diskless-workstation/serving/fleet-manager/cli.go

// cli.go — the `register-admin` bootstrap subcommand and the one-time enrolment
// token it mints.
//
// WebAuthn registration is a BROWSER ceremony (the credential is born on the
// YubiKey via navigator.credentials.create) — a CLI can't do it directly. So the
// CLI's job is to AUTHORISE an enrolment: it mints a short-lived HMAC-signed token
// (over a shared enrol key on the mgmt host) and prints an enrolment URL. The
// browser completes the ceremony; the register endpoint verifies the token. This
// keeps registration closed (no anyone-can-register endpoint) while the actual
// key material never leaves the YubiKey. Single-use is enforced by the credStore
// jti ledger.
package main

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"flag"
	"fmt"
	"os"
	"strconv"
	"strings"
	"time"
)

type enrollClaims struct {
	Name string
	JTI  string
	Exp  int64
}

func b64(b []byte) string            { return base64.RawURLEncoding.EncodeToString(b) }
func unb64(s string) ([]byte, error) { return base64.RawURLEncoding.DecodeString(s) }

func hmacSum(key []byte, msg string) []byte {
	m := hmac.New(sha256.New, key)
	m.Write([]byte(msg))
	return m.Sum(nil)
}

// mintEnrollToken -> base64url(name|exp|jti) "." base64url(hmac).
func mintEnrollToken(key []byte, name string, ttl time.Duration) (string, error) {
	jb := make([]byte, 12)
	if _, err := rand.Read(jb); err != nil {
		return "", err
	}
	jti := hex.EncodeToString(jb)
	exp := time.Now().Add(ttl).Unix()
	msg := name + "|" + strconv.FormatInt(exp, 10) + "|" + jti
	return b64([]byte(msg)) + "." + b64(hmacSum(key, msg)), nil
}

func verifyEnrollToken(key []byte, token string) (enrollClaims, error) {
	parts := strings.SplitN(token, ".", 2)
	if len(parts) != 2 {
		return enrollClaims{}, fmt.Errorf("malformed token")
	}
	msg, err := unb64(parts[0])
	if err != nil {
		return enrollClaims{}, fmt.Errorf("bad payload")
	}
	sig, err := unb64(parts[1])
	if err != nil {
		return enrollClaims{}, fmt.Errorf("bad signature encoding")
	}
	if subtle.ConstantTimeCompare(sig, hmacSum(key, string(msg))) != 1 {
		return enrollClaims{}, fmt.Errorf("bad signature")
	}
	f := strings.SplitN(string(msg), "|", 3)
	if len(f) != 3 {
		return enrollClaims{}, fmt.Errorf("bad claims")
	}
	exp, _ := strconv.ParseInt(f[1], 10, 64)
	if time.Now().Unix() > exp {
		return enrollClaims{}, fmt.Errorf("token expired")
	}
	return enrollClaims{Name: f[0], JTI: f[2], Exp: exp}, nil
}

// loadEnrollKey reads the shared enrol key (>=32 bytes). Used by the SERVICE — it
// must already exist (the same key the CLI mints with).
func loadEnrollKey(path string) ([]byte, error) {
	if path == "" {
		return nil, fmt.Errorf("no enrol key file configured (FLEET_MGR_ENROLL_KEY_FILE)")
	}
	b, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	if len(b) < 32 {
		return nil, fmt.Errorf("enrol key %s too short (need >=32 bytes)", path)
	}
	return b, nil
}

// loadOrCreateEnrollKey is the CLI's variant: it generates the key on first use.
func loadOrCreateEnrollKey(path string) ([]byte, error) {
	if path == "" {
		return nil, fmt.Errorf("set -key or FLEET_MGR_ENROLL_KEY_FILE")
	}
	if b, err := os.ReadFile(path); err == nil {
		if len(b) < 32 {
			return nil, fmt.Errorf("enrol key %s too short (need >=32 bytes)", path)
		}
		return b, nil
	} else if !os.IsNotExist(err) {
		return nil, err
	}
	key := make([]byte, 32)
	if _, err := rand.Read(key); err != nil {
		return nil, err
	}
	if err := os.WriteFile(path, key, 0o600); err != nil {
		return nil, err
	}
	fmt.Fprintf(os.Stderr, "note: generated a new enrol key at %s (mount the SAME file into the service)\n", path)
	return key, nil
}

// runRegisterAdmin implements: fleet-manager register-admin [-key F] [-origin URL] <name>
func runRegisterAdmin(args []string) int {
	fs := flag.NewFlagSet("register-admin", flag.ContinueOnError)
	keyFile := fs.String("key", env("FLEET_MGR_ENROLL_KEY_FILE", ""), "shared enrol key file")
	origin := fs.String("origin", env("FLEET_MGR_RP_ORIGIN", ""), "admin UI origin (for the enrolment URL)")
	ttl := fs.Duration("ttl", 10*time.Minute, "enrolment token validity")
	if err := fs.Parse(args); err != nil {
		return 2
	}
	name := fs.Arg(0)
	if name == "" {
		fmt.Fprintln(os.Stderr, "usage: fleet-manager register-admin [-key F] [-origin URL] [-ttl D] <name>")
		return 2
	}
	key, err := loadOrCreateEnrollKey(*keyFile)
	if err != nil {
		fmt.Fprintf(os.Stderr, "enrol key: %v\n", err)
		return 1
	}
	tok, err := mintEnrollToken(key, name, *ttl)
	if err != nil {
		fmt.Fprintf(os.Stderr, "mint: %v\n", err)
		return 1
	}
	base := strings.TrimRight(*origin, "/")
	if base == "" {
		base = "https://<admin-ui-origin>"
	}
	fmt.Printf("Enrolment link for %q (valid %s) — open on the mgmt network, then tap the YubiKey:\n\n  %s/#/enrol?token=%s\n\n", name, ttl.String(), base, tok)
	return 0
}