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>

keys.go

@@ -0,0 +1,63 @@
+package usertoken
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"os"
+)
+
+func readPrivateKeyFile(path string) (*rsa.PrivateKey, error) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read private key: %w", err)
+	}
+	return parsePrivateKey(b)
+}
+
+func readPublicKeyFile(path string) (*rsa.PublicKey, error) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read public key: %w", err)
+	}
+	return parsePublicKey(b)
+}
+
+func parsePrivateKey(pemBytes []byte) (*rsa.PrivateKey, error) {
+	block, _ := pem.Decode(pemBytes)
+	if block == nil {
+		return nil, fmt.Errorf("private key pem")
+	}
+	if key, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
+		return key, nil
+	}
+	parsed, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		return nil, fmt.Errorf("parse private key: %w", err)
+	}
+	key, ok := parsed.(*rsa.PrivateKey)
+	if !ok {
+		return nil, fmt.Errorf("private key is not RSA")
+	}
+	return key, nil
+}
+
+func parsePublicKey(pemBytes []byte) (*rsa.PublicKey, error) {
+	block, _ := pem.Decode(pemBytes)
+	if block == nil {
+		return nil, fmt.Errorf("public key pem")
+	}
+	if key, err := x509.ParsePKCS1PublicKey(block.Bytes); err == nil {
+		return key, nil
+	}
+	parsed, err := x509.ParsePKIXPublicKey(block.Bytes)
+	if err != nil {
+		return nil, fmt.Errorf("parse public key: %w", err)
+	}
+	key, ok := parsed.(*rsa.PublicKey)
+	if !ok {
+		return nil, fmt.Errorf("public key is not RSA")
+	}
+	return key, nil
+}