/tls/tls_version.h
+++ b/src/tls/tls_version.h
@@ -129,6 +129,14 @@ class BOTAN_DLL Protocol_Version
       */
       bool operator>(const Protocol_Version& other) const;
 
+      /**
+      * @return if this version is later than or equal to other
+      */
+      bool operator>=(const Protocol_Version& other) const
+         {
+         return (*this == other || *this > other);
+         }
+
    private:
       u16bit m_version;
    };