diff --git a/bencode.ml b/bencode.ml
new file mode 100644
index 00000000..a8c1fec5
--- /dev/null
+++ b/bencode.ml
@@ -0,0 +1,269 @@
+(*
+Copyright (c) 2013, Simon Cruanes
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+Redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  Redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {6 B-encoding} *)
+
+
+module SMap = Map.Make(String)
+
+type t =
+  | I of int
+  | S of string
+  | L of t list
+  | D of t SMap.t
+
+let rec eq t1 t2 = match t1, t2 with
+  | I i1, I i2 -> i1 = i2
+  | S s1, S s2 -> s1 = s2
+  | L l1, L l2 ->
+    (try List.for_all2 eq l1 l2 with Invalid_argument _ -> false)
+  | D d1, D d2 ->
+    SMap.equal eq d1 d2
+  | _ -> false
+
+let hash t = Hashtbl.hash t
+
+let dict_of_list l =
+  let d = List.fold_left
+    (fun d (k, v) -> SMap.add k v d)
+    SMap.empty l
+  in
+  D d
+
+(** {2 Serialization (encoding)} *)
+
+let rec to_buf buf t = match t with
+  | I i -> Printf.bprintf buf "i%de" i
+  | S s ->
+    Printf.bprintf buf "%d:" (String.length s);
+    Buffer.add_string buf s
+  | L l ->
+    Buffer.add_char buf 'l';
+    List.iter (fun t' -> to_buf buf t') l;
+    Buffer.add_char buf 'e'
+  | D m ->
+    Buffer.add_char buf 'd';
+    SMap.iter (fun key t' -> to_buf buf (S key); to_buf buf t') m;
+    Buffer.add_char buf 'e'
+
+let to_string t =
+  let b = Buffer.create 25 in
+  to_buf b t;
+  Buffer.contents b
+
+let to_chan ch t =
+  let b = Buffer.create 25 in
+  to_buf b t;
+  Buffer.output_buffer ch b
+
+let fmt formatter t =
+  let b = Buffer.create 25 in
+  to_buf b t;
+  Format.pp_print_string formatter (Buffer.contents b)
+
+(** {2 Deserialization (decoding)} *)
+
+(** Deserialization is based on the {! decoder} type. Parsing can be
+    incremental, in which case the input is provided chunk by chunk and
+    the decoder contains the parsing state. Once a B-encoded value
+    has been parsed, other values can still be read. *)
+
+type decoder = {
+  mutable buf : string;  (* buffer *)
+  mutable i : int;  (* index in buf *)
+  mutable len : int;  (* length of substring to read *)
+  mutable c : int;  (* line *)
+  mutable l : int;  (* column *)
+  mutable state : parse_result;
+  mutable stack : partial_state list;
+}
+
+(** Result of parsing *)
+and parse_result =
+  | ParseOk of t
+  | ParseError of string
+  | ParsePartial
+
+(** Partial state of the parser *)
+and partial_state =
+  | PS_I of bool * int (* sign and integer *)
+  | PS_S of int ref * string   (* index in string, plus string *)
+  | PS_L of t list
+  | PS_D of t SMap.t  (* in dictionary *)
+  | PS_D_key of string * t SMap.t  (* parsed key, wait for value *) 
+  | PS_return of t  (* bottom of stack *)
+  | PS_error of string (* error *)
+
+let mk_decoder () =
+  let dec = {
+    buf = "";
+    i = 0;
+    len = 0;
+    c = 0;
+    l = 0;
+    state = ParsePartial;
+    stack = [];
+  } in
+  dec
+
+let is_empty dec = dec.len = 0
+let cur dec = dec.buf.[dec.i]
+
+let junk dec =
+  (* update line/column *)
+  (if cur dec = '\n'
+    then (dec.c <- 0; dec.l <- dec.l + 1)
+    else dec.c <- dec.c + 1);
+  dec.i <- dec.i + 1;
+  dec.len <- dec.len - 1
+
+let next dec =
+  let c = cur dec in
+  junk dec;
+  c
+
+(* parse value *)
+let rec parse_rec dec =
+  match dec.stack with
+  | [PS_return v] ->  (* return value *)
+    dec.stack <- [];
+    dec.state <- ParseOk v;
+    dec.state
+  | [PS_error s] -> (* failure *)
+    dec.stack <- [];
+    dec.state <- ParseError s;
+    dec.state
+  | _ ->
+    if is_empty dec then ParsePartial (* wait *)
+    else begin
+      let c = next dec in
+      (match dec.stack, c with
+      | (PS_I (sign, i)) :: stack, '0' .. '9' ->
+        dec.stack <- PS_I (sign, (Char.code c - Char.code '0') + 10 * i) :: stack;
+      | (PS_I (_, 0)) :: stack, '-' ->
+        dec.stack <- PS_I (false, 0) :: stack  (* negative number *)
