diff --git a/src/bin_client/test/contracts/hash_consistency_checker.tz b/src/bin_client/test/contracts/hash_consistency_checker.tz
index 91d49a0ec..456c66507 100644
--- a/src/bin_client/test/contracts/hash_consistency_checker.tz
+++ b/src/bin_client/test/contracts/hash_consistency_checker.tz
@@ -1,3 +1,3 @@
 parameter (pair mutez (pair timestamp int)) ;
-storage string ;
+storage bytes ;
 code { CAR ; BLAKE2B ; NIL operation ; PAIR }
\ No newline at end of file
diff --git a/src/bin_client/test/contracts/hash_string.tz b/src/bin_client/test/contracts/hash_string.tz
index 76805aae3..531432992 100644
--- a/src/bin_client/test/contracts/hash_string.tz
+++ b/src/bin_client/test/contracts/hash_string.tz
@@ -1,3 +1,3 @@
 parameter string;
-storage string;
+storage bytes;
 code {CAR; BLAKE2B; NIL operation; PAIR};
diff --git a/src/bin_client/test/test_contracts.sh b/src/bin_client/test/test_contracts.sh
index 0316ffca1..69f6fc37d 100755
--- a/src/bin_client/test/test_contracts.sh
+++ b/src/bin_client/test/test_contracts.sh
@@ -230,8 +230,8 @@ assert_storage $contract_dir/first.tz '111' '{ 4 }' '4'
 
 # Hash input string
 # Test assumed to be correct -- hash is based on encoding of AST
-assert_storage $contract_dir/hash_string.tz '"?"' '"abcdefg"' '"exprv3MnhXvjthGzZ7jDtXRRFremZyey9rsGtL7JRkeaQX1fThN7WF"'
-assert_storage $contract_dir/hash_string.tz '"?"' '"12345"' '"expru81QVHsW2qaWLNHnMHSxDNhqtat17ajadri6mKUvXyc2EWHZC3"'
+assert_storage $contract_dir/hash_string.tz '0x00' '"abcdefg"' '0xc8e816fa5921f08d25ead933aedc189a7b0abfa97c649a8d2eb9e0f323f7909e'
+assert_storage $contract_dir/hash_string.tz '0x00' '"12345"' '0x4fc401c158dad6482d96f45bb2dc10bc445b1ff127a485814d735e5c3dcb36ec'
 
 # Test ASSERT
 assert_storage $contract_dir/assert.tz Unit True Unit
@@ -315,11 +315,11 @@ assert_storage  $contract_dir/map_caddaadr.tz \
 
 # Did the given key sign the string? (key is bootstrap1)
 assert_success $client run script $contract_dir/check_signature.tz \
-               on storage '(Pair 0x1f19f8f37e80d96797b019f30d23ede6a26a0f698220f942103a3401f047623746e51a9c6e77e269b5df9593994ab96b001aae0f73728a2259187cb640b61e01 "hello")' \
+               on storage '(Pair "edsigtursTM9zynxFfyiLyTLuVh369H7Cd1nkH63dyc3jKBoujWeQRE9hZFRhqgTrY3BzdZenDc3D6v7gZEShpdPDMQ7YswgEL3" "hello")' \
                and input '"edpkuBknW28nW72KG6RoHtYW7p12T6GKc7nAbwYX5m8Wd9sDVC9yav"'
 
 assert_fails $client run script $contract_dir/check_signature.tz \
-             on storage '(Pair 0x1f19f8f37e80d96797b019f30d23ede6a26a0f698220f942103a3401f047623746e51a9c6e77e269b5df9593994ab96b001aae0f73728a2259187cb640b61e01 "abcd")' \
+             on storage '(Pair "edsigtursTM9zynxFfyiLyTLuVh369H7Cd1nkH63dyc3jKBoujWeQRE9hZFRhqgTrY3BzdZenDc3D6v7gZEShpdPDMQ7YswgEL3" "abcd")' \
              and input '"edpkuBknW28nW72KG6RoHtYW7p12T6GKc7nAbwYX5m8Wd9sDVC9yav"'
 
 
@@ -449,12 +449,12 @@ assert_fails $client typecheck data '{ "A" ; "B" ; "B" }' against type '(set str
 
 # Test hash consistency between Michelson and the CLI
 hash_result=`$client hash data '(Pair 22220000000 (Pair "2017-12-13T04:49:00Z" 034))' \
-                     of type '(pair mutez (pair timestamp int))' | grep expr`
+                     of type '(pair mutez (pair timestamp int))' | grep 0x | sed 's/.*: *//'`
 
