Feat(go): add Signer interface + License Server for multi-customer deployments

server/go/licensing/token.go

@@ -0,0 +1,114 @@
+package licensing
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// LicenseClaims is the JWT payload the operator signs and ships to each
+// customer. The operator picks "sub" (a unique customer ID), "tier" (trial
+// or paid), "max_devices" (concurrent device cap), and "exp" (token
+// expiry — independent of any in-memory cache TTL on the RemoteSigner
+// side).
+type LicenseClaims struct {
+	Tier       string `json:"tier"`
+	MaxDevices int    `json:"max_devices"`
+	jwt.RegisteredClaims
+}
+
+// LoadRSAPublicKey parses an RSA public key from a PEM file. The License
+// Server loads this once at startup to verify incoming customer JWTs.
+// Accepts both PKCS#1 ("RSA PUBLIC KEY") and PKIX ("PUBLIC KEY") PEM
+// encodings — openssl emits PKIX by default; "openssl rsa -RSAPublicKey_out"
+// emits PKCS#1.
+func LoadRSAPublicKey(pemPath string) (*rsa.PublicKey, error) {
+	data, err := os.ReadFile(pemPath)
+	if err != nil {
+		return nil, fmt.Errorf("read public key %s: %w", pemPath, err)
+	}
+	block, _ := pem.Decode(data)
+	if block == nil {
+		return nil, fmt.Errorf("no PEM block in %s", pemPath)
+	}
+
+	// Try PKIX first (most common output of openssl genrsa | openssl rsa -pubout).
+	if pub, err := x509.ParsePKIXPublicKey(block.Bytes); err == nil {
+		rsaPub, ok := pub.(*rsa.PublicKey)
+		if !ok {
+			return nil, fmt.Errorf("PKIX key in %s is not RSA", pemPath)
+		}
+		return rsaPub, nil
+	}
+
+	// Fall back to PKCS#1.
+	if pub, err := x509.ParsePKCS1PublicKey(block.Bytes); err == nil {
+		return pub, nil
+	}
+
+	return nil, fmt.Errorf("failed to parse %s as PKIX or PKCS#1 RSA public key", pemPath)
+}
+
+// VerifyJWT validates the customer's JWT against the License Server's
+// public key. Returns the parsed claims on success. Caller enforces the
+// tier-specific quota using claims.MaxDevices.
+//
+// Validation done by jwt.ParseWithClaims:
+//   - signature (RS256, using the supplied public key)
+//   - "exp" claim (expiry)
+//   - "iat" / "nbf" if present
+//
+// We additionally require tier ∈ {trial, paid}, max_devices > 0, and "sub"
+// non-empty.
+func VerifyJWT(tokenStr string, pubKey *rsa.PublicKey) (*LicenseClaims, error) {
+	claims := &LicenseClaims{}
+	token, err := jwt.ParseWithClaims(tokenStr, claims, func(t *jwt.Token) (any, error) {
+		// Lock to exactly RS256. The wider *SigningMethodRSA check (which
+		// would also accept RS384/RS512) is technically still safe because
+		// all RSA variants require the private key — but pinning the exact
+		// alg matches the docs and avoids surprises if Issue() ever changes.
+		// "alg":"none" tampering is blocked because none isn't SigningMethodRS256.
+		if t.Method != jwt.SigningMethodRS256 {
+			return nil, fmt.Errorf("unexpected JWT alg: %v (need RS256)", t.Header["alg"])
+		}
+		return pubKey, nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if !token.Valid {
+		return nil, errors.New("invalid JWT")
+	}
+
+	if claims.Subject == "" {
+		return nil, errors.New("JWT missing 'sub' claim")
+	}
+	switch claims.Tier {
+	case TierTrial:
+		if claims.MaxDevices <= 0 {
+			claims.MaxDevices = TrialMaxDevices
+		}
+	case TierPaid:
+		if claims.MaxDevices <= 0 {
+			return nil, errors.New("paid-tier JWT missing max_devices")
+		}
+	default:
+		return nil, fmt.Errorf("unsupported tier: %q", claims.Tier)
+	}
+
+	return claims, nil
+}
+
+// ttlSinceNow returns the time.Duration until exp; useful for logging.
+func (c *LicenseClaims) ttlSinceNow() time.Duration {
+	if c.ExpiresAt == nil {
+		return 0
+	}
+	return time.Until(c.ExpiresAt.Time)
+}