goods/api/internal/auth/auth.go

// Package auth provides minimal single-account authentication for the admin
// console: a bcrypt-verified login and a stdlib HMAC-SHA256 signed token
// (JWT-compatible) plus a chi middleware that guards write routes.
package auth

import (
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"errors"
	"net/http"
	"strings"
	"time"

	"golang.org/x/crypto/bcrypt"
)

// Authenticator holds the single admin credential and token signing secret.
type Authenticator struct {
	username     string
	passwordHash []byte
	secret       []byte
	ttl          time.Duration
}

// New builds an Authenticator. passwordHash must be a bcrypt hash.
func New(username string, passwordHash, secret []byte, ttl time.Duration) *Authenticator {
	return &Authenticator{username: username, passwordHash: passwordHash, secret: secret, ttl: ttl}
}

// ErrInvalidCredentials is returned when login fails.
var ErrInvalidCredentials = errors.New("invalid credentials")

// Login verifies the username/password and returns a signed token on success.
func (a *Authenticator) Login(username, password string) (string, error) {
	if username != a.username {
		// Still run bcrypt to keep timing roughly constant.
		_ = bcrypt.CompareHashAndPassword(a.passwordHash, []byte(password))
		return "", ErrInvalidCredentials
	}
	if err := bcrypt.CompareHashAndPassword(a.passwordHash, []byte(password)); err != nil {
		return "", ErrInvalidCredentials
	}
	return a.issue(username)
}

type claims struct {
	Sub string `json:"sub"`
	Exp int64  `json:"exp"`
}

func b64(b []byte) string { return base64.RawURLEncoding.EncodeToString(b) }

func (a *Authenticator) sign(signingInput string) string {
	mac := hmac.New(sha256.New, a.secret)
	mac.Write([]byte(signingInput))
	return b64(mac.Sum(nil))
}

func (a *Authenticator) issue(sub string) (string, error) {
	header := b64([]byte(`{"alg":"HS256","typ":"JWT"}`))
	payloadJSON, err := json.Marshal(claims{Sub: sub, Exp: time.Now().Add(a.ttl).Unix()})
	if err != nil {
		return "", err
	}
	payload := b64(payloadJSON)
	signingInput := header + "." + payload
	return signingInput + "." + a.sign(signingInput), nil
}

// Verify checks a token's signature and expiry, returning the subject.
func (a *Authenticator) Verify(token string) (string, error) {
	parts := strings.Split(token, ".")
	if len(parts) != 3 {
		return "", errors.New("malformed token")
	}
	signingInput := parts[0] + "." + parts[1]
	if !hmac.Equal([]byte(a.sign(signingInput)), []byte(parts[2])) {
		return "", errors.New("bad signature")
	}
	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
	if err != nil {
		return "", err
	}
	var c claims
	if err := json.Unmarshal(payload, &c); err != nil {
		return "", err
	}
	if time.Now().Unix() >= c.Exp {
		return "", errors.New("token expired")
	}
	return c.Sub, nil
}

type ctxKey int

const userKey ctxKey = 0

// UserFrom returns the authenticated subject from the request context.
func UserFrom(ctx context.Context) string {
	if v, ok := ctx.Value(userKey).(string); ok {
		return v
	}
	return ""
}

// Middleware rejects requests without a valid Bearer token.
func (a *Authenticator) Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		h := r.Header.Get("Authorization")
		token := strings.TrimPrefix(h, "Bearer ")
		if token == h || token == "" {
			http.Error(w, `{"error":{"code":"unauthorized","message":"missing token"}}`, http.StatusUnauthorized)
			return
		}
		sub, err := a.Verify(token)
		if err != nil {
			http.Error(w, `{"error":{"code":"unauthorized","message":"invalid token"}}`, http.StatusUnauthorized)
			return
		}
		ctx := context.WithValue(r.Context(), userKey, sub)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}