-assert_storage $contract_dir/hash_consistency_checker.tz '"?"' \
+assert_storage $contract_dir/hash_consistency_checker.tz '0x00' \
               '(Pair 22220000000 (Pair "2017-12-13T04:49:00Z" 034))' "$hash_result"
 
-assert_storage $contract_dir/hash_consistency_checker.tz '"?"' \
+assert_storage $contract_dir/hash_consistency_checker.tz '0x00' \
               '(Pair 22220000000 (Pair "2017-12-13T04:49:00+00:00" 34))' "$hash_result"
 
 # Test for big maps
diff --git a/src/proto_alpha/lib_client/client_proto_programs.ml b/src/proto_alpha/lib_client/client_proto_programs.ml
index 76a95fb32..575493c6b 100644
--- a/src/proto_alpha/lib_client/client_proto_programs.ml
+++ b/src/proto_alpha/lib_client/client_proto_programs.ml
@@ -43,11 +43,11 @@ let print_big_map_diff ppf = function
         (Format.pp_print_list
            ~pp_sep:Format.pp_print_space
            (fun ppf (key, value) ->
-              Format.fprintf ppf "%s %s%a"
+              Format.fprintf ppf "%s %a%a"
                 (match value with
                  | None -> "-"
                  | Some _ -> "+")
-                key
+                Script_expr_hash.pp key
                 (fun ppf -> function
                    | None -> ()
                    | Some x -> Format.fprintf ppf "-> %a" print_expr x)
@@ -125,8 +125,8 @@ let hash_and_sign
     sk =
   Alpha_services.Helpers.Scripts.hash_data
     cctxt (chain, block) (data.expanded, typ.expanded, gas) >>=? fun (hash, gas) ->
-  Client_keys.sign cctxt sk (MBytes.of_string hash) >>=? fun signature ->
-  return (hash, Signature.to_b58check signature, gas)
+  Client_keys.sign cctxt sk (Script_expr_hash.to_bytes hash) >>=? fun signature ->
+  return (hash, signature, gas)
 
 let typecheck_data
     cctxt
diff --git a/src/proto_alpha/lib_client/client_proto_programs.mli b/src/proto_alpha/lib_client/client_proto_programs.mli
index cd9222a83..9711796f4 100644
--- a/src/proto_alpha/lib_client/client_proto_programs.mli
+++ b/src/proto_alpha/lib_client/client_proto_programs.mli
@@ -69,7 +69,7 @@ val hash_and_sign :
   Michelson_v1_parser.parsed ->
   Michelson_v1_parser.parsed ->
   Client_keys.sk_uri ->
-  (string * string * Gas.t) tzresult Lwt.t
+  (Script_expr_hash.t * Signature.t * Gas.t) tzresult Lwt.t
 
 val typecheck_data :
   #Proto_alpha.rpc_context ->
diff --git a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
index 89000524e..d5af7e92d 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
@@ -205,7 +205,9 @@ let commands () =
          Alpha_services.Helpers.Scripts.hash_data cctxt (`Main, cctxt#block)
            (data.expanded, typ.expanded, Some original_gas) >>= function
          | Ok (hash, remaining_gas) ->
-             cctxt#message "%S@,Gas remaining: %a" hash
+             cctxt#message "Hash: %a@,Raw hash bytes: 0x%a@,Gas remaining: %a"
+               Script_expr_hash.pp hash
+               MBytes.pp_hex (Script_expr_hash.to_bytes hash)
                Proto_alpha.Alpha_context.Gas.pp remaining_gas >>= fun () ->
              return ()
          | Error errs ->
@@ -238,8 +240,10 @@ let commands () =
          Client_proto_programs.hash_and_sign cctxt cctxt#block
            ~gas data typ sk >>= begin function
            | Ok (hash, signature, current_gas) ->
-               cctxt#message "@[<v 0>Hash: %S@,Signature: %S@,Remaining gas: %a@]"
-                 hash signature
+               cctxt#message "@[<v 0>Hash: %a@,Raw hash bytes: 0x%a@,Signature: %a@,Remaining gas: %a@]"
+                 Script_expr_hash.pp hash
+                 MBytes.pp_hex (Script_expr_hash.to_bytes hash)
+                 Signature.pp signature
                  Proto_alpha.Alpha_context.Gas.pp current_gas
            | Error errs ->
                cctxt#warning "%a"
diff --git a/src/proto_alpha/lib_protocol/src/alpha_context.mli b/src/proto_alpha/lib_protocol/src/alpha_context.mli
index 16490dec0..453878bb4 100644
--- a/src/proto_alpha/lib_protocol/src/alpha_context.mli
+++ b/src/proto_alpha/lib_protocol/src/alpha_context.mli
@@ -540,7 +540,7 @@ module Contract : sig
   val fresh_contract_from_current_nonce : context -> (context * t) tzresult Lwt.t
   val originated_from_current_nonce: since: context -> until:context -> contract list tzresult Lwt.t
 
