diff --git a/bench/speed.ml b/bench/speed.ml
index 90d44425..3b1c90e5 100644
--- a/bench/speed.ml
+++ b/bench/speed.ml
@@ -45,6 +45,15 @@ let throughput title f =
     Printf.printf "    % 5d:  %04f MB/s  (%d iters in %.03f s)\n%!"
       size (bw /. mb) iters time
 
+let throughput_into ?(add = 0) title f =
+  Printf.printf "\n* [%s]\n%!" title ;
+  sizes |> List.iter @@ fun size ->
+    Gc.full_major () ;
+    let dst = Bytes.create (size + add) in
+    let (iters, time, bw) = burn (f dst) size in
+    Printf.printf "    % 5d:  %04f MB/s  (%d iters in %.03f s)\n%!"
+      size (bw /. mb) iters time
+
 let count_period = 10.
 
 let count f n =
@@ -347,55 +356,128 @@ let benchmarks = [
         fst ecdh_shares);
 
   bm "chacha20-poly1305" (fun name ->
-      let key = Mirage_crypto.Chacha20.of_secret (Mirage_crypto_rng.generate 32)
+      let key = Chacha20.of_secret (Mirage_crypto_rng.generate 32)
       and nonce = Mirage_crypto_rng.generate 8 in
-      throughput name (Mirage_crypto.Chacha20.authenticate_encrypt ~key ~nonce)) ;
+      throughput_into ~add:Chacha20.tag_size name
+        (fun dst cs -> Chacha20.authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs))) ;
+
+  bm "chacha20-poly1305-unsafe" (fun name ->
+      let key = Chacha20.of_secret (Mirage_crypto_rng.generate 32)
+      and nonce = Mirage_crypto_rng.generate 8 in
+      throughput_into ~add:Chacha20.tag_size name
+        (fun dst cs -> Chacha20.unsafe_authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs))) ;
 
   bm "aes-128-ecb" (fun name ->
     let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 16) in
-    throughput name (fun cs -> AES.ECB.encrypt ~key cs)) ;
+    throughput_into name
+      (fun dst cs -> AES.ECB.encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-192-ecb" (fun name ->
+    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 24) in
+    throughput_into name (fun dst cs -> AES.ECB.encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-192-ecb-unsafe" (fun name ->
+    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 24) in
+    throughput_into name (fun dst cs -> AES.ECB.unsafe_encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-256-ecb" (fun name ->
+    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 32) in
+    throughput_into name (fun dst cs -> AES.ECB.encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-256-ecb-unsafe" (fun name ->
+    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 32) in
+    throughput_into name (fun dst cs -> AES.ECB.unsafe_encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-128-ecb-unsafe" (fun name ->
+    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 16) in
+    throughput_into name
+      (fun dst cs -> AES.ECB.unsafe_encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
 
   bm "aes-128-cbc-e" (fun name ->
     let key = AES.CBC.of_secret (Mirage_crypto_rng.generate 16)
     and iv  = Mirage_crypto_rng.generate 16 in
-    throughput name (fun cs -> AES.CBC.encrypt ~key ~iv cs)) ;
+    throughput_into name
+      (fun dst cs -> AES.CBC.encrypt_into ~key ~iv cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-128-cbc-e-unsafe" (fun name ->
+    let key = AES.CBC.of_secret (Mirage_crypto_rng.generate 16)
+    and iv  = Mirage_crypto_rng.generate 16 in
+    throughput_into name
+      (fun dst cs -> AES.CBC.unsafe_encrypt_into ~key ~iv cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-128-cbc-e-unsafe-inplace" (fun name ->
+    let key = AES.CBC.of_secret (Mirage_crypto_rng.generate 16)
+    and iv  = Mirage_crypto_rng.generate 16 in
+    throughput name
+      (fun cs ->
+         let b = Bytes.unsafe_of_string cs in
+         AES.CBC.unsafe_encrypt_into_inplace ~key ~iv b ~dst_off:0 (String.length cs))) ;
 
