add support for CLI flag for mTLS client certificate key passphrase

It adds support for encrypted client certificate key during install/enroll, which done by the cli flag `--elastic-agent-cert-key-passphrase`.

changelog/fragments/1726040648-Add-support-for-passphrase-protected-mTLS-client-certificate-key.yaml

@@ -0,0 +1,35 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: feature
+
+# Change summary; a 80ish characters long description of the change.
+summary: Add support for passphrase protected mTLS client certificate key during install/enroll
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+description: |
+  Adds `--elastic-agent-cert-key-passphrase` command line flag for the `install`
+  and `enroll` commands. The new flag accepts a absolute path for a file containing
+  a passphrase to be used to decrypt the mTLS client certificate key.
+
+# Affected component; a word indicating the component this changeset affects.
+component:
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+#pr: https://github.com/owner/repo/1234
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+#issue: https://github.com/owner/repo/1234

internal/pkg/agent/application/actions/handlers/handler_action_policy_change_test.go

@@ -741,6 +741,46 @@ func TestPolicyChangeHandler_handlePolicyChange_FleetClientSettings(t *testing.T
 						"unexpected error when applying fleet.ssl.certificate and key")
 				},
 			},
+			{
+				name: "certificate and key without passphrase clear out previous passphrase",
+				originalCfg: &configuration.Configuration{
+					Fleet: &configuration.FleetAgentConfig{
+						Client: remote.Config{
+							Host: fleetmTLSServer.URL,
+							Transport: httpcommon.HTTPTransportSettings{
+								TLS: &tlscommon.Config{
+									CAs: []string{string(fleetRootPair.Cert)},
+									Certificate: tlscommon.CertificateConfig{
+										Certificate:    "some certificate",
+										Key:            "some key",
+										Passphrase:     "",
+										PassphrasePath: "/path/to/passphrase",
+									},
+								},
+							},
+						},
+						AccessAPIKey: "ignore",
+					},
+					Settings: configuration.DefaultSettingsConfig(),
+				},
+				newCfg: map[string]interface{}{
+					"fleet.ssl.enabled":     true,
+					"fleet.ssl.certificate": string(agentChildPair.Cert),
+					"fleet.ssl.key":         string(agentChildPair.Key),
+				},
+				setterCalledCount: 1,
+				wantCAs:           []string{string(fleetRootPair.Cert)},
+				wantCertificateConfig: tlscommon.CertificateConfig{
+					Certificate:    string(agentChildPair.Cert),
+					Key:            string(agentChildPair.Key),
+					Passphrase:     "",
+					PassphrasePath: "",
+				},
+				assertErr: func(t *testing.T, err error) {
+					assert.NoError(t, err,
+						"unexpected error when applying fleet.ssl.certificate and key")
+				},
+			},
 			{
 				name: "certificate and key with passphrase_path is applied when present",
 				originalCfg: &configuration.Configuration{

