diff --git a/src/core/CCSexp.ml b/src/core/CCSexp.ml
index 1c75df73..9a2b69b8 100644
--- a/src/core/CCSexp.ml
+++ b/src/core/CCSexp.ml
@@ -8,12 +8,14 @@ type 'a or_error = ('a, string) result
 type 'a gen = unit -> 'a option
 
 module type SEXP = CCSexp_intf.SEXP
+module type BASIC_SEXP = CCSexp_intf.BASIC_SEXP
 module type S = CCSexp_intf.S
+module type S0 = CCSexp_intf.S0
 
 let equal_string (a : string) b = Stdlib.(=) a b
 let compare_string (a : string) b = Stdlib.compare a b
 
-module Make(Sexp : SEXP) = struct
+module MakeBasic(Sexp : BASIC_SEXP) = struct
   type t = Sexp.t
   type sexp = t
 
@@ -33,18 +35,32 @@ module Make(Sexp : SEXP) = struct
   let of_field name t = Sexp.list [Sexp.atom name; t]
   let of_record l =
     Sexp.list (List.map (fun (n,x) -> of_field n x) l)
+end
 
-  (** {2 Printing} *)
+let _with_in filename f =
+  let ic = open_in filename in
+  try
+    let x = f ic in
+    close_in ic;
+    x
+  with e ->
+    close_in ic;
+    Error (Printexc.to_string e)
 
-  let _with_out filename f =
-    let oc = open_out filename in
-    try
-      let x = f oc in
-      close_out oc;
-      x
-    with e ->
-      close_out oc;
-      raise e
+let _with_out filename f =
+  let oc = open_out filename in
+  try
+    let x = f oc in
+    close_out oc;
+    x
+  with e ->
+    close_out oc;
+    raise e
+
+module Make(Sexp : SEXP) = struct
+  include MakeBasic(Sexp)
+
+  (** {3 Printing} *)
 
   (* shall we escape the string because of one of its chars? *)
   let _must_escape s =
@@ -127,16 +143,6 @@ module Make(Sexp : SEXP) = struct
 
   (** {2 Parsing} *)
 
-  let _with_in filename f =
-    let ic = open_in filename in
-    try
-      let x = f ic in
-      close_in ic;
-      x
-    with e ->
-      close_in ic;
-      Error (Printexc.to_string e)
-
   (** A parser of ['a] can return [Yield x] when it parsed a value,
       or [Fail e] when a parse error was encountered, or
       [End] if the input was empty *)
@@ -190,7 +196,7 @@ module Make(Sexp : SEXP) = struct
           let _u = expr() in (* discard next sexp *)
           expr()
         | L.ATOM s ->
-          junk t; 
+          junk t;
           begin match Sexp.make_loc with
             | None -> Sexp.atom s
             | Some f ->
@@ -213,7 +219,7 @@ module Make(Sexp : SEXP) = struct
                   let loc =
                     f (pair_of_pos_ pos_start)
                       (pair_of_pos_ t.buf.lex_curr_p)
-                      t.buf.lex_curr_p.pos_fname in 
+                      t.buf.lex_curr_p.pos_fname in
                   Sexp.list_with_loc ~loc l
               end
             | _ -> error_ t.buf "expected ')'"
@@ -293,6 +299,189 @@ module Make(Sexp : SEXP) = struct
   let parse_file_list filename = _with_in filename (parse_chan_list_ ~file:filename)
 end
 