-  type big_map_diff = (string * Script.expr option) list
+  type big_map_diff = (Script_expr_hash.t * Script.expr option) list
 
   val originate:
     context -> contract ->
@@ -585,9 +585,9 @@ module Contract : sig
 
   module Big_map : sig
     val mem:
-      context -> contract -> string -> (context * bool) tzresult Lwt.t
+      context -> contract -> Script_expr_hash.t -> (context * bool) tzresult Lwt.t
     val get_opt:
-      context -> contract -> string -> (context * Script_repr.expr option) tzresult Lwt.t
+      context -> contract -> Script_expr_hash.t -> (context * Script_repr.expr option) tzresult Lwt.t
   end
 
   (**/**)
diff --git a/src/proto_alpha/lib_protocol/src/contract_storage.ml b/src/proto_alpha/lib_protocol/src/contract_storage.ml
index 7d8219856..e01220a6f 100644
--- a/src/proto_alpha/lib_protocol/src/contract_storage.ml
+++ b/src/proto_alpha/lib_protocol/src/contract_storage.ml
@@ -186,7 +186,7 @@ let () =
 
 let failwith msg = fail (Failure msg)
 
-type big_map_diff = (string * Script_repr.expr option) list
+type big_map_diff = (Script_expr_hash.t * Script_repr.expr option) list
 
 let update_script_big_map c contract = function
   | None -> return (c, Z.zero)
diff --git a/src/proto_alpha/lib_protocol/src/contract_storage.mli b/src/proto_alpha/lib_protocol/src/contract_storage.mli
index ed6d03802..7bece6eda 100644
--- a/src/proto_alpha/lib_protocol/src/contract_storage.mli
+++ b/src/proto_alpha/lib_protocol/src/contract_storage.mli
@@ -60,7 +60,7 @@ val get_script:
 val get_storage:
   Raw_context.t -> Contract_repr.t -> (Raw_context.t * Script_repr.expr option) tzresult Lwt.t
 
-type big_map_diff = (string * Script_repr.expr option) list
+type big_map_diff = (Script_expr_hash.t * Script_repr.expr option) list
 
 val update_script_storage:
   Raw_context.t -> Contract_repr.t ->
@@ -108,7 +108,7 @@ val pay_for_storage_space: Raw_context.t -> Contract_repr.t -> Tez_repr.t -> Raw
 
 module Big_map : sig
   val mem :
-    Raw_context.t -> Contract_repr.t -> string -> (Raw_context.t * bool) tzresult Lwt.t
+    Raw_context.t -> Contract_repr.t -> Script_expr_hash.t -> (Raw_context.t * bool) tzresult Lwt.t
   val get_opt :
-    Raw_context.t -> Contract_repr.t -> string -> (Raw_context.t * Script_repr.expr option) tzresult Lwt.t
+    Raw_context.t -> Contract_repr.t -> Script_expr_hash.t -> (Raw_context.t * Script_repr.expr option) tzresult Lwt.t
 end
diff --git a/src/proto_alpha/lib_protocol/src/helpers_services.ml b/src/proto_alpha/lib_protocol/src/helpers_services.ml
index 54c327327..efcb92e0f 100644
--- a/src/proto_alpha/lib_protocol/src/helpers_services.ml
+++ b/src/proto_alpha/lib_protocol/src/helpers_services.ml
@@ -60,7 +60,9 @@ module Scripts = struct
         ~output: (obj3
                     (req "storage" Script.expr_encoding)
                     (req "operations" (list Operation.internal_operation_encoding))
-                    (opt "big_map_diff" (list (tup2 string (option Script.expr_encoding)))))
+                    (opt "big_map_diff" (list (tup2
+                                                 Script_expr_hash.encoding
+                                                 (option Script.expr_encoding)))))
         RPC_path.(path / "run_code")
 
