key-cape/src/internal/server/oidc/userinfo.go

package oidc

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

	"keycape/internal/domain"
	"keycape/internal/server/telemetry"
)

// UserinfoHandler implements GET /userinfo (OIDC Core §5.3).
//
// The endpoint validates the Bearer token, extracts the subject, looks up
// the user, and returns claims that are consistent with those in the ID token
// for the same scope set.
type UserinfoHandler struct {
	Users      domain.UserRepository
	SigningKey *rsa.PublicKey // used to verify the incoming access token
	Issuer     string
	Emitter    telemetry.Emitter
}

// ServeHTTP handles GET /userinfo.
func (h *UserinfoHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	// 1. Extract Bearer token.
	tokenStr, ok := bearerToken(r)
	if !ok {
		http.Error(w, `{"error":"missing_token","description":"Authorization: Bearer <token> required"}`, http.StatusUnauthorized)
		return
	}

	// 2. Validate token (algorithm, signature, expiry, issuer, purpose) and
	// extract claims.
	claims, err := validateAccessToken(tokenStr, h.SigningKey, h.Issuer)
	if err != nil {
		http.Error(w, `{"error":"invalid_token","description":"token validation failed"}`, http.StatusUnauthorized)
		return
	}

	// 3. Extract sub claim (which is the username in our model).
	sub, _ := claims["sub"].(string)
	if sub == "" {
		http.Error(w, `{"error":"invalid_token","description":"missing sub claim"}`, http.StatusUnauthorized)
		return
	}

	// 4. Resolve the canonical subject. Human tokens use the stable directory
	// ID (the LDAP DN) as sub, while LookupUser accepts an LDAP uid. Prefer the
	// scoped username hint but require it to resolve back to the signed subject;
	// fall back to an ID scan for openid-only tokens and renamed users.
	user, err := lookupUserBySubject(ctx, h.Users, claims, sub)
	if err != nil {
		// User referenced in token but not found → treat as invalid token.
		http.Error(w, `{"error":"invalid_token","description":"subject not found"}`, http.StatusUnauthorized)
		return
	}
	if isSuspended(user) {
		http.Error(w, `{"error":"invalid_token","description":"subject is suspended"}`, http.StatusUnauthorized)
		return
	}

	// 5. Build response claims filtered by the scopes embedded in the token.
	scopeStr, _ := claims["scope"].(string)
	scopeSet := parseScopeSet(scopeStr)

	resp := map[string]interface{}{
		"sub": sub,
	}

	if scopeSet["profile"] {
		resp["preferred_username"] = user.Username
		resp["name"] = user.DisplayName
	}
	if scopeSet["email"] {
		resp["email"] = user.Email
	}
	if scopeSet["groups"] {
		resp["groups"] = user.Groups
	}

	// 6. Emit telemetry.
	h.Emitter.Emit(ctx, telemetry.Event{
		Timestamp: time.Now(),
		EventType: telemetry.EventAuthSuccess,
		Endpoint:  "/userinfo",
		Result:    "success",
	})

	// 7. Write JSON response.
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusOK)
	_ = json.NewEncoder(w).Encode(resp)
}

func lookupUserBySubject(
	ctx context.Context,
	users domain.UserRepository,
	claims map[string]interface{},
	sub string,
) (*domain.User, error) {
	if username, _ := claims["preferred_username"].(string); username != "" {
		if user, err := users.LookupUser(ctx, username); err == nil && user.ID == sub {
			return user, nil
		}
	}

	// Retain compatibility with older signed tokens whose subject was the uid.
	if user, err := users.LookupUser(ctx, sub); err == nil &&
		(user.ID == sub || user.Username == sub) {
		return user, nil
	}

	all, err := users.ListUsers(ctx)
	if err != nil {
		return nil, err
	}
	for i := range all {
		if all[i].ID == sub {
			return &all[i], nil
		}
	}
	return nil, domain.ErrUserNotFound
}

// ---------------------------------------------------------------------------
// JWT validation (stdlib only — no external JWT library)
// ---------------------------------------------------------------------------

