go-usertoken/auth.go

package usertoken

import (
	"context"
	"crypto/rsa"
	"fmt"
	"net/http"
	"os"
	"strings"
	"time"
)

// Config selects one verify mode.
// Set PublicKey for static RS256, or OIDCIssuer for discovery. Not both.
// Skew of zero uses 1 minute. GroupsClaim empty uses "groups" (OIDC only).
// Audience empty is allowed only for OIDC, where it skips the audience check.
type Config struct {
	PublicKey    *rsa.PublicKey
	Issuer       string
	Audience     string
	OIDCIssuer   string
	OIDCAudience string
	GroupsClaim  string
	Skew         time.Duration
}

// Auth verifies bearers for one Microservice process.
type Auth struct {
	v tokenVerifier
}

// FromEnv reads the process environment and builds an Auth.
// Set USER_TOKEN_PUBLIC_KEY_FILE or OIDC_ISSUER, not both and not neither.
// Static mode also reads USER_TOKEN_ISSUER and USER_TOKEN_AUDIENCE.
// OIDC mode reads OIDC_AUDIENCE and optional OIDC_GROUPS_CLAIM.
// Optional USER_TOKEN_SKEW is a time.Duration; empty uses 1 minute.
// It returns an error when the mode is ambiguous, the key file is unreadable,
// USER_TOKEN_SKEW cannot be parsed, or OIDC discovery fails.
// ctx is used for OIDC discovery.
func FromEnv(ctx context.Context) (*Auth, error) {
	pubFile := strings.TrimSpace(os.Getenv("USER_TOKEN_PUBLIC_KEY_FILE"))
	oidcIss := strings.TrimSpace(os.Getenv("OIDC_ISSUER"))
	switch {
	case pubFile != "" && oidcIss != "":
		return nil, fmt.Errorf("set USER_TOKEN_PUBLIC_KEY_FILE or OIDC_ISSUER, not both")
	case pubFile == "" && oidcIss == "":
		return nil, fmt.Errorf("set USER_TOKEN_PUBLIC_KEY_FILE or OIDC_ISSUER")
	}
	var skew time.Duration
	if raw := strings.TrimSpace(os.Getenv("USER_TOKEN_SKEW")); raw != "" {
		var err error
		skew, err = time.ParseDuration(raw)
		if err != nil {
			return nil, fmt.Errorf("USER_TOKEN_SKEW: %w", err)
		}
	}
	cfg := Config{
		Issuer:       os.Getenv("USER_TOKEN_ISSUER"),
		Audience:     os.Getenv("USER_TOKEN_AUDIENCE"),
		OIDCIssuer:   oidcIss,
		OIDCAudience: os.Getenv("OIDC_AUDIENCE"),
		GroupsClaim:  os.Getenv("OIDC_GROUPS_CLAIM"),
		Skew:         skew,
	}
	if pubFile != "" {
		key, err := readPublicKeyFile(pubFile)
		if err != nil {
			return nil, err
		}
		cfg.PublicKey = key
	}
	return New(ctx, cfg)
}

// New builds an Auth from explicit configuration. ctx is used for OIDC discovery.
func New(ctx context.Context, cfg Config) (*Auth, error) {
	static := cfg.PublicKey != nil
	oidcOn := strings.TrimSpace(cfg.OIDCIssuer) != ""
	switch {
	case static && oidcOn:
		return nil, fmt.Errorf("set a public key or an OIDC issuer, not both")
	case !static && !oidcOn:
		return nil, fmt.Errorf("set a public key or an OIDC issuer")
	}
	var v tokenVerifier
	var err error
	if static {
		v, err = newStaticVerifier(cfg.PublicKey, cfg.Issuer, cfg.Audience, cfg.Skew)
	} else {
		v, err = newOIDCVerifier(ctx, cfg.OIDCIssuer, cfg.OIDCAudience, cfg.GroupsClaim, cfg.Skew)
	}
	if err != nil {
		return nil, err
	}
	return &Auth{v: v}, nil
}

// Verify checks a raw JWT. raw has no "Bearer " prefix.
// A failed check wraps ErrUnauthorized.
// Startup and configuration failures do not.
func (a *Auth) Verify(ctx context.Context, raw string) (Caller, error) {
	return a.v.verify(ctx, raw)
}

// Middleware requires a bearer on every path except /health.
// Success stores the Caller and the raw JWT on the request context.
func (a *Auth) Middleware() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			if r.URL.Path == "/health" {
				next.ServeHTTP(w, r)
				return
			}
			raw, ok := bearerToken(r.Header.Get("Authorization"))
			if !ok {
				http.Error(w, "unauthorized", http.StatusUnauthorized)
				return
			}
			caller, err := a.Verify(r.Context(), raw)
			if err != nil {
				http.Error(w, "unauthorized", http.StatusUnauthorized)
				return
			}
			next.ServeHTTP(w, r.WithContext(withAuth(r.Context(), caller, raw)))
		})
	}
}

func bearerToken(h string) (string, bool) {
	scheme, rest, ok := strings.Cut(h, " ")
	if !ok || !strings.EqualFold(scheme, "Bearer") {
		return "", false
	}
	rest = strings.TrimSpace(rest)
	if rest == "" {
		return "", false
	}
	return rest, true
}