     let trace_code =
@@ -81,7 +83,9 @@ module Scripts = struct
                                 (obj2
                                    (req "item" (Script.expr_encoding))
                                    (opt "annot" string))))))
-                    (opt "big_map_diff" (list (tup2 string (option Script.expr_encoding)))))
+                    (opt "big_map_diff" (list (tup2
+                                                 Script_expr_hash.encoding
+                                                 (option Script.expr_encoding)))))
         RPC_path.(path / "trace_code")
 
     let typecheck_code =
@@ -118,7 +122,7 @@ module Scripts = struct
                    (req "type" Script.expr_encoding)
                    (opt "gas" z))
         ~output: (obj2
-                    (req "hash" string)
+                    (req "hash" Script_expr_hash.encoding)
                     (req "gas" Gas.encoding))
         ~query: RPC_query.empty
         RPC_path.(path / "hash_data")
diff --git a/src/proto_alpha/lib_protocol/src/helpers_services.mli b/src/proto_alpha/lib_protocol/src/helpers_services.mli
index 6445583d3..3f59384a6 100644
--- a/src/proto_alpha/lib_protocol/src/helpers_services.mli
+++ b/src/proto_alpha/lib_protocol/src/helpers_services.mli
@@ -50,7 +50,7 @@ module Scripts : sig
 
   val hash_data:
     'a #RPC_context.simple ->
-    'a -> Script.expr * Script.expr * Z.t option -> (string * Gas.t) shell_tzresult Lwt.t
+    'a -> Script.expr * Script.expr * Z.t option -> (Script_expr_hash.t * Gas.t) shell_tzresult Lwt.t
 
   val run_operation:
     'a #RPC_context.simple ->
diff --git a/src/proto_alpha/lib_protocol/src/script_interpreter.ml b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
index 43651a40a..686bb54f9 100644
--- a/src/proto_alpha/lib_protocol/src/script_interpreter.ml
+++ b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
@@ -678,7 +678,6 @@ let rec interp
             logged_return (Item (now, rest), ctxt)
         | Check_signature, Item (key, Item (signature, Item (message, rest))) ->
             Lwt.return (Gas.consume ctxt Interp_costs.check_signature) >>=? fun ctxt ->
-            let message = MBytes.of_string message in
             let res = Signature.check key signature message in
             logged_return (Item (res, rest), ctxt)
         | Hash_key, Item (key, rest) ->
@@ -687,6 +686,7 @@ let rec interp
         | Blake2b ty, Item (v, rest) ->
             Lwt.return (Gas.consume ctxt (Interp_costs.hash v)) >>=? fun ctxt ->
             hash_data ctxt ty v >>=? fun (hash, ctxt) ->
+            let hash = Script_expr_hash.to_bytes hash in
             logged_return (Item (hash, rest), ctxt)
         | Steps_to_quota, rest ->
             Lwt.return (Gas.consume ctxt Interp_costs.steps_to_quota) >>=? fun ctxt ->
diff --git a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
index 3a1694872..12c18dc2f 100644
--- a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
+++ b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
@@ -2341,7 +2341,7 @@ and parse_instr
         typed ctxt loc Hash_key
           (Item_t (Key_hash_t None, rest, annot))
     | Prim (loc, I_CHECK_SIGNATURE, [], annot),
-      Item_t (Key_t _, Item_t (Signature_t _, Item_t (String_t _, rest, _), _), _) ->
+      Item_t (Key_t _, Item_t (Signature_t _, Item_t (Bytes_t _, rest, _), _), _) ->
         parse_var_annot loc annot >>=? fun annot ->
         typed ctxt loc Check_signature
           (Item_t (Bool_t None, rest, annot))
@@ -2349,7 +2349,7 @@ and parse_instr
       Item_t (t, rest, _) ->
         parse_var_annot loc annot >>=? fun annot ->
         typed ctxt loc (Blake2b t)
-          (Item_t (String_t None, rest, annot))
+          (Item_t (Bytes_t None, rest, annot))
     | Prim (loc, I_STEPS_TO_QUOTA, [], annot),
       stack ->
         parse_var_annot loc annot ~default:default_steps_annot >>=? fun annot ->
@@ -2806,7 +2806,7 @@ let hash_data ctxt typ data =
   unparse_data ctxt Optimized typ data >>=? fun (data, ctxt) ->
   let unparsed = strip_annotations @@ data in
   let bytes = Data_encoding.Binary.to_bytes_exn expr_encoding (Micheline.strip_locations unparsed) in
-  return (Script_expr_hash.(hash_bytes [ bytes ] |> to_b58check), ctxt)
+  return (Script_expr_hash.(hash_bytes [ bytes ]), ctxt)
 