   bm "aes-128-cbc-d" (fun name ->
     let key = AES.CBC.of_secret (Mirage_crypto_rng.generate 16)
     and iv  = Mirage_crypto_rng.generate 16 in
-    throughput name (fun cs -> AES.CBC.decrypt ~key ~iv cs)) ;
+    throughput_into name
+      (fun dst cs -> AES.CBC.decrypt_into ~key ~iv cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-128-cbc-d-unsafe" (fun name ->
+    let key = AES.CBC.of_secret (Mirage_crypto_rng.generate 16)
+    and iv  = Mirage_crypto_rng.generate 16 in
+    throughput_into name
+      (fun dst cs -> AES.CBC.unsafe_decrypt_into ~key ~iv cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
 
   bm "aes-128-ctr" (fun name ->
     let key = Mirage_crypto_rng.generate 16 |> AES.CTR.of_secret
     and ctr = Mirage_crypto_rng.generate 16 |> AES.CTR.ctr_of_octets in
-    throughput name (fun cs -> AES.CTR.encrypt ~key ~ctr cs)) ;
+    throughput_into name (fun dst cs -> AES.CTR.encrypt_into ~key ~ctr cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "aes-128-ctr-unsafe" (fun name ->
+    let key = Mirage_crypto_rng.generate 16 |> AES.CTR.of_secret
+    and ctr = Mirage_crypto_rng.generate 16 |> AES.CTR.ctr_of_octets in
+    throughput_into name (fun dst cs -> AES.CTR.unsafe_encrypt_into ~key ~ctr cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
 
   bm "aes-128-gcm" (fun name ->
     let key = AES.GCM.of_secret (Mirage_crypto_rng.generate 16)
     and nonce = Mirage_crypto_rng.generate 12 in
-    throughput name (fun cs -> AES.GCM.authenticate_encrypt ~key ~nonce cs));
+    throughput_into ~add:AES.GCM.tag_size name
+      (fun dst cs -> AES.GCM.authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs)));
+
+  bm "aes-128-gcm-unsafe" (fun name ->
+    let key = AES.GCM.of_secret (Mirage_crypto_rng.generate 16)
+    and nonce = Mirage_crypto_rng.generate 12 in
+    throughput_into ~add:AES.GCM.tag_size name
+      (fun dst cs -> AES.GCM.unsafe_authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs)));
 
   bm "aes-128-ghash" (fun name ->
     let key = AES.GCM.of_secret (Mirage_crypto_rng.generate 16)
     and nonce = Mirage_crypto_rng.generate 12 in
-    throughput name (fun cs -> AES.GCM.authenticate_encrypt ~key ~nonce ~adata:cs ""));
+    throughput_into ~add:AES.GCM.tag_size name
+      (fun dst cs -> AES.GCM.authenticate_encrypt_into ~key ~nonce ~adata:cs "" ~src_off:0 dst ~dst_off:0 ~tag_off:0 0));
+
+  bm "aes-128-ghash-unsafe" (fun name ->
+    let key = AES.GCM.of_secret (Mirage_crypto_rng.generate 16)
+    and nonce = Mirage_crypto_rng.generate 12 in
+    throughput_into ~add:AES.GCM.tag_size name
+      (fun dst cs -> AES.GCM.unsafe_authenticate_encrypt_into ~key ~nonce ~adata:cs "" ~src_off:0 dst ~dst_off:0 ~tag_off:0 0));
 
   bm "aes-128-ccm" (fun name ->
     let key   = AES.CCM16.of_secret (Mirage_crypto_rng.generate 16)
     and nonce = Mirage_crypto_rng.generate 10 in
-    throughput name (fun cs -> AES.CCM16.authenticate_encrypt ~key ~nonce cs));
-
-  bm "aes-192-ecb" (fun name ->
-    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 24) in
-    throughput name (fun cs -> AES.ECB.encrypt ~key cs)) ;
+    throughput_into ~add:AES.CCM16.tag_size name
+      (fun dst cs -> AES.CCM16.authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs)));
 