+      | (PS_I (sign, i)) :: stack, 'e' ->
+        dec.stack <- stack;
+        push_value dec (I (if sign then i else ~- i))
+      | ((PS_D _ | PS_D_key _ | PS_L _) :: _ | []), '0' .. '9' ->
+        (* initial length of string *)
+        dec.stack <- (PS_I (true, Char.code c - Char.code '0')) :: dec.stack
+      | (PS_I (sign, i)) :: stack, ':' ->
+        if i < 0
+          then error dec "string length cannot be negative"
+        else if i = 0 then  (* empty string *)
+          let _ = dec.stack <- stack in
+          push_value dec (S "")
+        else (* prepare to parse a string *)
+          dec.stack <- (PS_S (ref 0, String.create i)) :: stack;
+      | (PS_S (n, s)) :: stack, _ ->
+        s.[!n] <- c;
+        incr n;
+        (* value completed *)
+        (if !n = String.length s
+          then
+            let _ = dec.stack <- stack in
+            push_value dec (S s));
+      | stack, 'i' ->
+        dec.stack <- (PS_I (true, 0)) :: stack
+      | stack, 'l' ->
+        dec.stack <- PS_L [] :: stack;
+      | stack, 'd' ->
+        dec.stack <- PS_D SMap.empty :: stack
+      | (PS_L l) :: stack, 'e' -> (* end of list *)
+        dec.stack <- stack;
+        push_value dec (L (List.rev l))
+      | (PS_D d) :: stack, 'e' -> (* end of dict *)
+        dec.stack <- stack;
+        push_value dec (D d)
+      | (PS_D_key _) :: _, 'e' -> (* error *)
+        error dec "missing value in dict"
+      | _ -> (* generic error *)
+        error dec (Printf.sprintf "expected value, got %c" c));
+      parse_rec dec
+    end
+(* When a value is parsed, push it on the stack (possibly collapsing it) *)
+and push_value dec v =
+  match v, dec.stack with
+  | _, [] ->
+    dec.stack <- [PS_return v] (* finished *)
+  | _, (PS_L l) :: stack ->
+    (* add to list *)
+    dec.stack <- (PS_L (v :: l)) :: stack;
+  | S key, ((PS_D d) :: stack) ->
+    (* new key for the map *)
+    dec.stack <- (PS_D_key (key, d)) :: stack;
+  | _, ((PS_D d) :: _) ->
+    (* error: key must be string *)
+    error dec "dict keys must be strings"
+  | _, (PS_D_key (key, d)) :: stack ->
+    (* new binding for the map *)
+    dec.stack <- (PS_D (SMap.add key v d)) :: stack;
+  | _ -> assert false
+(* signal error *)
+and error dec msg =
+  let msg = Printf.sprintf "Bencode: error at line %d, column %d: %s"
+    dec.l dec.c msg in
+  dec.stack <- [PS_error msg]
+
+(* exported parse function *)
+let parse dec s i len =
+  (if i < 0 || i+len > String.length s
+    then invalid_arg "Bencode.parse: not a valid substring");
+  (* add the input to [dec] *)
+  if dec.len = 0
+    then begin
+      dec.buf <- s;
+      dec.i <- i;
+      dec.len <- len;
+    end else begin
+      (* use a buffer to merge the stored input and the new input *)
+      let b = Buffer.create (dec.len + len) in
+      Buffer.add_substring b dec.buf dec.i dec.len;
+      Buffer.add_substring b s i len;
+      dec.buf <- Buffer.contents b;
+      dec.i <- 0;
+      dec.len <- dec.len + len;
+    end;
+  (* state machine *)
+  parse_rec dec
+
+let state dec = dec.state
+
+let rest dec =
+  String.sub dec.buf dec.i dec.len
+
+let rest_size dec =
+  dec.len
+
+let parse_string s =
+  let dec = mk_decoder () in
+  parse dec s 0 (String.length s)
+
+let of_string s =
+  match parse_string s with
+  | ParseOk t -> t
+  | ParsePartial -> invalid_arg "Bencode: partial parse"
+  | ParseError msg -> invalid_arg msg
diff --git a/bencode.mli b/bencode.mli
new file mode 100644
index 00000000..3cb8c1f9
--- /dev/null
+++ b/bencode.mli
@@ -0,0 +1,92 @@
+(*
+Copyright (c) 2013, Simon Cruanes
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+Redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  Redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {6 B-encoding} *)
+
+(** This implements encoding and decoding using the {i B-encode} format.
