boc/backend/auth/rs256.go

package auth

import (
	"crypto/rsa"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"fmt"
	"math/big"
	"net/http"
	"os"
	"strings"
	"time"

	"github.com/golang-jwt/jwt/v5"
	"github.com/rs/zerolog/log"
)

// RS256Service validates RS256 JWT tokens using a public key
// Compatible with ouroboros-identity (port 3208) and aamos-admin-v2
type RS256Service struct {
	publicKey *rsa.PublicKey
	issuer    string
	audience  string
}

// JWKS represents a JSON Web Key Set
type JWKS struct {
	Keys []JWK `json:"keys"`
}

// JWK represents a JSON Web Key
type JWK struct {
	Kty string `json:"kty"`
	N   string `json:"n"`
	E   string `json:"e"`
	Use string `json:"use"`
	Alg string `json:"alg"`
	Kid string `json:"kid"`
}

// NewRS256ServiceFromURL fetches JWKS from URL and creates RS256Service
func NewRS256ServiceFromURL(jwksURL string) (*RS256Service, error) {
	client := &http.Client{Timeout: 10 * time.Second}
	resp, err := client.Get(jwksURL)
	if err != nil {
		return nil, fmt.Errorf("failed to fetch JWKS: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("JWKS endpoint returned %d", resp.StatusCode)
	}

	var jwks JWKS
	if err := json.NewDecoder(resp.Body).Decode(&jwks); err != nil {
		return nil, fmt.Errorf("failed to decode JWKS: %w", err)
	}

	if len(jwks.Keys) == 0 {
		return nil, fmt.Errorf("no keys in JWKS")
	}

	// Use first signing key
	key := jwks.Keys[0]
	nBytes, err := base64.RawURLEncoding.DecodeString(key.N)
	if err != nil {
		return nil, fmt.Errorf("failed to decode N: %w", err)
	}
	eBytes, err := base64.RawURLEncoding.DecodeString(key.E)
	if err != nil {
		return nil, fmt.Errorf("failed to decode E: %w", err)
	}

	pub := &rsa.PublicKey{
		N: new(big.Int).SetBytes(nBytes),
		E: int(new(big.Int).SetBytes(eBytes).Int64()),
	}

	return &RS256Service{
		publicKey: pub,
		issuer:    "prexo-identity",
		audience:  "prexo",
	}, nil
}

// NewRS256Service loads the public key from a PEM file
func NewRS256Service(publicKeyPath string) (*RS256Service, error) {
	pemData, err := os.ReadFile(publicKeyPath)
	if err != nil {
		return nil, fmt.Errorf("failed to read public key: %w", err)
	}

	block, _ := pem.Decode(pemData)
	if block == nil {
		return nil, fmt.Errorf("failed to decode PEM block")
	}

	pub, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		// Try PKCS1 format
		pub, err = x509.ParsePKCS1PublicKey(block.Bytes)
		if err != nil {
			return nil, fmt.Errorf("failed to parse public key: %w", err)
		}
	}

	rsaPub, ok := pub.(*rsa.PublicKey)
	if !ok {
		return nil, fmt.Errorf("not an RSA public key")
	}

	return &RS256Service{
		publicKey: rsaPub,
		issuer:    "prexo-identity",
		audience:  "prexo",
	}, nil
}

// Middleware returns HTTP middleware that validates Bearer tokens using RS256
func (s *RS256Service) Middleware() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			authHeader := r.Header.Get("Authorization")
			if authHeader == "" {
				http.Error(w, `{"error":"missing authorization header"}`, http.StatusUnauthorized)
				return
			}

			if !strings.HasPrefix(authHeader, "Bearer ") {
				http.Error(w, `{"error":"invalid authorization format"}`, http.StatusUnauthorized)
				return
			}

			tokenString := strings.TrimPrefix(authHeader, "Bearer ")
			claims, err := s.ValidateToken(tokenString)
			if err != nil {
				log.Warn().Err(err).Msg("token validation failed")
				http.Error(w, `{"error":"invalid token"}`, http.StatusUnauthorized)
				return
			}

			ctx := WithClaims(r.Context(), claims)
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	}
}

// ValidateToken verifies an RS256 JWT token
func (s *RS256Service) ValidateToken(tokenString string) (*Claims, error) {
	token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
		if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {
			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
		}
		return s.publicKey, nil
	})
	if err != nil {
		return nil, fmt.Errorf("token parse error: %w", err)
	}
	if !token.Valid {
		return nil, fmt.Errorf("token invalid")
	}

	mapClaims, ok := token.Claims.(jwt.MapClaims)
	if !ok {
		return nil, fmt.Errorf("invalid claims format")
	}

	claims := &Claims{
		Sub: getStringClaim(mapClaims, "sub"),
		Iss: getStringClaim(mapClaims, "iss"),
		Aud: getStringClaim(mapClaims, "aud"),
		Exp: getInt64Claim(mapClaims, "exp"),
		Iat: getInt64Claim(mapClaims, "iat"),
	}

	if email, ok := mapClaims["email"].(string); ok {
		claims.Email = email
	}
	if orgID, ok := mapClaims["org_id"].(string); ok {
		claims.OrgID = orgID
	}
	if roles, ok := mapClaims["roles"].([]interface{}); ok {
		claims.Roles = make([]string, len(roles))
		for i, r := range roles {
			claims.Roles[i] = fmt.Sprint(r)
		}
	}
	if scopes, ok := mapClaims["scopes"].([]interface{}); ok {
		claims.Scopes = make([]string, len(scopes))
		for i, sc := range scopes {
			claims.Scopes[i] = fmt.Sprint(sc)
		}
	}

	if err := claims.Valid(); err != nil {
		return nil, err
	}

	return claims, nil
}