-  bm "aes-256-ecb" (fun name ->
-    let key = AES.ECB.of_secret (Mirage_crypto_rng.generate 32) in
-    throughput name (fun cs -> AES.ECB.encrypt ~key cs)) ;
+  bm "aes-128-ccm-unsafe" (fun name ->
+    let key   = AES.CCM16.of_secret (Mirage_crypto_rng.generate 16)
+    and nonce = Mirage_crypto_rng.generate 10 in
+    throughput_into ~add:AES.CCM16.tag_size name
+      (fun dst cs -> AES.CCM16.unsafe_authenticate_encrypt_into ~key ~nonce cs ~src_off:0 dst ~dst_off:0 ~tag_off:(String.length cs) (String.length cs)));
 
   bm "d3des-ecb" (fun name ->
     let key = DES.ECB.of_secret (Mirage_crypto_rng.generate 24) in
-    throughput name (fun cs -> DES.ECB.encrypt ~key cs)) ;
+    throughput_into name (fun dst cs -> DES.ECB.encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
+
+  bm "d3des-ecb-unsafe" (fun name ->
+    let key = DES.ECB.of_secret (Mirage_crypto_rng.generate 24) in
+    throughput_into name (fun dst cs -> DES.ECB.unsafe_encrypt_into ~key cs ~src_off:0 dst ~dst_off:0 (String.length cs))) ;
 
   bm "fortuna" (fun name ->
     let open Mirage_crypto_rng.Fortuna in
diff --git a/src/aead.ml b/src/aead.ml
index a03214e1..30b716a1 100644
--- a/src/aead.ml
+++ b/src/aead.ml
@@ -10,4 +10,16 @@ module type AEAD = sig
     string -> string * string
   val authenticate_decrypt_tag : key:key -> nonce:string -> ?adata:string ->
     tag:string -> string -> string option
+  val authenticate_encrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> bytes -> dst_off:int ->
+    tag_off:int -> int -> unit
+  val authenticate_decrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> tag_off:int -> bytes ->
+    dst_off:int -> int -> bool
+  val unsafe_authenticate_encrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> bytes -> dst_off:int ->
+    tag_off:int -> int -> unit
+  val unsafe_authenticate_decrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> tag_off:int -> bytes ->
+    dst_off:int -> int -> bool
 end
diff --git a/src/ccm.ml b/src/ccm.ml
index 8b68b7f3..ecee28ec 100644
--- a/src/ccm.ml
+++ b/src/ccm.ml
@@ -10,7 +10,7 @@ let encode_len buf ~off size value =
     | 0 -> Bytes.set_uint8 buf off num
     | m ->
       Bytes.set_uint8 buf (off + m) (num land 0xff);
-      ass (num lsr 8) (pred m)
+      (ass [@tailcall]) (num lsr 8) (pred m)
   in
   ass value (pred size)
 
@@ -74,10 +74,8 @@ let prepare_header nonce adata plen tlen =
 
 type mode = Encrypt | Decrypt
 
-let crypto_core ~cipher ~mode ~key ~nonce ~maclen ~adata data =
-  let datalen = String.length data in
-  let cbcheader = prepare_header nonce adata datalen maclen in
-  let dst = Bytes.create datalen in
+let crypto_core_into ~cipher ~mode ~key ~nonce ~adata src ~src_off dst ~dst_off len =
+  let cbcheader = prepare_header nonce adata len block_size in
 
   let small_q = 15 - String.length nonce in
   let ctr_flag_val = flags 0 0 (small_q - 1) in
@@ -93,65 +91,62 @@ let crypto_core ~cipher ~mode ~key ~nonce ~maclen ~adata data =
     cipher ~key (Bytes.unsafe_to_string block) ~src_off:dst_off block ~dst_off
   in
 
-  let cbcprep =
+  let iv =
     let rec doit iv iv_off block block_off =
       match Bytes.length block - block_off with
       | 0 -> Bytes.sub iv iv_off block_size
       | _ ->
          cbc (Bytes.unsafe_to_string iv) iv_off block block_off;
-         doit block block_off block (block_off + block_size)
+         (doit [@tailcall]) block block_off block (block_off + block_size)
     in
     doit (Bytes.make block_size '\x00') 0 cbcheader 0
   in
 
