bsky-feedgen/pkg/api/middleware/auth.go

package middleware

import (
	"context"
	"crypto"
	"errors"
	"fmt"
	"net/http"
	"slices"
	"strings"
	"time"

	"github.com/bluesky-social/indigo/atproto/identity"
	"github.com/bluesky-social/indigo/atproto/syntax"
	"github.com/golang-jwt/jwt/v5"
	"github.com/whyrusleeping/go-did"
)

const UserDIDKey ContextKey = "user_did"

const (
	authorizationHeaderName        = "Authorization"
	authorizationHeaderValuePrefix = "Bearer "
)

// Global (or dependency-injected) DID resolver with caching.
var didResolver *identity.CacheDirectory

func init() {
	baseDir := identity.BaseDirectory{}

	// Configure cache with appropriate TTLs.
	// Capacity 0 means unlimited cache size.
	// hitTTL: 24 hours for successful resolutions.
	// errTTL: 5 minutes for failed resolutions.
	// invalidHandleTTL: also 5 minutes for invalid handles.
	resolver := identity.NewCacheDirectory(
		&baseDir,
		0,             // Unlimited capacity
		24*time.Hour,  // hitTTL
		5*time.Minute, // errTTL
		5*time.Minute, // invalidHandleTTL
	)
	didResolver = &resolver
}

type AuthorizationError struct {
	Message string
	Err     error
}

func (e *AuthorizationError) Error() string {
	if e.Err != nil {
		return fmt.Sprintf("%s: %v", e.Message, e.Err)
	}
	return e.Message
}

func (e *AuthorizationError) Unwrap() error {
	return e.Err
}

type Auth struct {
	serviceDID *did.DID
}

func NewAuth(serviceDID *did.DID) *Auth {
	return &Auth{serviceDID}
}

func (auth *Auth) JWTAuthMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		authHeader := r.Header.Get("Authorization")
		if authHeader == "" {
			// No auth header, continue without authentication
			next.ServeHTTP(w, r)
			return
		}

		userDID, _ := auth.validateAuth(r.Context(), r)
		ctx := context.WithValue(r.Context(), UserDIDKey, userDID)

		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

// getDIDSigningKey resolves a DID and extracts its public signing key.
// It leverages indigo's identity package which handles multibase decoding and key parsing.
func (auth *Auth) getDIDSigningKey(ctx context.Context, did string) (crypto.PublicKey, error) {
	atID, err := syntax.ParseAtIdentifier(did)
	if err != nil {
		return nil, fmt.Errorf("invalid DID syntax: %w", err)
	}

	// Use Lookup for bi-directional verification (handle -> DID -> handle).
	// The `Lookup` method returns an `Identity` struct which contains `PublicKey()` method
	// to get the signing key.
	identity, err := didResolver.Lookup(ctx, *atID)
	if err != nil {
		return nil, fmt.Errorf("DID resolution failed for %s: %w", did, err)
	}
	if identity == nil || identity.DID.String() == "" {
		return nil, fmt.Errorf("DID resolution returned empty identity for %s", did)
	}

	publicKey, err := identity.PublicKey()
	if err != nil {
		return nil, fmt.Errorf("failed to get signing key for DID %s: %w", did, err)
	}

	return publicKey, nil
}

// ValidateAuth validates the authorization header and returns the requester's DID.
func (auth *Auth) validateAuth(ctx context.Context, r *http.Request) (string, error) {
	authHeader := r.Header.Get(authorizationHeaderName)
	if authHeader == "" {
		return "", &AuthorizationError{Message: "Authorization header is missing"}
	}

	if !strings.HasPrefix(authHeader, authorizationHeaderValuePrefix) {
		return "", &AuthorizationError{Message: "Invalid authorization header format"}
	}

	jwtString := strings.TrimPrefix(authHeader, authorizationHeaderValuePrefix)
	jwtString = strings.TrimSpace(jwtString)

	claims := jwt.RegisteredClaims{}

	keyFunc := func(token *jwt.Token) (any, error) {
		regClaims, ok := token.Claims.(*jwt.RegisteredClaims)
		if !ok {
			return nil, fmt.Errorf("invalid JWT claims type")
		}

		issuerDID := regClaims.Issuer
		if issuerDID == "" {
			return nil, fmt.Errorf("JWT 'iss' claim is missing")
		}

		publicKey, err := auth.getDIDSigningKey(ctx, issuerDID)
		if err != nil {
			return nil, fmt.Errorf("failed to get signing key for DID %s: %w", issuerDID, err)
		}

		return publicKey, nil
	}

	token, err := jwt.ParseWithClaims(jwtString, &claims, keyFunc)
	if err != nil {
		if errors.Is(err, jwt.ErrTokenSignatureInvalid) {
			return "", &AuthorizationError{Message: "Invalid signature", Err: err}
		}
		if errors.Is(err, jwt.ErrTokenExpired) {
			return "", &AuthorizationError{Message: "Token expired", Err: err}
		}
		if errors.Is(err, jwt.ErrTokenNotValidYet) {
			return "", &AuthorizationError{Message: "Token not valid yet", Err: err}
		}
		if errors.Is(err, jwt.ErrTokenMalformed) {
			return "", &AuthorizationError{Message: "Malformed token", Err: err}
		}
		return "", &AuthorizationError{Message: "Failed to parse or validate JWT", Err: err}
	}

	if !token.Valid {
		return "", &AuthorizationError{Message: "Token is invalid"}
	}

	if slices.Contains(claims.Audience, auth.serviceDID.String()) {
		return "", &AuthorizationError{Message: fmt.Sprintf("Invalid audience (expected %s)", auth.serviceDID)}
	}

	// Return the issuer's DID.
	return claims.Issuer, nil
}