diff --git a/src/utils/crypto_box.ml b/src/utils/crypto_box.ml
index 7620a6aad..fb06f3fed 100644
--- a/src/utils/crypto_box.ml
+++ b/src/utils/crypto_box.ml
@@ -15,6 +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
 
 let random_keypair = Sodium.Box.random_keypair
 let random_nonce = Sodium.Box.random_nonce
@@ -24,6 +25,13 @@ let box_open sk pk msg nonce =
   try Some (Sodium.Box.Bigbytes.box_open sk pk msg nonce) with
     | Sodium.Verification_failure -> None
 
+let check_proof_of_work pk nonce difficulty = assert false
+let generate_proof_of_work pk difficulty =
+  let rec loop nonce =
+    if check_proof_of_work pk nonce difficulty then nonce
+    else loop (increment_nonce nonce) in
+  loop (random_nonce ())
+
 let public_key_encoding =
   let open Data_encoding in
     conv
@@ -44,4 +52,3 @@ let nonce_encoding =
       Sodium.Box.Bigbytes.of_nonce
       Sodium.Box.Bigbytes.to_nonce
       (Fixed.bytes Sodium.Box.nonce_size)
-
diff --git a/src/utils/crypto_box.mli b/src/utils/crypto_box.mli
index e286ff0aa..a31fb6380 100644
--- a/src/utils/crypto_box.mli
+++ b/src/utils/crypto_box.mli
@@ -10,6 +10,7 @@
 (** Tezos - X25519/XSalsa20-Poly1305 cryptography *)
 
 type nonce
+type difficulty
 
 val random_nonce : unit -> nonce
 val increment_nonce : ?step:int -> nonce -> nonce
@@ -28,3 +29,5 @@ val box : secret_key -> public_key -> MBytes.t -> nonce -> MBytes.t
 
 val box_open : secret_key -> public_key -> MBytes.t -> nonce -> MBytes.t option
 
+val check_proof_of_work : public_key -> nonce -> difficulty -> bool
+val generate_proof_of_work : public_key -> difficulty -> nonce