-  let rec loop iv ctr src src_off dst dst_off=
+  let rec loop ctr src src_off dst dst_off len =
     let cbcblock, cbc_off =
       match mode with
       | Encrypt -> src, src_off
       | Decrypt -> Bytes.unsafe_to_string dst, dst_off
     in
-    match String.length src - src_off with
-    | 0 -> iv
-    | x when x < block_size ->
+    if len = 0 then
+      ()
+    else if len < block_size then begin
       let buf = Bytes.make block_size '\x00' in
-      Bytes.unsafe_blit dst dst_off buf 0 x;
+      Bytes.unsafe_blit dst dst_off buf 0 len ;
       ctrblock ctr buf ;
-      Bytes.unsafe_blit buf 0 dst dst_off x ;
-      unsafe_xor_into src ~src_off dst ~dst_off x ;
-      Bytes.unsafe_blit_string cbcblock cbc_off buf 0 x;
-      Bytes.unsafe_fill buf x (block_size - x) '\x00';
-      cbc (Bytes.unsafe_to_string buf) cbc_off iv 0 ;
-      iv
-    | _ ->
+      Bytes.unsafe_blit buf 0 dst dst_off len ;
+      unsafe_xor_into src ~src_off dst ~dst_off len ;
+      Bytes.unsafe_blit_string cbcblock cbc_off buf 0 len ;
+      Bytes.unsafe_fill buf len (block_size - len) '\x00';
+      cbc (Bytes.unsafe_to_string buf) cbc_off iv 0
+    end else begin
       ctrblock ctr dst ;
       unsafe_xor_into src ~src_off dst ~dst_off block_size ;
       cbc cbcblock cbc_off iv 0 ;
-      loop iv (succ ctr) src (src_off + block_size) dst (dst_off + block_size)
+      (loop [@tailcall]) (succ ctr) src (src_off + block_size) dst (dst_off + block_size) (len - block_size)
+    end
   in
-  let last = loop cbcprep 1 data 0 dst 0 in
-  let t = Bytes.sub last 0 maclen in
-  (dst, t)
+  loop 1 src src_off dst dst_off len;
+  iv
+
+let crypto_core ~cipher ~mode ~key ~nonce ~adata data =
+  let datalen = String.length data in
+  let dst = Bytes.create datalen in
+  let t = crypto_core_into ~cipher ~mode ~key ~nonce ~adata data ~src_off:0 dst ~dst_off:0 datalen in
+  dst, t
 
 let crypto_t t nonce cipher key =
   let ctr = gen_ctr nonce 0 in
   cipher ~key (Bytes.unsafe_to_string ctr) ~src_off:0 ctr ~dst_off:0 ;
   unsafe_xor_into (Bytes.unsafe_to_string ctr) ~src_off:0 t ~dst_off:0 (Bytes.length t)
 
-let valid_nonce nonce =
-  let nsize = String.length nonce in
-  if nsize < 7 || nsize > 13 then
-    invalid_arg "CCM: nonce length not between 7 and 13: %u" nsize
-
-let generation_encryption ~cipher ~key ~nonce ~maclen ~adata data =
-  valid_nonce nonce;
-  let cdata, t = crypto_core ~cipher ~mode:Encrypt ~key ~nonce ~maclen ~adata data in
+let unsafe_generation_encryption_into ~cipher ~key ~nonce ~adata src ~src_off dst ~dst_off ~tag_off len =
+  let t = crypto_core_into ~cipher ~mode:Encrypt ~key ~nonce ~adata src ~src_off dst ~dst_off len in
   crypto_t t nonce cipher key ;
-  Bytes.unsafe_to_string cdata, Bytes.unsafe_to_string t
+  Bytes.unsafe_blit t 0 dst tag_off block_size
 
