command/ca/token: support custom "user" claim

Add the `--set` and `--set-file` flags to the `step ca token` command,
allowing the user to set keys in the "user" claim in the resulting JWT.

Signed-off-by: Dan Fuhry <dan@fuhry.com>

command/ca/token.go

@@ -34,7 +34,8 @@ func tokenCommand() cli.Command {
 [**--sshpop-cert**=<file>] [**--sshpop-key**=<file>]
 [**--cnf**=<fingerprint>] [**--cnf-file**=<file>]
 [**--ssh**] [**--host**] [**--principal**=<name>] [**--k8ssa-token-path**=<file>]
-[**--ca-url**=<uri>] [**--root**=<file>] [**--context**=<name>]`,
+[**--ca-url**=<uri>] [**--root**=<file>] [**--context**=<name>]
+[**--set**=<key=value>] [**--set-file**=<file>]`,
 		Description: `**step ca token** command generates a one-time token granting access to the
 certificates authority.
 
@@ -174,6 +175,18 @@ add the intermediate and the root in the provisioner configuration:
 $ step ca token --kms yubikey:pin-value=123456 \
   --x5c-cert yubikey:slot-id=82 --x5c-key yubikey:slot-id=82 \
   internal.example.com
+'''
+
+Generate a token with custom data in the "user" claim. The example below can be
+accessed in a template as **{{ .Token.user.field }}**, rendering to the string
+"value".
+
+This is distinct from **.Insecure.User**: any attributes set using this option
+are added to a claim named "user" in the signed JWT produced by this command.
+This data may therefore be considered trusted (insofar as the token itself is
+trusted).
+'''
+$ step ca token --set field=value internal.example.com
 '''`,
 		Flags: []cli.Flag{
 			provisionerKidFlag,
@@ -244,6 +257,8 @@ be invalid for any other API request.`,
 			flags.CaURL,
 			flags.Root,
 			flags.Context,
+			flags.TemplateSet,
+			flags.TemplateSetFile,
 		},
 	}
 }
@@ -350,11 +365,29 @@ func tokenAction(ctx *cli.Context) error {
 		tokenOpts = append(tokenOpts, cautils.WithConfirmationFingerprint(cnf))
 	}
 
+	templateData, err := flags.GetTemplateData(ctx)
+	if err != nil {
+		return err
+	}
+	if templateData != nil {
+		tokenOpts = append(tokenOpts, cautils.WithCustomAttributes(templateData))
+	}
+
 	// --san and --type revoke are incompatible. Revocation tokens do not support SANs.
 	if typ == cautils.RevokeType && len(sans) > 0 {
 		return errs.IncompatibleFlagWithFlag(ctx, "san", "revoke")
 	}
 
+	// --offline doesn't support tokenOpts, so reject set/set-file
+	if offline {
+		if len(ctx.StringSlice("set")) > 0 {
+			return errs.IncompatibleFlagWithFlag(ctx, "offline", "set")
+		}
+		if ctx.String("set-file") != "" {
+			return errs.IncompatibleFlagWithFlag(ctx, "offline", "set-file")
+		}
+	}
+
 	// parse times or durations
 	notBefore, ok := flags.ParseTimeOrDuration(ctx.String("not-before"))
 	if !ok {

token/options.go

@@ -80,6 +80,25 @@ func WithStep(v interface{}) Options {
 	}
 }
 
+// WithUserData returns an Option function that merges the provided map with the
+// existing user claim in the payload.
+func WithUserData(v map[string]interface{}) Options {
+	return func(c *Claims) error {
+		if _, ok := c.ExtraClaims[UserClaim]; !ok {
+			c.Set(UserClaim, make(map[string]interface{}))
+		}
+		s := c.ExtraClaims[UserClaim]
+		sm, ok := s.(map[string]interface{})
+		if !ok {
+			return fmt.Errorf("%q claim is %T, not map[string]interface{}", UserClaim, s)
+		}
+		for k, val := range v {
+			sm[k] = val
+		}
+		return nil
+	}
+}
+
 // WithSSH returns an Options function that sets the step claim with the ssh
 // property in the value.
 func WithSSH(v interface{}) Options {

token/token.go

@@ -32,6 +32,9 @@ const SANSClaim = "sans"
 // StepClaim is the property name for a JWT claim the stores the custom information in the certificate.
 const StepClaim = "step"
 
+// UserClaim is the property name for a JWT claim that stores user-provided custom information.
+const UserClaim = "user"
+
 // ConfirmationClaim is the property name for a JWT claim that stores a JSON
 // object used as Proof-Of-Possession.
 const ConfirmationClaim = "cnf"

utils/cautils/certificate_flow.go

@@ -43,6 +43,7 @@ type flowContext struct {
 	SSHPublicKey            ssh.PublicKey
 	CertificateRequest      *x509.CertificateRequest
 	ConfirmationFingerprint string
+	CustomAttributes        map[string]interface{}
 }
 
 // sharedContext is used to share information between commands.
@@ -88,6 +89,18 @@ func WithConfirmationFingerprint(fp string) Option {
 	})
 }
 
+// WithCustomAttributes adds custom attributes to be set in the "user" claim.
+func WithCustomAttributes(v map[string]interface{}) Option {
+	return newFuncFlowOption(func(fo *flowContext) {
+		if fo.CustomAttributes == nil {
+			fo.CustomAttributes = make(map[string]interface{})
+		}
+		for k, val := range v {
+			fo.CustomAttributes[k] = val
+		}
+	})
+}
+
 // NewCertificateFlow initializes a cli flow to get a new certificate.
 func NewCertificateFlow(ctx *cli.Context, opts ...Option) (*CertificateFlow, error) {
 	var err error

utils/cautils/token_generator.go

@@ -108,6 +108,11 @@ func (t *TokenGenerator) SignToken(sub string, sans []string, opts ...token.Opti
 		opts = append(opts, token.WithConfirmationFingerprint(sharedContext.ConfirmationFingerprint))
 	}
 
+	// Add custom user data, if set.
+	if sharedContext.CustomAttributes != nil {
+		opts = append(opts, token.WithUserData(sharedContext.CustomAttributes))
+	}
+
 	return t.Token(sub, opts...)
 }
 
@@ -126,6 +131,11 @@ func (t *TokenGenerator) SignSSHToken(sub, certType string, principals []string,
 		ValidBefore: notAfter,
 	})}, opts...)
 
+	// Add custom user data, if set.
+	if sharedContext.CustomAttributes != nil {
+		opts = append(opts, token.WithUserData(sharedContext.CustomAttributes))
+	}
+
 	return t.Token(sub, opts...)
 }