add support for TLS 1.2

This commit adds support for TLS 1.2 in
addition to TLS 1.3.

Some legacy language versions (like Java 8)
do not support TLS 1.3, and therefore, fail
to connect to KES.

However, we want to support only PFS TLS connections.
Therefore, we limit the supported cipher suites to
ECDHE key exchange and AEAD ciphers.

Further, we have to drop ChaCha20-Poly1305 in case
of FIPS 140.

cmd/kes/server.go

@@ -27,6 +27,7 @@ import (
 	"github.com/fatih/color"
 	"github.com/minio/kes"
 	"github.com/minio/kes/internal/auth"
+	"github.com/minio/kes/internal/fips"
 	xhttp "github.com/minio/kes/internal/http"
 	xlog "github.com/minio/kes/internal/log"
 	"github.com/minio/kes/internal/metric"
@@ -269,13 +270,39 @@ func server(args []string) {
 		Addr:    config.Addr,
 		Handler: mux,
 		TLSConfig: &tls.Config{
-			MinVersion: tls.VersionTLS13,
+			MinVersion: tls.VersionTLS12,
 		},
 		ErrorLog: errorLog.Log(),
 
 		ReadTimeout:  5 * time.Second,
 		WriteTimeout: 0 * time.Second, // explicitly set no write timeout - see timeout handler.
 	}
+
+	// Limit the supported cipher suites to the secure TLS 1.2/1.3 subset - i.e. only ECDHE key exchange and only AEAD ciphers.
+	if fips.Enabled {
+		server.TLSConfig.CipherSuites = []uint16{
+			tls.TLS_AES_128_GCM_SHA256, // TLS 1.3
+			tls.TLS_AES_256_GCM_SHA384,
+
+			tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, // TLS 1.2
+			tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+			tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+			tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+		}
+	} else {
+		server.TLSConfig.CipherSuites = []uint16{
+			tls.TLS_AES_128_GCM_SHA256, // TLS 1.3
+			tls.TLS_AES_256_GCM_SHA384,
+			tls.TLS_CHACHA20_POLY1305_SHA256,
+
+			tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, // TLS 1.2
+			tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+			tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
+			tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+			tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+			tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
+		}
+	}
 	switch strings.ToLower(mtlsAuthFlag) {
 	case "on":
 		server.TLSConfig.ClientAuth = tls.RequireAndVerifyClientCert