feat: add --intermediate-file flag to step ca certificate

command/ca/certificate.go

@@ -31,6 +31,7 @@ func certificateCommand() cli.Command {
 [**--contact**=<email>] [**--http-listen**=<address>]
 [**--kty**=<type>] [**--curve**=<curve>] [**--size**=<size>] [**--console**]
 [**--x5c-cert**=<file>] [**--x5c-key**=<file>] [**--k8ssa-token-path**=<file>]
+[**--intermediate-file**=<file>]
 [**--offline**] [**--password-file**] [**--ca-url**=<uri>] [**--root**=<file>]
 [**--context**=<name>]`,
 		Description: `**step ca certificate** command generates a new certificate pair
@@ -156,6 +157,11 @@ step CA does not impose this requirement.
 '''
 $ step ca certificate foo.internal foo.crt foo.key \
 --acme https://acme-staging-v02.api.letsencrypt.org/directory --san bar.internal
+'''
+
+Request a new certificate and write the intermediate chain to a separate file:
+'''
+$ step ca certificate foo.internal foo.crt foo.key --intermediate-file intermediate.crt
 '''`,
 		Flags: []cli.Flag{
 			cli.StringSliceFlag{
@@ -164,6 +170,7 @@ $ step ca certificate foo.internal foo.crt foo.key \
 that should be authorized. Use the '--san' flag multiple times to configure
 multiple SANs. The '--san' flag and the '--token' flag are mutually exclusive.`,
 			},
+			flags.IntermediateFile,
 			cli.StringFlag{
 				Name:  "attestation-ca-url",
 				Usage: "The base url of the Attestation CA to use",
@@ -292,7 +299,7 @@ func certificateAction(ctx *cli.Context) error {
 		return errors.New("token is not supported")
 	}
 
-	if err := flow.Sign(ctx, tok, req.CsrPEM, crtFile); err != nil {
+	if err := flow.SignWithIntermediate(ctx, tok, req.CsrPEM, crtFile, ctx.String("intermediate-file")); err != nil {
 		return err
 	}
 
@@ -303,5 +310,8 @@ func certificateAction(ctx *cli.Context) error {
 
 	ui.PrintSelected("Certificate", crtFile)
 	ui.PrintSelected("Private Key", keyFile)
+	if intermediateFile := ctx.String("intermediate-file"); intermediateFile != "" {
+		ui.PrintSelected("Intermediate Certificate", intermediateFile)
+	}
 	return nil
 }

flags/flags.go

@@ -119,6 +119,13 @@ unset, default is P-256 for EC keys and Ed25519 for OKP keys.
 		Usage: `The path to the <file> containing the password to encrypt or decrypt the private key.`,
 	}
 
+	// IntermediateFile is a cli.Flag used to write the intermediate certificate
+	// chain to a separate file.
+	IntermediateFile = cli.StringFlag{
+		Name:  "intermediate-file",
+		Usage: `The path to the <file> where the intermediate certificate chain will be written.`,
+	}
+
 	// NoPassword is a cli.Flag used to avoid using a password to encrypt private
 	// keys.
 	NoPassword = cli.BoolFlag{

utils/cautils/certificate_flow.go

@@ -248,6 +248,12 @@ func (f *CertificateFlow) GenerateIdentityToken(ctx *cli.Context) (string, error
 
 // Sign signs the CSR using the online or the offline certificate authority.
 func (f *CertificateFlow) Sign(ctx *cli.Context, tok string, csr api.CertificateRequest, crtFile string) error {
+	return f.SignWithIntermediate(ctx, tok, csr, crtFile, "")
+}
+
+// SignWithIntermediate signs the CSR and optionally writes the intermediate
+// certificate chain to a separate file.
+func (f *CertificateFlow) SignWithIntermediate(ctx *cli.Context, tok string, csr api.CertificateRequest, crtFile, intermediateFile string) error {
 	client, err := f.GetClient(ctx, tok)
 	if err != nil {
 		return err
@@ -281,6 +287,29 @@ func (f *CertificateFlow) Sign(ctx *cli.Context, tok string, csr api.Certificate
 	if len(resp.CertChainPEM) == 0 {
 		resp.CertChainPEM = []api.Certificate{resp.ServerPEM, resp.CaPEM}
 	}
+
+	if intermediateFile != "" && len(resp.CertChainPEM) > 1 {
+		// Write leaf certificate to crtFile
+		leafPEM, err := pemutil.Serialize(resp.CertChainPEM[0].Certificate)
+		if err != nil {
+			return errors.Wrap(err, "error serializing leaf certificate from step-ca API response")
+		}
+		if err := fileutil.WriteFile(crtFile, pem.EncodeToMemory(leafPEM), 0o600); err != nil {
+			return err
+		}
+
+		// Write intermediate chain to intermediateFile
+		var chainData []byte
+		for _, certPEM := range resp.CertChainPEM[1:] {
+			pemblk, err := pemutil.Serialize(certPEM.Certificate)
+			if err != nil {
+				return errors.Wrap(err, "error serializing intermediate certificate from step-ca API response")
+			}
+			chainData = append(chainData, pem.EncodeToMemory(pemblk)...)
+		}
+		return fileutil.WriteFile(intermediateFile, chainData, 0o600)
+	}
+
 	var data []byte
 	for _, certPEM := range resp.CertChainPEM {
 		pemblk, err := pemutil.Serialize(certPEM.Certificate)