-let decryption_verification ~cipher ~key ~nonce ~maclen ~adata ~tag data =
-  valid_nonce nonce;
-  let cdata, t = crypto_core ~cipher ~mode:Decrypt ~key ~nonce ~maclen ~adata data in
+let unsafe_decryption_verification_into ~cipher ~key ~nonce ~adata src ~src_off ~tag_off dst ~dst_off len =
+  let tag = String.sub src tag_off block_size in
+  let t = crypto_core_into ~cipher ~mode:Decrypt ~key ~nonce ~adata src ~src_off dst ~dst_off len in
   crypto_t t nonce cipher key ;
-  match Eqaf.equal tag (Bytes.unsafe_to_string t) with
-  | true  -> Some (Bytes.unsafe_to_string cdata)
-  | false -> None
+  Eqaf.equal tag (Bytes.unsafe_to_string t)
diff --git a/src/chacha20.ml b/src/chacha20.ml
index f0d97840..2c70251d 100644
--- a/src/chacha20.ml
+++ b/src/chacha20.ml
@@ -42,77 +42,121 @@ let init ctr ~key ~nonce =
   Bytes.unsafe_blit_string nonce 0 state nonce_off (String.length nonce) ;
   state, inc
 
-let crypt ~key ~nonce ?(ctr = 0L) data =
+let crypt_into ~key ~nonce ~ctr src ~src_off dst ~dst_off len =
   let state, inc = init ctr ~key ~nonce in
-  let l = String.length data in
-  let block_count = l // block in
+  let block_count = len // block in
   let last_len =
-    let last = l mod block in
+    let last = len mod block in
     if last = 0 then block else last
   in
-  let res = Bytes.create l in
   let rec loop i = function
     | 0 -> ()
     | 1 ->
       if last_len = block then begin
-        chacha20_block state i res ;
-        Native.xor_into_bytes data i res i block
+        chacha20_block state (dst_off + i) dst ;
+        Native.xor_into_bytes src (src_off + i) dst (dst_off + i) block
       end else begin
         let buf = Bytes.create block in
         chacha20_block state 0 buf ;
-        Native.xor_into_bytes data i buf 0 last_len ;
-        Bytes.unsafe_blit buf 0 res i last_len
+        Native.xor_into_bytes src (src_off + i) buf 0 last_len ;
+        Bytes.unsafe_blit buf 0 dst (dst_off + i) last_len
       end
     | n ->
-      chacha20_block state i res ;
-      Native.xor_into_bytes data i res i block ;
+      chacha20_block state (dst_off + i) dst ;
+      Native.xor_into_bytes src (src_off + i) dst (dst_off + i) block ;
       inc state;
-      loop (i + block) (n - 1)
+      (loop [@tailcall]) (i + block) (n - 1)
   in
-  loop 0 block_count ;
+  loop 0 block_count
+
+let crypt ~key ~nonce ?(ctr = 0L) data =
+  let l = String.length data in
+  let res = Bytes.create l in
+  crypt_into ~key ~nonce ~ctr data ~src_off:0 res ~dst_off:0 l;
   Bytes.unsafe_to_string res
 
 module P = Poly1305.It
 
+let tag_size = P.mac_size
+
 let generate_poly1305_key ~key ~nonce =
   crypt ~key ~nonce (String.make 32 '\000')
 
-let mac ~key ~adata ciphertext =
-  let pad16 b =
-    let len = String.length b mod 16 in
+let mac_into ~key ~adata src ~src_off len dst ~dst_off =
+  let pad16 l =
+    let len = l mod 16 in
     if len = 0 then "" else String.make (16 - len) '\000'
-  and len =
+  and len_buf =
     let data = Bytes.create 16 in
     Bytes.set_int64_le data 0 (Int64.of_int (String.length adata));
-    Bytes.set_int64_le data 8 (Int64.of_int (String.length ciphertext));
+    Bytes.set_int64_le data 8 (Int64.of_int len);
     Bytes.unsafe_to_string data
   in
-  P.macl ~key [ adata ; pad16 adata ; ciphertext ; pad16 ciphertext ; len ]
+  let p1 = pad16 (String.length adata) and p2 = pad16 len in
+  P.unsafe_mac_into ~key [ adata, 0, String.length adata ;
+                           p1, 0, String.length p1 ;
+                           src, src_off, len ;
+                           p2, 0, String.length p2 ;
+                           len_buf, 0, String.length len_buf ]
+    dst ~dst_off
 