+module MakeCanonical(Sexp : BASIC_SEXP) = struct
+  include MakeBasic(Sexp)
+
+  (** {3 Printing} *)
+
+  let rec to_buf b t =
+    Sexp.match_ t
+      ~atom:(fun s -> Printf.bprintf b "%d:%s" (String.length s) s)
+      ~list:(function
+          | [] -> Buffer.add_string b "()"
+          | [x] -> Printf.bprintf b "(%a)" to_buf x
+          | l ->
+            Buffer.add_char b '(';
+            List.iter (to_buf b) l;
+            Buffer.add_char b ')')
+
+  let to_string t =
+    let b = Buffer.create 128 in
+    to_buf b t;
+    Buffer.contents b
+
+  let rec pp_noindent fmt t =
+    Sexp.match_ t
+      ~atom:(fun s -> Format.fprintf fmt "%d:%s" (String.length s) s)
+      ~list:(function
+          | [] -> Format.pp_print_string fmt "()"
+          | [x] -> Format.fprintf fmt "(%a)" pp_noindent x
+          | l ->
+            Format.fprintf fmt "(";
+            List.iter (pp_noindent fmt) l;
+            Format.fprintf fmt ")")
+
+  let pp = pp_noindent
+
+  let rec to_chan oc t =
+    Sexp.match_ t
+      ~atom:(fun s -> Printf.fprintf oc "%d:%s" (String.length s) s)
+      ~list:(function
+          | [] -> output_string oc "()"
+          | [x] -> Printf.fprintf oc "(%a)" to_chan x
+          | l ->
+            output_char oc '(';
+            List.iter (to_chan oc) l;
+            output_char oc ')')
+
+  let to_file_iter filename iter =
+    _with_out filename
+      (fun oc -> iter (fun t -> to_chan oc t))
+
+  let to_file filename t = to_file_iter filename (fun k -> k t)
+
+  (** {3 Parsing} *)
+
+  module type INPUT = sig
+    exception EOF
+    val read_char : unit -> char
+    val read_string : int -> string
+  end
+
+  module Decoder(I:INPUT) = struct
+    let[@inline] is_num_ c = Char.code c >= Char.code '0' && Char.code c <= Char.code '9'
+    let[@inline] as_num_ c = Char.code c - Char.code '0'
+
+    let next_ () : sexp or_error * bool =
+      let rec read_string_len n =
+        match I.read_char () with
+        | c when is_num_ c -> read_string_len (n * 10 + as_num_ c)
+        | ':' ->
+          let s = I.read_string n in
+          atom s
+        | _ -> failwith "expected string length"
+
+      and eat_colon () =
+        match I.read_char () with
+        | ':' -> ()
+        | _ -> failwith "expected ':'"
+
+      and read_in_paren acc =
+        match I.read_char () with
+        | ')' -> list (List.rev acc)
+        | c when is_num_ c ->
+          let sexp = read_string_len (as_num_ c) in
+          read_in_paren (sexp::acc)
+        | '(' ->
+          let sexp = read_in_paren [] in
+          read_in_paren (sexp::acc)
+        | _ -> failwith "expected list of sexprs"
+      in
+      (* read a S-expr *)
+      try
+        begin match I.read_char () with
+          | exception I.EOF -> Error "unexpected EOF", true
+          | '(' -> Ok (read_in_paren []), false
+          | '0' -> eat_colon (); Ok (atom ""), false
+          | c when is_num_ c -> Ok (read_string_len (as_num_ c)), false
+          | _ -> Error "unexpected char, expected toplevel sexpr", false
+        end
+      with Failure e -> Error e, false
+
+    let to_list () : _ or_error =
+      let rec iter acc =
+        match next_ () with
+        | Error _, true -> Ok (List.rev acc)
+        | Ok x, _ -> iter (x::acc)
+        | Error _ as res, _ -> res
+      in
+      try iter []
+      with e -> Error (Printexc.to_string e)
+
+    let[@inline] next_or_error () : _ or_error = fst (next_ ())
+  end[@@inline]
+
+  module Decoder_str(X : sig val s : string end) =
+    Decoder(struct
+      exception EOF
+      let i = ref 0
+      let n = String.length X.s
+      let read_char () =
+        if !i >= n then raise_notrace EOF;
+        let c = String.unsafe_get X.s !i in
+        incr i;
+        c
+      let read_string len =
+        if !i + len > n then raise_notrace EOF;
+        let res = String.sub X.s !i len in
+        i := !i + len;
+        res
+    end)[@@inline]
+
+  let parse_string s : t or_error =
+    let module D = Decoder_str(struct let s=s end) in
+    D.next_or_error ()
+
+  let parse_string_list s : t list or_error =
+    let module D = Decoder_str(struct let s=s end) in
+    D.to_list ()
+
+  module Decoder_ic(X : sig val ic : in_channel end) =
+    Decoder(struct
+      exception EOF = End_of_file
+      let[@inline] read_char () = input_char X.ic
+      let read_string n =
+        match n with
+        | 0 -> ""
+        | 1 -> String.make 1 (read_char ())
+        | _ ->
+          let buf = Bytes.make n '\000' in
+          let i = ref 0 in
+          while !i < n do
+            let len = input X.ic buf !i (n - !i) in
+            i := !i + len;
+          done;
+          Bytes.unsafe_to_string buf
+    end)[@@inline]
+
+  let parse_chan_ ?file ic : sexp or_error =
+    let module D = Decoder_ic(struct let ic=ic end) in
+    match D.next_or_error(), file with
+    | Error s, Some file -> Error (Printf.sprintf "%s in '%s'" s file)
+    | r, _ -> r
+
+  let parse_chan_list_ ?file ic =
+    let module D = Decoder_ic(struct let ic=ic end) in
+    match D.to_list (), file with
+    | Error s, Some file -> Error (Printf.sprintf "%s in '%s'" s file)
+    | r, _ -> r
+
+  let parse_chan ic = parse_chan_ ic
+  let parse_chan_list ic = parse_chan_list_ ic
+
+  let parse_chan_gen ic =
+    let module D = Decoder_ic(struct let ic=ic end) in
+    fun () ->
+      match D.next_ () with
+      | _, true -> None
+      | Error e, _ -> Some (Error e)
+      | Ok x, _ -> Some (Ok x)
+
+  let parse_file filename = _with_in filename (parse_chan_ ~file:filename)
+
+  let parse_file_list filename = _with_in filename (parse_chan_list_ ~file:filename)
+end
+
 type t = [
   | `Atom of string
   | `List of t list
@@ -321,20 +510,22 @@ and compare a b = match a, b with
   | `Atom _, _ -> -1
   | `List _, _ -> 1
 
