diff --git a/src/crypto.ml b/src/crypto.ml
index 4a9a378d..140d99c1 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -259,15 +259,10 @@ end
 module RsaSignature : sig
   type t
 
-  val sign : key:Mirage_crypto_pk.Rsa.priv -> string -> t
   val jsont : t Jsont.t
 end = struct
   type t = string
 
-  (* TODO rsa sign *)
-  let sign ~key s =
-    Mirage_crypto_pk.Rsa.decrypt ~crt_hardening:true ~mask:`Yes ~key s
-
   let jsont =
     let of_b32 s = B32.decode s in
     let to_b32 t = B32.encode t in