-// VerifySignature returns true if the signature on the signed_locator
-// can be verified using the given api_token.
-func VerifySignature(signed_locator string, api_token string) bool {
+// VerifySignature returns nil if the signature on the signed_locator
+// can be verified using the given api_token. Otherwise it returns
+// either ExpiredError (if the timestamp has expired, which is
+// something the client could have figured out independently) or
+// PermissionError.
+func VerifySignature(signed_locator string, api_token string) error {