-include (Make(struct
-    type t_ = t
-    type t = t_
-    type loc = unit
-    let make_loc = None
-    let atom x = `Atom x
-    let list x = `List x
-    let atom_with_loc ~loc:_ s = atom s
-    let list_with_loc ~loc:_ l = list l
+module Basic_ = struct
+  type nonrec t = t
+  type loc = unit
+  let make_loc = None
+  let atom x = `Atom x
+  let list x = `List x
+  let atom_with_loc ~loc:_ s = atom s
+  let list_with_loc ~loc:_ l = list l
 
-    let match_ x ~atom ~list = match x with
-      | `Atom x -> atom x
-      | `List l -> list l
-  end) : S with type t := t)
+  let match_ x ~atom ~list = match x with
+    | `Atom x -> atom x
+    | `List l -> list l
+end
+
+include (Make(Basic_) : S with type t := t)
+module Canonical = MakeCanonical(Basic_)
 
 (*$T
   CCResult.to_opt (parse_string "(abc d/e/f \"hello \\\" () world\" )") <> None
@@ -361,11 +552,18 @@ include (Make(struct
 
 (*$inject
   let sexp_bijective s = to_string s |> parse_string = Ok s
+  let csexp_bijective s = Canonical.to_string s |> Canonical.parse_string = Ok s
+*)
+
+(*$= & ~printer:CCFormat.(to_string (Dump.result Canonical.pp))
+  (Ok (`List [`Atom ""])) (parse_string "(\"\")")
+  (Ok (`List [`Atom ""])) (Canonical.parse_string {|(0:)|})
+  (Ok (`List [`Atom "a"; `Atom "b "])) (Canonical.parse_string {|(1:a2:b )|})
 *)
 
 (*$T
-  (Ok (`List [`Atom ""]) = (parse_string "(\"\")"))
   sexp_bijective (`List [`Atom ""])
+  csexp_bijective (`List [`Atom ""])
 *)
 
 (*$inject
@@ -398,6 +596,17 @@ include (Make(struct
 
 (*$Q & ~count:100
     sexp_gen sexp_bijective
+    sexp_gen csexp_bijective
+*)
+
+(*$R
+  let s1 =
+    `List (CCList.init 100_000
+      (fun i -> `List [`Atom "-"; `Atom (string_of_int i); `Atom ")(\n]"])) in
+  let str = Canonical.to_string s1 in
+  match Canonical.parse_string str with
+    | Ok s2 -> assert_equal s1 s2
+    | Error e -> assert_failure e
 *)
 
 let atom s : t = `Atom s
diff --git a/src/core/CCSexp.mli b/src/core/CCSexp.mli
index 449cd5ce..697e486f 100644
--- a/src/core/CCSexp.mli
+++ b/src/core/CCSexp.mli
@@ -10,6 +10,11 @@ type 'a gen = unit -> 'a option
 
 (** {2 Abstract representation of S-expressions}
 
+    @since NEXT_RELEASE *)
+module type BASIC_SEXP = CCSexp_intf.BASIC_SEXP
+
+(** {2 Abstract representation of S-expressions (extended)}
+
     @since 2.7 *)
 module type SEXP = CCSexp_intf.SEXP
 
@@ -18,6 +23,10 @@ module type SEXP = CCSexp_intf.SEXP
     @since 2.7 *)
 module type S = CCSexp_intf.S
 
+(** Subset of {!S}
+    @since NEXT_RELEASE *)
+module type S0 = CCSexp_intf.S0
+
 (** {2 Functorized operations}
 
     This builds a parser and printer for S-expressions represented as
@@ -26,6 +35,12 @@ module type S = CCSexp_intf.S
     @since 2.7 *)
 module Make(Sexp : SEXP) : S with type t = Sexp.t
 
