Dialectic.Backend/internal/auth/oidc_bearer.go

package auth

import (
	"context"
	"encoding/json"
	"errors"
	"net/http"
	"strings"
	"sync"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
)

var (
	errNoSub       = errors.New("token missing sub")
	errBadAudience = errors.New("token audience does not match")
)

// TesseraBearer middleware: accepts access tokens issued by the external
// "Tessera" OIDC provider (Keycloak-compatible) as API bearer tokens.
//
// This is ADDITIVE to the existing agent-key and browser-session auth
// paths. It only acts when the request carries `Authorization: Bearer
// <jwt>` AND that bearer is a parseable, verifiable Tessera JWT. Agent
// keys are opaque (not JWTs), so they fail verification here and the
// chain falls through to the next auth step — nothing breaks.
//
// On success it attaches a CallerUser:
//
//	Caller{
//	  Kind:  CallerUser,
//	  ID:    sub,
//	  Email: email,
//	  Name:  preferred_username,
//	  Roles: realm_access.roles ++ resource_access[audience].roles,
//	}
//
// On any failure (missing/opaque bearer, bad signature, wrong issuer,
// wrong audience, expired) it 401s — the caller composes this into the
// auth chain the same way as AgentAPIKey (capture 401 → fall through).
func TesseraBearer(issuer, audience string) func(http.Handler) http.Handler {
	v := newTesseraVerifier(issuer, audience)
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			raw := bearerToken(r)
			if raw == "" {
				http.Error(w, "missing bearer token", http.StatusUnauthorized)
				return
			}
			caller, err := v.verify(r.Context(), raw)
			if err != nil {
				http.Error(w, "invalid tessera bearer", http.StatusUnauthorized)
				return
			}
			ctx := WithCaller(r.Context(), caller)
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	}
}

// tesseraVerifier wraps a go-oidc IDTokenVerifier over the issuer's JWKS.
// The provider (and thus its cached KeySet) is lazily fetched on first
// use and refreshed hourly — same defensive TTL the OIDC login service
// uses. go-oidc's RemoteKeySet handles per-kid JWKS fetching + caching
// internally; this wrapper just adds the discovery cache + claim mapping.
type tesseraVerifier struct {
	issuer   string
	audience string

	mu        sync.Mutex
	verifier  *oidc.IDTokenVerifier
	fetchedAt time.Time
}

func newTesseraVerifier(issuer, audience string) *tesseraVerifier {
	return &tesseraVerifier{issuer: issuer, audience: audience}
}

func (t *tesseraVerifier) getVerifier(ctx context.Context) (*oidc.IDTokenVerifier, error) {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.verifier != nil && time.Since(t.fetchedAt) < time.Hour {
		return t.verifier, nil
	}
	p, err := oidc.NewProvider(ctx, t.issuer)
	if err != nil {
		return nil, err
	}
	// SkipClientIDCheck: Keycloak access tokens carry the app client id in
	// `aud`, but go-oidc's default aud check compares against ClientID. We
	// do the audience check ourselves in verify() so we can accept aud as
	// either a string or an array, so disable the built-in one here.
	v := p.Verifier(&oidc.Config{
		SkipClientIDCheck:    true,
		SupportedSigningAlgs: []string{oidc.RS256},
	})
	t.verifier = v
	t.fetchedAt = time.Now()
	return v, nil
}

// tesseraClaims is the projection of a Tessera/Keycloak access token we
// read. aud is decoded loosely (string or array) via audience.
type tesseraClaims struct {
	Sub               string   `json:"sub"`
	Email             string   `json:"email"`
	PreferredUsername string   `json:"preferred_username"`
	Aud               audience `json:"aud"`
	RealmAccess       struct {
		Roles []string `json:"roles"`
	} `json:"realm_access"`
	ResourceAccess map[string]struct {
		Roles []string `json:"roles"`
	} `json:"resource_access"`
}

func (t *tesseraVerifier) verify(ctx context.Context, raw string) (Caller, error) {
	v, err := t.getVerifier(ctx)
	if err != nil {
		return Caller{}, err
	}
	// Verify() checks RS256 signature against JWKS, iss == issuer, and exp.
	tok, err := v.Verify(ctx, raw)
	if err != nil {
		return Caller{}, err
	}
	var c tesseraClaims
	if err := tok.Claims(&c); err != nil {
		return Caller{}, err
	}
	if c.Sub == "" {
		return Caller{}, errNoSub
	}
	if !c.Aud.contains(t.audience) {
		return Caller{}, errBadAudience
	}
	roles := append([]string{}, c.RealmAccess.Roles...)
	if ra, ok := c.ResourceAccess[t.audience]; ok {
		roles = append(roles, ra.Roles...)
	}
	name := c.PreferredUsername
	return Caller{
		Kind:  CallerUser,
		ID:    c.Sub,
		Email: c.Email,
		Name:  name,
		Roles: roles,
	}, nil
}

// audience decodes the JWT `aud` claim, which may be a single string or
// an array of strings.
type audience []string

func (a *audience) UnmarshalJSON(b []byte) error {
	b = []byte(strings.TrimSpace(string(b)))
	if len(b) == 0 || string(b) == "null" {
		*a = nil
		return nil
	}
	if b[0] == '[' {
		var arr []string
		if err := json.Unmarshal(b, &arr); err != nil {
			return err
		}
		*a = arr
		return nil
	}
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}
	*a = []string{s}
	return nil
}

func (a audience) contains(want string) bool {
	for _, v := range a {
		if v == want {
			return true
		}
	}
	return false
}