Mint and verify short-lived RS256 user tokens.

ExApps sign after AppAPI auth; Microservices check a static public key or an OIDC issuer and forward the same bearer.

Co-authored-by: Cursor <cursoragent@cursor.com>

verify.go

@@ -0,0 +1,84 @@
+package usertoken
+
+import (
+	"context"
+	"crypto/rsa"
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+const defaultSkew = time.Minute
+
+type tokenVerifier interface {
+	verify(ctx context.Context, raw string) (Caller, error)
+}
+
+type staticVerifier struct {
+	key  *rsa.PublicKey
+	iss  string
+	aud  string
+	skew time.Duration
+}
+
+func newStaticVerifier(key *rsa.PublicKey, iss, aud string, skew time.Duration) (tokenVerifier, error) {
+	if key == nil {
+		return nil, fmt.Errorf("public key is nil")
+	}
+	iss = strings.TrimSpace(iss)
+	aud = strings.TrimSpace(aud)
+	if iss == "" || aud == "" {
+		return nil, fmt.Errorf("issuer and audience are required")
+	}
+	if skew <= 0 {
+		skew = defaultSkew
+	}
+	return &staticVerifier{key: key, iss: iss, aud: aud, skew: skew}, nil
+}
+
+func (v *staticVerifier) verify(_ context.Context, raw string) (Caller, error) {
+	if strings.TrimSpace(raw) == "" {
+		return Caller{}, unauthorized("empty token")
+	}
+	parser := jwt.NewParser(
+		jwt.WithValidMethods([]string{jwt.SigningMethodRS256.Alg()}),
+		jwt.WithIssuer(v.iss),
+		jwt.WithAudience(v.aud),
+		jwt.WithLeeway(v.skew),
+		jwt.WithExpirationRequired(),
+		jwt.WithIssuedAt(),
+	)
+	var claims tokenClaims
+	tok, err := parser.ParseWithClaims(raw, &claims, func(t *jwt.Token) (any, error) {
+		if t.Method == nil || t.Method.Alg() != jwt.SigningMethodRS256.Alg() {
+			return nil, unauthorized("unexpected alg")
+		}
+		return v.key, nil
+	})
+	if err != nil || tok == nil || !tok.Valid {
+		if err == nil {
+			err = errors.New("invalid token")
+		}
+		return Caller{}, fmt.Errorf("verify: %w", errors.Join(err, ErrUnauthorized))
+	}
+	return callerFromClaims(claims.Subject, claims.PreferredUsername, claims.Groups)
+}
+
+func callerFromClaims(subject, username string, groups *[]string) (Caller, error) {
+	subject = strings.TrimSpace(subject)
+	if subject == "" {
+		return Caller{}, unauthorized("empty sub")
+	}
+	username = strings.TrimSpace(username)
+	if username == "" {
+		username = subject
+	}
+	c := Caller{Subject: subject, Username: username}
+	if groups != nil {
+		c.Groups = append([]string{}, (*groups)...)
+	}
+	return c, nil
+}