// validateAccessToken parses and validates a KeyCape-issued access token.
//
// Beyond signature and expiry it enforces the bindings the caller CLI already
// requires (KEY-WP-0016-T03): the JOSE header algorithm must be exactly RS256,
// so an "alg":"none" or HMAC-shaped token can never bypass the RSA check; the
// issuer claim must equal this issuer, so a correctly-signed token from another
// deployment is refused; and the token must be an access token.
//
// Purpose is decided on the presence of the `scope` claim, which the token
// endpoint sets on access tokens and never on ID tokens. That keeps the check
// verification-side: it does not add a claim to the issued token contract, which
// consumers pin exactly.
func validateAccessToken(tokenStr string, pubKey *rsa.PublicKey, issuer string) (map[string]interface{}, error) {
	parts := strings.Split(tokenStr, ".")
	if len(parts) != 3 {
		return nil, errors.New("malformed JWT: expected 3 parts")
	}

	// Verify the JOSE header algorithm before trusting the signature check.
	headerJSON, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil {
		return nil, errors.New("malformed JWT: invalid header encoding")
	}
	var header struct {
		Alg string `json:"alg"`
	}
	if err := json.Unmarshal(headerJSON, &header); err != nil {
		return nil, errors.New("malformed JWT: header is not valid JSON")
	}
	if header.Alg != "RS256" {
		return nil, errors.New("unsupported JWT algorithm")
	}

	// Verify signature over header.payload.
	signingInput := parts[0] + "." + parts[1]
	digest := sha256.Sum256([]byte(signingInput))

	sigBytes, decodeErr := base64.RawURLEncoding.DecodeString(parts[2])
	if decodeErr != nil {
		return nil, errors.New("malformed JWT: invalid signature encoding")
	}

	if err := rsa.VerifyPKCS1v15(pubKey, crypto.SHA256, digest[:], sigBytes); err != nil {
		return nil, errors.New("JWT signature verification failed")
	}

	// Decode payload.
	payloadJSON, payloadErr := base64.RawURLEncoding.DecodeString(parts[1])
	if payloadErr != nil {
		return nil, errors.New("malformed JWT: invalid payload encoding")
	}

	var claims map[string]interface{}
	if err := json.Unmarshal(payloadJSON, &claims); err != nil {
		return nil, errors.New("malformed JWT: payload is not valid JSON")
	}

	// Check exp claim.
	exp, ok := claims["exp"].(float64)
	if !ok {
		return nil, errors.New("JWT missing exp claim")
	}
	if time.Now().Unix() > int64(exp) {
		return nil, errors.New("JWT has expired")
	}

	// Require this issuer. An empty configured issuer would make the check
	// vacuous, so treat that as a misconfiguration rather than skipping it.
	if issuer == "" {
		return nil, errors.New("issuer is not configured")
	}
	if tokenIssuer, _ := claims["iss"].(string); tokenIssuer != issuer {
		return nil, errors.New("JWT issuer mismatch")
	}

	// Require an access token. ID tokens carry no scope claim.
	if _, ok := claims["scope"]; !ok {
		return nil, errors.New("JWT is not an access token")
	}

	return claims, nil
}

// ---------------------------------------------------------------------------
// Helpers
// ---------------------------------------------------------------------------

// bearerToken extracts the token from the Authorization header.
// Returns ("", false) when the header is missing or not a Bearer token.
func bearerToken(r *http.Request) (string, bool) {
	hdr := r.Header.Get("Authorization")
	if hdr == "" {
		return "", false
	}
	const prefix = "Bearer "
	if !strings.HasPrefix(hdr, prefix) {
		return "", false
	}
	tok := strings.TrimSpace(hdr[len(prefix):])
	if tok == "" {
		return "", false
	}
	return tok, true
}

// parseScopeSet converts a space-separated scope string to a set.
func parseScopeSet(scope string) map[string]bool {
	set := make(map[string]bool)
	for _, s := range strings.Fields(scope) {
		set[s] = true
	}
	return set
}

// ---------------------------------------------------------------------------
// BuildJWT — exported for test helpers
// ---------------------------------------------------------------------------

// BuildJWT is an exported wrapper around the internal buildJWT function so
// that tests in the oidc_test package can construct valid tokens for the
// UserinfoHandler without importing an external JWT library.
func BuildJWT(claims map[string]interface{}, kid string, key *rsa.PrivateKey) (string, error) {
	return buildJWT(claims, kid, key)
}