internal/pkg/agent/cmd/enroll.go

@@ -84,6 +84,7 @@ func addEnrollFlags(cmd *cobra.Command) {
 	cmd.Flags().StringP("ca-sha256", "p", "", "Comma-separated list of certificate authority hash pins for server verification used by Elastic Agent and Fleet Server")
 	cmd.Flags().StringP("elastic-agent-cert", "", "", "Elastic Agent client certificate to use with Fleet Server during mTLS authentication")
 	cmd.Flags().StringP("elastic-agent-cert-key", "", "", "Elastic Agent client private key to use with Fleet Server during mTLS authentication")
+	cmd.Flags().StringP("elastic-agent-cert-key-passphrase", "", "", "Path for private key passphrase file used to decrypt Elastic Agent client certificate key")
 	cmd.Flags().BoolP("insecure", "i", false, "Allow insecure connection made by the Elastic Agent. It's also required to use a Fleet Server on a HTTP endpoint")
 	cmd.Flags().StringP("staging", "", "", "Configures Elastic Agent to download artifacts from a staging build")
 	cmd.Flags().StringP("proxy-url", "", "", "Configures the proxy URL: when bootstrapping Fleet Server, it's the proxy used by Fleet Server to connect to Elasticsearch; when enrolling the Elastic Agent to Fleet Server, it's the proxy used by the Elastic Agent to connect to Fleet Server")
@@ -111,6 +112,16 @@ func validateEnrollFlags(cmd *cobra.Command) error {
 	if key != "" && !filepath.IsAbs(key) {
 		return errors.New("--elastic-agent-cert-key must be provided as an absolute path", errors.M("path", key), errors.TypeConfig)
 	}
+	keyPassphrase, _ := cmd.Flags().GetString("elastic-agent-cert-key-passphrase")
+	if keyPassphrase != "" {
+		if !filepath.IsAbs(keyPassphrase) {
+			return errors.New("--elastic-agent-cert-key-passphrase must be provided as an absolute path", errors.M("path", keyPassphrase), errors.TypeConfig)
+		}
+
+		if cert == "" || key == "" {
+			return errors.New("--elastic-agent-cert and --elastic-agent-cert-key must be provided when using --elastic-agent-cert-key-passphrase", errors.M("path", keyPassphrase), errors.TypeConfig)
+		}
+	}
 	esCa, _ := cmd.Flags().GetString("fleet-server-es-ca")
 	if esCa != "" && !filepath.IsAbs(esCa) {
 		return errors.New("--fleet-server-es-ca must be provided as an absolute path", errors.M("path", esCa), errors.TypeConfig)
@@ -180,6 +191,7 @@ func buildEnrollmentFlags(cmd *cobra.Command, url string, token string) []string
 	ca, _ := cmd.Flags().GetString("certificate-authorities")
 	cert, _ := cmd.Flags().GetString("elastic-agent-cert")
 	key, _ := cmd.Flags().GetString("elastic-agent-cert-key")
+	keyPassphrase, _ := cmd.Flags().GetString("elastic-agent-cert-key-passphrase")
 	sha256, _ := cmd.Flags().GetString("ca-sha256")
 	insecure, _ := cmd.Flags().GetBool("insecure")
 	staging, _ := cmd.Flags().GetString("staging")
@@ -285,6 +297,10 @@ func buildEnrollmentFlags(cmd *cobra.Command, url string, token string) []string
 		args = append(args, "--elastic-agent-cert-key")
 		args = append(args, key)
 	}
+	if keyPassphrase != "" {
+		args = append(args, "--elastic-agent-cert-key-passphrase")
+		args = append(args, keyPassphrase)
+	}
 	if sha256 != "" {
 		args = append(args, "--ca-sha256")
 		args = append(args, sha256)
@@ -422,6 +438,7 @@ func enroll(streams *cli.IOStreams, cmd *cobra.Command) error {
 	caSHA256 := cli.StringToSlice(caSHA256str)
 	cert, _ := cmd.Flags().GetString("elastic-agent-cert")
 	key, _ := cmd.Flags().GetString("elastic-agent-cert-key")
+	keyPassphrase, _ := cmd.Flags().GetString("elastic-agent-cert-key-passphrase")
 
 	ctx := handleSignal(context.Background())
 
@@ -449,6 +466,7 @@ func enroll(streams *cli.IOStreams, cmd *cobra.Command) error {
 		CASha256:             caSHA256,
 		Certificate:          cert,
 		Key:                  key,
+		KeyPassphrasePath:    keyPassphrase,
 		Insecure:             insecure,
 		UserProvidedMetadata: make(map[string]interface{}),
 		Staging:              staging,

internal/pkg/agent/cmd/enroll_cmd.go

@@ -111,6 +111,7 @@ type enrollCmdOption struct {
 	CASha256             []string                   `yaml:"ca_sha256,omitempty"`
 	Certificate          string                     `yaml:"certificate,omitempty"`
 	Key                  string                     `yaml:"key,omitempty"`
+	KeyPassphrasePath    string                     `yaml:"key_passphrase_path,omitempty"`
 	Insecure             bool                       `yaml:"insecure,omitempty"`
 	EnrollAPIKey         string                     `yaml:"enrollment_key,omitempty"`
 	Staging              string                     `yaml:"staging,omitempty"`
@@ -149,8 +150,9 @@ func (e *enrollCmdOption) remoteConfig() (remote.Config, error) {
 	}
 	if e.Certificate != "" || e.Key != "" {
 		tlsCfg.Certificate = tlscommon.CertificateConfig{
-			Certificate: e.Certificate,
-			Key:         e.Key,
+			Certificate:    e.Certificate,
+			Key:            e.Key,
+			PassphrasePath: e.KeyPassphrasePath,
 		}
 	}