 (* ---------------- Big map -------------------------------------------------*)
 
diff --git a/src/proto_alpha/lib_protocol/src/script_ir_translator.mli b/src/proto_alpha/lib_protocol/src/script_ir_translator.mli
index 7c2e1ae33..323743dee 100644
--- a/src/proto_alpha/lib_protocol/src/script_ir_translator.mli
+++ b/src/proto_alpha/lib_protocol/src/script_ir_translator.mli
@@ -95,7 +95,7 @@ val parse_contract :
   context -> Script.location -> 'a Script_typed_ir.ty -> Contract.t ->
   (context * 'a Script_typed_ir.typed_contract) tzresult Lwt.t
 
-val hash_data : context -> 'a Script_typed_ir.ty -> 'a -> (string * context) tzresult Lwt.t
+val hash_data : context -> 'a Script_typed_ir.ty -> 'a -> (Script_expr_hash.t * context) tzresult Lwt.t
 
 val extract_big_map :
   'a Script_typed_ir.ty -> 'a -> Script_typed_ir.ex_big_map option
diff --git a/src/proto_alpha/lib_protocol/src/script_typed_ir.ml b/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
index 460bb7a2c..442d6d8cf 100644
--- a/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
+++ b/src/proto_alpha/lib_protocol/src/script_typed_ir.ml
@@ -336,11 +336,11 @@ and ('bef, 'aft) instr =
   | Balance :
       ('rest, Tez.t * 'rest) instr
   | Check_signature :
-      (public_key * (signature * (string * 'rest)), bool * 'rest) instr
+      (public_key * (signature * (MBytes.t * 'rest)), bool * 'rest) instr
   | Hash_key :
       (public_key * 'rest, public_key_hash * 'rest) instr
   | Blake2b : 'a ty ->
-    ('a * 'rest, string * 'rest) instr
+    ('a * 'rest, MBytes.t * 'rest) instr
   | Steps_to_quota : (* TODO: check that it always returns a nat *)
       ('rest, n num * 'rest) instr
   | Source :
diff --git a/src/proto_alpha/lib_protocol/src/storage.ml b/src/proto_alpha/lib_protocol/src/storage.ml
index c5eacf536..1731cf9f3 100644
--- a/src/proto_alpha/lib_protocol/src/storage.ml
+++ b/src/proto_alpha/lib_protocol/src/storage.ml
@@ -41,21 +41,6 @@ module Int_index = struct
     }
 end
 
-module String_index = struct
-  type t = string
-  let path_length = 1
-  let to_path c l = c :: l
-  let of_path = function
-    | [ c ] -> Some c
-    | [] | _ :: _ :: _ -> None
-  type 'a ipath = 'a * t
-  let args = Storage_description.One {
-      rpc_arg = RPC_arg.string ;
-      encoding = Data_encoding.string ;
-      compare = Compare.String.compare ;
-    }
-end
-
 module Make_index(H : Storage_description.INDEX)
   : INDEX with type t = H.t and type 'a ipath = 'a * H.t = struct
   include H
@@ -183,7 +168,7 @@ module Contract = struct
       (Make_subcontext
          (Indexed_context.Raw_context)
          (struct let name = ["big_map"] end))
-      (String_index)
+      (Make_index(Script_expr_hash))
       (struct
         type t = Script_repr.expr
         let encoding = Script_repr.expr_encoding
diff --git a/src/proto_alpha/lib_protocol/src/storage.mli b/src/proto_alpha/lib_protocol/src/storage.mli
index 1e88942de..ee435f661 100644
--- a/src/proto_alpha/lib_protocol/src/storage.mli
+++ b/src/proto_alpha/lib_protocol/src/storage.mli
@@ -188,7 +188,7 @@ module Contract : sig
   type bigmap_key = Raw_context.t * Contract_repr.t
 
   module Big_map : Non_iterable_indexed_carbonated_data_storage
-    with type key = string
+    with type key = Script_expr_hash.t
      and type value = Script_repr.expr
      and type t := bigmap_key