-let authenticate_encrypt_tag ~key ~nonce ?(adata = "") data =
+let unsafe_authenticate_encrypt_into ~key ~nonce ?(adata = "") src ~src_off dst ~dst_off ~tag_off len =
   let poly1305_key = generate_poly1305_key ~key ~nonce in
-  let ciphertext = crypt ~key ~nonce ~ctr:1L data in
-  let mac = mac ~key:poly1305_key ~adata ciphertext in
-  ciphertext, mac
+  crypt_into ~key ~nonce ~ctr:1L src ~src_off dst ~dst_off len;
+  mac_into ~key:poly1305_key ~adata (Bytes.unsafe_to_string dst) ~src_off:dst_off len dst ~dst_off:tag_off
+
+let authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len =
+  if String.length src - src_off < len then
+    invalid_arg "Chacha20: src length %u - src_off %u < len %u"
+      (String.length src) src_off len;
+  if Bytes.length dst - dst_off < len then
+    invalid_arg "Chacha20: dst length %u - dst_off %u < len %u"
+      (Bytes.length dst) dst_off len;
+  if Bytes.length dst - tag_off < tag_size then
+    invalid_arg "Chacha20: dst length %u - tag_off %u < tag_size %u"
+      (Bytes.length dst) tag_off tag_size;
+  unsafe_authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len
 
 let authenticate_encrypt ~key ~nonce ?adata data =
-  let cdata, ctag = authenticate_encrypt_tag ~key ~nonce ?adata data in
-  cdata ^ ctag
+  let l = String.length data in
+  let dst = Bytes.create (l + tag_size) in
+  unsafe_authenticate_encrypt_into ~key ~nonce ?adata data ~src_off:0 dst ~dst_off:0 ~tag_off:l l;
+  Bytes.unsafe_to_string dst
 
-let authenticate_decrypt_tag ~key ~nonce ?(adata = "") ~tag data =
+let authenticate_encrypt_tag ~key ~nonce ?adata data =
+  let r = authenticate_encrypt ~key ~nonce ?adata data in
+  String.sub r 0 (String.length data), String.sub r (String.length data) tag_size
+
+let unsafe_authenticate_decrypt_into ~key ~nonce ?(adata = "") src ~src_off ~tag_off dst ~dst_off len =
   let poly1305_key = generate_poly1305_key ~key ~nonce in
-  let ctag = mac ~key:poly1305_key ~adata data in
-  let plain = crypt ~key ~nonce ~ctr:1L data in
-  if Eqaf.equal tag ctag then Some plain else None
+  let ctag = Bytes.create tag_size in
+  mac_into ~key:poly1305_key ~adata src ~src_off len ctag ~dst_off:0;
+  crypt_into ~key ~nonce ~ctr:1L src ~src_off dst ~dst_off len;
+  Eqaf.equal (String.sub src tag_off tag_size) (Bytes.unsafe_to_string ctag)
+
+let authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len =
+  if String.length src - src_off < len then
+    invalid_arg "Chacha20: src length %u - src_off %u < len %u"
+      (String.length src) src_off len;
+  if Bytes.length dst - dst_off < len then
+    invalid_arg "Chacha20: dst length %u - dst_off %u < len %u"
+      (Bytes.length dst) dst_off len;
+  if String.length src - tag_off < tag_size then
+    invalid_arg "Chacha20: src length %u - tag_off %u < tag_size %u"
+      (String.length src) tag_off tag_size;
+  unsafe_authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len
 
 let authenticate_decrypt ~key ~nonce ?adata data =
-  if String.length data < P.mac_size then
+  if String.length data < tag_size then
     None
   else
-    let cipher, tag =
-      let p = String.length data - P.mac_size in
-      String.sub data 0 p, String.sub data p P.mac_size
-    in
-    authenticate_decrypt_tag ~key ~nonce ?adata ~tag cipher
+    let l = String.length data - tag_size in
+    let r = Bytes.create l in
+    if unsafe_authenticate_decrypt_into ~key ~nonce ?adata data ~src_off:0 ~tag_off:l r ~dst_off:0 l then
+      Some (Bytes.unsafe_to_string r)
+    else
+      None
 