+(** {2 Parser and printer for Canonical S-exprs}
+
+    See {{: https://en.wikipedia.org/wiki/Canonical_S-expressions} wikipedia}.
+    These S-expressions are binary safe. *)
+module MakeCanonical(Sexp : BASIC_SEXP) : S0 with type t = Sexp.t
+
 (** {2 Basics} *)
 
 (** A simple, structural representation of S-expressions. *)
@@ -36,6 +51,10 @@ type t = [
 
 include S with type t := t
 
+(** Canonical encoding
+    @since NEXT_RELEASE *)
+module Canonical : S0 with type t = t
+
 val equal : t -> t -> bool
 (** @since 3.0 *)
 
diff --git a/src/core/CCSexp_intf.ml b/src/core/CCSexp_intf.ml
index 6fc61e42..02458a0c 100644
--- a/src/core/CCSexp_intf.ml
+++ b/src/core/CCSexp_intf.ml
@@ -4,10 +4,22 @@ type 'a iter = ('a -> unit) -> unit
 type 'a gen = unit -> 'a option
 
 (** {2 Abstract representation of S-expressions}
+    @since NEXT_RELEASE *)
+module type BASIC_SEXP = sig
+  type t
+
+  val atom : string -> t
+  val list : t list -> t
+
+  val match_ : t -> atom:(string -> 'a) -> list:(t list -> 'a) -> 'a
+end
+
+(** {2 Abstract representation of S-expressions (extended)}
 
     @since 2.7 *)
 module type SEXP = sig
-  type t
+  include BASIC_SEXP
+
   type loc
 
   val make_loc : ((int * int) -> (int * int) -> string -> loc) option
@@ -16,17 +28,12 @@ module type SEXP = sig
 
   val atom_with_loc : loc:loc -> string -> t
   val list_with_loc : loc:loc -> t list -> t
-
-  val atom : string -> t
-  val list : t list -> t
-
-  val match_ : t -> atom:(string -> 'a) -> list:(t list -> 'a) -> 'a
 end
 
 (** {2 Operations over S-expressions}
 
     @since 2.7 *)
-module type S = sig
+module type S0 = sig
   type t
   type sexp = t
 
@@ -86,29 +93,6 @@ module type S = sig
 
   (** {2 Parsing} *)
 
-  (** A parser of ['a] can return [Yield x] when it parsed a value,
-      or [Fail e] when a parse error was encountered, or
-      [End] if the input was empty. *)
-  type 'a parse_result =
-    | Yield of 'a
-    | Fail of string
-    | End
-
-  module Decoder : sig
-    type t
-    (** Decoder *)
-
-    val of_lexbuf : Lexing.lexbuf -> t
-
-    val next : t -> sexp parse_result
-    (** Parse the next S-expression or return an error if the input isn't
-        long enough or isn't a proper S-expression. *)
-
-    val to_list : t -> sexp list or_error
-    (** Read all the values from this decoder.
-        @since 2.8 *)
-  end
-
   val parse_string : string -> t or_error
   (** Parse a string. *)
 
@@ -131,4 +115,37 @@ module type S = sig
 
   val parse_file_list : string -> t list or_error
   (** Open the file and read a S-exp from it. *)
+
+end
+
+(** {2 Operations over S-expressions (extended)}
+
+    @since 2.7 *)
+module type S = sig
+  include S0
+
+  (** {2 Parsing} *)
+
+  (** A parser of ['a] can return [Yield x] when it parsed a value,
+      or [Fail e] when a parse error was encountered, or
+      [End] if the input was empty. *)
+  type 'a parse_result =
+    | Yield of 'a
+    | Fail of string
+    | End
+
+  module Decoder : sig
+    type t
+    (** Decoder *)
+
+    val of_lexbuf : Lexing.lexbuf -> t
+
+    val next : t -> sexp parse_result
+    (** Parse the next S-expression or return an error if the input isn't
+        long enough or isn't a proper S-expression. *)
+
+    val to_list : t -> sexp list or_error
+    (** Read all the values from this decoder.
+        @since 2.8 *)
+  end
 end
diff --git a/src/core/CCSexp_lex.mll b/src/core/CCSexp_lex.mll
index 881338df..76e0ecca 100644
--- a/src/core/CCSexp_lex.mll
+++ b/src/core/CCSexp_lex.mll
@@ -88,3 +88,4 @@ rule token = parse
   | string { ATOM (remove_quotes lexbuf (Lexing.lexeme lexbuf)) }
   | _ as c
     { error lexbuf (Printf.sprintf "lexing failed on char `%c`" c) }
+