[security] rpc: Do not transfer access/secret key. (#4857)

This is an improvement upon existing implementation
by avoiding transfer of access and secret keys over
the network. This change only exchanges JWT tokens
generated by an rpc client. Even if the JWT can be
traced over the network on a non-TLS connection, this
change makes sure that we never really expose the
secret key over the network.

cmd/web-handlers_test.go

@@ -34,6 +34,7 @@ import (
 	"strings"
 	"testing"
 
+	jwtgo "github.com/dgrijalva/jwt-go"
 	humanize "github.com/dustin/go-humanize"
 	"github.com/minio/minio-go/pkg/policy"
 	"github.com/minio/minio-go/pkg/set"
@@ -667,6 +668,16 @@ func TestWebCreateURLToken(t *testing.T) {
 	ExecObjectLayerTest(t, testCreateURLToken)
 }
 
+func getTokenString(accessKey, secretKey string) (string, error) {
+	utcNow := UTCNow()
+	token := jwtgo.NewWithClaims(jwtgo.SigningMethodHS512, jwtgo.StandardClaims{
+		ExpiresAt: utcNow.Add(defaultJWTExpiry).Unix(),
+		IssuedAt:  utcNow.Unix(),
+		Subject:   accessKey,
+	})
+	return token.SignedString([]byte(secretKey))
+}
+
 func testCreateURLToken(obj ObjectLayer, instanceType string, t TestErrHandler) {
 	apiRouter := initTestWebRPCEndPoint(obj)
 	credentials := serverConfig.GetCredential()
@@ -700,6 +711,21 @@ func testCreateURLToken(obj ObjectLayer, instanceType string, t TestErrHandler)
 	if !isAuthTokenValid(tokenReply.Token) {
 		t.Fatalf("token is not valid")
 	}
+
+	// Token is invalid.
+	if isAuthTokenValid("") {
+		t.Fatalf("token shouldn't be valid, but it is")
+	}
+
+	token, err := getTokenString("invalid-access", credentials.SecretKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Token has invalid access key.
+	if isAuthTokenValid(token) {
+		t.Fatalf("token shouldn't be valid, but it is")
+	}
 }
 
 // Wrapper for calling Upload Handler