-let tag_size = P.mac_size
+let authenticate_decrypt_tag ~key ~nonce ?adata ~tag data =
+  let cdata = data ^ tag in
+  authenticate_decrypt ~key ~nonce ?adata cdata
diff --git a/src/cipher_block.ml b/src/cipher_block.ml
index 3dfa1fcb..d430492f 100644
--- a/src/cipher_block.ml
+++ b/src/cipher_block.ml
@@ -28,6 +28,10 @@ module Block = struct
     val block_size : int
     val encrypt : key:key -> string -> string
     val decrypt : key:key -> string -> string
+    val encrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    val decrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    val unsafe_encrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    val unsafe_decrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
   end
 
   module type CBC = sig
@@ -40,7 +44,19 @@ module Block = struct
 
     val encrypt : key:key -> iv:string -> string -> string
     val decrypt : key:key -> iv:string -> string -> string
-    val next_iv : iv:string -> string -> string
+    val next_iv : ?off:int -> string -> iv:string -> string
+
+    val encrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    val decrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+
+    val unsafe_encrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    val unsafe_decrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    val unsafe_encrypt_into_inplace : key:key -> iv:string ->
+      bytes -> dst_off:int -> int -> unit
   end
 
   module type CTR = sig
@@ -48,18 +64,29 @@ module Block = struct
     type key
     val of_secret : string -> key
 
-    type ctr
-
     val key_sizes  : int array
     val block_size : int
 
+    type ctr
+    val add_ctr        : ctr -> int64 -> ctr
+    val next_ctr       : ?off:int -> string -> ctr:ctr -> ctr
+    val ctr_of_octets  : string -> ctr
+
     val stream  : key:key -> ctr:ctr -> int -> string
     val encrypt : key:key -> ctr:ctr -> string -> string
     val decrypt : key:key -> ctr:ctr -> string -> string
 
-    val add_ctr        : ctr -> int64 -> ctr
-    val next_ctr       : ctr:ctr -> string -> ctr
-    val ctr_of_octets  : string -> ctr
+    val stream_into  : key:key -> ctr:ctr -> bytes -> off:int -> int -> unit
+    val encrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    val decrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+
+    val unsafe_stream_into  : key:key -> ctr:ctr -> bytes -> off:int -> int -> unit
+    val unsafe_encrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    val unsafe_decrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
   end
 
   module type GCM = sig
@@ -83,7 +110,7 @@ module Counters = struct
     val size : int
     val add  : ctr -> int64 -> ctr
     val of_octets : string -> ctr
-    val unsafe_count_into : ctr -> bytes -> blocks:int -> unit
+    val unsafe_count_into : ctr -> bytes -> off:int -> blocks:int -> unit
   end
 
   module C64be = struct
@@ -91,10 +118,10 @@ module Counters = struct
     let size = 8
     let of_octets cs = String.get_int64_be cs 0
     let add = Int64.add
-    let unsafe_count_into t buf ~blocks =
-      let tmp = Bytes.create 8 in
-      Bytes.set_int64_be tmp 0 t;
-      Native.count8be tmp buf ~blocks
+    let unsafe_count_into t buf ~off ~blocks =
+      let ctr = Bytes.create 8 in
+      Bytes.set_int64_be ctr 0 t;
+      Native.count8be ~ctr buf ~off ~blocks
   end
 
   module C128be = struct
@@ -107,10 +134,10 @@ module Counters = struct
       let w0'  = Int64.add w0 n in
       let flip = if Int64.logxor w0 w0' < 0L then w0' > w0 else w0' < w0 in
       ((if flip then Int64.succ w1 else w1), w0')
-    let unsafe_count_into (w1, w0) buf ~blocks =
-      let tmp = Bytes.create 16 in
-      Bytes.set_int64_be tmp 0 w1; Bytes.set_int64_be tmp 8 w0;
-      Native.count16be tmp buf ~blocks
+    let unsafe_count_into (w1, w0) buf ~off ~blocks =
+      let ctr = Bytes.create 16 in
+      Bytes.set_int64_be ctr 0 w1; Bytes.set_int64_be ctr 8 w0;
+      Native.count16be ~ctr buf ~off ~blocks
   end
 
