Re-add curve fallback detection

cipherscan

@@ -63,6 +63,8 @@ CAPATH=""
 SAVECRT=""
 TEST_CURVES="False"
 has_curves="False"
+# openssl formated list of curves that will cause server to select ECC suite
+ecc_ciphers=""
 unset known_certs
 declare -A known_certs
 unset cert_checksums
@@ -458,7 +460,11 @@ test_cipher_on_target() {
         if [[ $pfs =~ ECDH ]]; then
             has_curves="True"
             if [ $TEST_CURVES == "True" ]; then
-                test_ecc_curves
+                test_curves
+                if [ "$ecc_ciphers" != "" ]; then
+                    ecc_ciphers+=":"
+                fi
+                ecc_ciphers+="$cipher"
             else
                 # resolve the openssl curve to the proper IANA name
                 current_curves="$(get_curve_name $(echo $pfs|cut -d ',' -f2))"
@@ -622,6 +628,7 @@ display_results_in_terminal() {
     fi
     if [ $TEST_CURVES == "True" ]; then
         echo "Curves ordering: $curvesordering"
+        echo "Curves fallback: $fallback_supported"
     fi
 }
 
@@ -656,7 +663,11 @@ display_results_in_json() {
         echo -n "}"
         ctr=$((ctr+1))
     done
-    echo ']}'
+    echo -n ']'
+    if [ $TEST_CURVES == "True" ]; then
+        echo -n ",\"curves_fallback\":\"$fallback_supported\""
+    fi
+    echo '}'
 }
 
 test_serverside_ordering() {
@@ -712,9 +723,7 @@ test_serverside_ordering() {
     fi
 }
 
-test_ecc_curves() {
-    # openssl formated list of curves that will cause server to select ECC suite
-    local ecc_ciphers=""
+test_curves() {
     # "True" if server supports ciphers that don't use ECC at a lower priority
     local fallback_available="False"
 
@@ -817,8 +826,7 @@ test_ecc_curves() {
         local tmp=$(echo Q | $sslcommand -curves $test_curves 2>/dev/null)
         parse_openssl_output <<<"$tmp"
 
-        if [[ -z $current_protocol || $current_cipher == "(NONE)" \
-            || $current_cipher == '0000' ]]; then
+        if [[ -z $current_protocol || $current_cipher == "(NONE)" || $current_cipher == '0000' ]]; then
                 fallback_supported="order-specific"
                 verbose "Server aborted connection"
         else
@@ -829,18 +837,14 @@ test_ecc_curves() {
             if [[ ${ephem_data[0]} =~ ECDH ]]; then
                 verbose "Server did select ${ephem_data[1]} curve"
                 curves_ordering="inconclusive-${ephem_data[1]}"
-                for id in "${!curve_names[@]}"; do
-                    if [[ ${curve_names[$id]} =~ ${ephem_data[1]} ]]; then
-                        local canonic_name=(${curve_names[$id]})
-                        if [[ ${canonic_name[0]} == $most_wanted ]]; then
-                            curves_ordering="client"
-                            break
-                        else
-                            curves_ordering="server"
-                            break
-                        fi
-                    fi
-                done
+                local cname="$(get_curve_name ${ephem_data[1]})"
+                if [ "$cname" == "$most_wanted" ]; then
+                    curves_ordering="client"
+                    break
+                else
+                    curves_ordering="server"
+                    break
+                fi
             else
                 # some servers downgrade to non ECDH when curve order is changed
                 curves_ordering="inconclusive-noecc"
@@ -849,6 +853,76 @@ test_ecc_curves() {
     fi
 }
 
+test_curves_fallback() {
+    # "True" if server supports ciphers that don't use ECC at a lower priority
+    local fallback_available="False"
+    # return variable: whatever a server will fall back to non ECC suite when
+    # client doesn't advertise support for curves the server needs
+    fallback_supported="False"
+
+    # prepare the ssl command we'll be using
+    local sslcommand=""
+    sslcommand="$TIMEOUTBIN $TIMEOUT $OPENSSLBIN s_client"
+    if [ -n "$CAPATH" ]; then
+        sslcommand+=" -CApath $CAPATH -showcerts"
+    elif [ -e "$CACERTS" ]; then
+        sslcommand+=" -CAfile $CACERTS"
+    fi
+    sslcommand+=" -status $SCLIENTARGS -connect $TARGET -cipher $ecc_ciphers"
+    # force the TLS to send a TLS1.0 client hello at least, as with SSLv2
+    # ciphers present it will try to send a SSLv2 compatible client hello
+    sslcommand+=" -no_ssl2 -no_ssl3"
+
+    #
+    # here we use the same logic as with detecting cipher suites: first
+    # advertise all curves as supported, then remove curves one by one until we
+    # either get a fallback to a non ECC cipher, we run of curves or server
+    # tries to negotiate a curve we didn't advertise
+    #
+    local curves=(${CURVES[*]})
+    while [[ ${#curves[@]} -gt 0 ]]; do
+        OLDIFS="$IFS"
+        IFS=':'
+        local test_curves="${curves[*]}"
+        IFS="$OLDIFS"
+        verbose "Testing $test_curves"
+
+        ratelimit
+        local tmp=$(echo Q | $sslcommand -curves $test_curves 2>/dev/null)
+        parse_openssl_output <<<"$tmp"
+
+        if [[ -z $current_protocol || $current_cipher == "(NONE)" || $current_cipher == '0000' ]]; then
+                # server aborted connection
+                if [[ $fallback_available == "True" ]]; then
+                    fallback_supported="False"
+                else
+                    fallback_supported="unknown"
+                fi
+                break
+        else
+            # server accepted connection
+            local ephem_data=(${current_pfs//,/ })
+
+            if [[ ${ephem_data[0]} =~ ECDH ]]; then
+                # we got an ecc connection, remove the curve from the list of testable curves
+                local cname="$(get_curve_name ${ephem_data[1]})"
+                verbose "Server selected curve $cname"
+                for id in "${!curves[@]}"; do
+                    if [ "${curves[id]}" == "$cname" ]; then
+                        unset curves[$id]
+                        break
+                    fi
+                done
+            else
+                verbose "Server fell back to $current_cipher"
+                # ok, we got a fallback
+                fallback_supported="True"
+                break
+            fi
+        fi
+    done
+}
+
 # If no options are given, give usage information and exit (with error code)
 if [ $# -eq 0 ]; then
    usage;
@@ -979,7 +1053,7 @@ get_cipher_pref $CIPHERSUITE
 test_serverside_ordering
 
 if [[ $TEST_CURVES == "True" ]]; then
-    test_ecc_curves
+    test_curves_fallback
 fi
 
 if [ "$OUTPUTFORMAT" == "json" ]; then