+    See {{: http://en.wikipedia.org/wiki/Bencode} wikipedia} for more details
+    *)
+
+module SMap : Map.S with type key = string
+
+type t =
+  | I of int
+  | S of string
+  | L of t list
+  | D of t SMap.t
+
+val eq : t -> t -> bool
+val hash : t -> int
+
+val dict_of_list : (string * t) list -> t
+
+(** {2 Serialization (encoding)} *)
+
+val to_buf : Buffer.t -> t -> unit
+val to_string : t -> string
+val to_chan : out_channel -> t -> unit
+val fmt : Format.formatter -> t -> unit
+
+(** {2 Deserialization (decoding)} *)
+
+(** Deserialization is based on the {! decoder} type. Parsing can be
+    incremental, in which case the input is provided chunk by chunk and
+    the decoder contains the parsing state. Once a B-encoded value
+    has been parsed, other values can still be read.
+    
+    This implementation does accept leading zeros, because it simplifies
+    the code. *)
+
+type decoder
+  (** Decoding state *)
+
+val mk_decoder : unit -> decoder
+  (** Create a new decoder *)
+
+type parse_result =
+  | ParseOk of t
+  | ParseError of string
+  | ParsePartial
+
+val parse : decoder -> string -> int -> int -> parse_result
+  (** [parse dec s i len] uses the partial state stored in [dec] and
+      the substring of [s] starting at index [i] with length [len].
+      It can return an error, a value or just [ParsePartial] if
+      more input is needed *)
+
+val state : decoder -> parse_result
+  (** Current state of the decoder *)
+
+val rest : decoder -> string
+  (** What remains after parsing (the additional, unused input) *)
+
+val rest_size : decoder -> int
+  (** Length of [rest d]. 0 indicates that the whole input has been consumed. *)
+
+val parse_string : string -> parse_result
+  (** Parse a full value from this string. *)
+
+val of_string : string -> t
+  (** Parse the string. @raise Invalid_argument if it fails to parse. *)
diff --git a/tests/test_bencode.ml b/tests/test_bencode.ml
new file mode 100644
index 00000000..17ebfd8c
--- /dev/null
+++ b/tests/test_bencode.ml
@@ -0,0 +1,46 @@
+
+open OUnit
+
+module B = Bencode
+
+let test1 () =
+  let s = "li42ei0ei-200ee" in
+  match B.parse_string s with
+  | B.ParseError msg ->
+    OUnit.assert_failure (Printf.sprintf "should parse, got %s" msg)
+  | B.ParsePartial ->
+    OUnit.assert_failure "should parse, got partial"
+  | B.ParseOk b ->
+    OUnit.assert_equal (B.L [B.I 42; B.I 0; B.I ~-200]) b
+
+let test2 () =
+  let b =
+    B.dict_of_list [
+      "foo", B.I 42;
+      "bar", B.L [B.I 0; B.S "caramba si"];
+      "", B.S "";
+    ]
+  in
+  let s = B.to_string b in
+  (* Printf.printf "serialized to %s\n" s; *)
+  let b' = B.of_string s in
+  OUnit.assert_equal ~cmp:B.eq ~printer:B.to_string b b'
+
+let test3 () =
+  let b = B.dict_of_list [
+    "a", B.I 1;
+    "b", B.S "bbbb";
+    "l", B.L [B.I 0; B.I 0; B.S "zero\n\t \x00"];
+    "d", B.dict_of_list ["foo", B.S "bar"];
+  ] in
+  let s = B.to_string b in
+  (* Printf.printf "serialized to %s\n" s; *)
+  let b' = B.of_string s in
+  OUnit.assert_equal ~cmp:B.eq ~printer:B.to_string b b'
+
+let suite =
+  "test_bencode" >:::
+    [ "test1" >:: test1;
+      "test2" >:: test2;
+      "test3" >:: test3;
+    ]