diff --git a/src/utils/crypto_box.ml b/src/utils/crypto_box.ml
index 97db6c028..bf4586799 100644
--- a/src/utils/crypto_box.ml
+++ b/src/utils/crypto_box.ml
@@ -15,7 +15,7 @@ type secret_key = Sodium.Box.secret_key
 type public_key = Sodium.Box.public_key
 type channel_key = Sodium.Box.channel_key
 type nonce = Sodium.Box.nonce
-type difficulty = int
+type difficulty = int64
 
 let random_keypair = Sodium.Box.random_keypair
 let random_nonce = Sodium.Box.random_nonce
@@ -35,13 +35,9 @@ let check_proof_of_work pk nonce difficulty =
       [ Sodium.Box.Bigbytes.of_public_key pk ;
         Sodium.Box.Bigbytes.of_nonce nonce ] in
   let bytes = MBytes.of_string hash in
-  let len = MBytes.length bytes * 8 in
-  try
-    for i = len - 1 downto (len - difficulty) do
-      if MBytes.get_bool bytes i then raise Exit
-    done;
-    true
-  with Exit -> false
+  let last_int64 =
+    EndianBigstring.BigEndian.get_int64 bytes (MBytes.length bytes - 8) in
+  Int64.logand last_int64 (Int64.of_int 1) < difficulty
 let generate_proof_of_work pk difficulty =
   let rec loop nonce =
     if check_proof_of_work pk nonce difficulty then nonce