diff --git a/src/crypto.ml b/src/crypto.ml
index a223a606..afc0276b 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -284,10 +284,10 @@ end = struct
     let cipher = cipher_to_int32 RSA in
     let pub = RsaPublicKey.to_octets pub in
     let pub_len = String.length pub in
-    let b = Bytes.create (2 + 2 + pub_len) in
+    let b = Bytes.create (4 + 4 + pub_len) in
     Bytes.set_int32_be b 0 age_mask;
-    Bytes.set_int32_be b 2 cipher;
-    Bytes.blit_string pub 0 b 4 pub_len;
+    Bytes.set_int32_be b 4 cipher;
+    Bytes.blit_string pub 0 b (4 + 4) pub_len;
     SHA512.(digest_bytes b)
 
   let of_octets s =