   module C128be32 = struct
@@ -118,13 +145,22 @@ module Counters = struct
     let add (w1, w0) n =
       let hi = 0xffffffff00000000L and lo = 0x00000000ffffffffL in
       (w1, Int64.(logor (logand hi w0) (add n w0 |> logand lo)))
-    let unsafe_count_into (w1, w0) buf ~blocks =
-      let tmp = Bytes.create 16 in
-      Bytes.set_int64_be tmp 0 w1; Bytes.set_int64_be tmp 8 w0;
-      Native.count16be4 tmp buf ~blocks
+    let unsafe_count_into (w1, w0) buf ~off ~blocks =
+      let ctr = Bytes.create 16 in
+      Bytes.set_int64_be ctr 0 w1; Bytes.set_int64_be ctr 8 w0;
+      Native.count16be4 ~ctr buf ~off ~blocks
   end
 end
 
+let check_offset ~tag ~buf ~off ~len actual_len =
+  if off < 0 then
+    invalid_arg "%s: %s off %u < 0"
+      tag buf off;
+  if actual_len - off < len then
+    invalid_arg "%s: %s length %u - off %u < len %u"
+      tag buf actual_len off len
+[@@inline]
+
 module Modes = struct
   module ECB_of (Core : Block.Core) : Block.ECB = struct
 
@@ -134,17 +170,39 @@ module Modes = struct
 
     let of_secret = Core.of_secret
 
-    let (encrypt, decrypt) =
-      let ecb xform key src =
-        let n = String.length src in
-        if n mod block_size <> 0 then invalid_arg "ECB: length %u" n;
-        let dst = Bytes.create n in
-        xform ~key ~blocks:(n / block_size) src 0 dst 0 ;
-        Bytes.unsafe_to_string dst
-      in
-      (fun ~key:(key, _) src -> ecb Core.encrypt key src),
-      (fun ~key:(_, key) src -> ecb Core.decrypt key src)
+    let unsafe_ecb xform key src src_off dst dst_off len =
+      xform ~key ~blocks:(len / block_size) src src_off dst dst_off
+
+    let ecb xform key src src_off dst dst_off len =
+      if len mod block_size <> 0 then
+        invalid_arg "ECB: length %u not of block size" len;
+      check_offset ~tag:"ECB" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"ECB" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      unsafe_ecb xform key src src_off dst dst_off len
+
+    let encrypt_into ~key:(key, _) src ~src_off dst ~dst_off len =
+      ecb Core.encrypt key src src_off dst dst_off len
 
+    let unsafe_encrypt_into ~key:(key, _) src ~src_off dst ~dst_off len =
+      unsafe_ecb Core.encrypt key src src_off dst dst_off len
+
+    let decrypt_into ~key:(_, key) src ~src_off dst ~dst_off len =
+      ecb Core.decrypt key src src_off dst dst_off len
+
+    let unsafe_decrypt_into ~key:(_, key) src ~src_off dst ~dst_off len =
+      unsafe_ecb Core.decrypt key src src_off dst dst_off len
+
+    let encrypt ~key src =
+      let len = String.length src in
+      let dst = Bytes.create len in
+      encrypt_into ~key src ~src_off:0 dst ~dst_off:0 len;
+      Bytes.unsafe_to_string dst
+
+    let decrypt ~key src =
+      let len = String.length src in
+      let dst = Bytes.create len in
+      decrypt_into ~key src ~src_off:0 dst ~dst_off:0 len;
+      Bytes.unsafe_to_string dst
   end
 
   module CBC_of (Core : Block.Core) : Block.CBC = struct
@@ -156,40 +214,64 @@ module Modes = struct
 
     let of_secret = Core.of_secret
 
-    let bounds_check ~iv cs =
-      if String.length iv <> block then invalid_arg "CBC: IV length %u" (String.length iv);
-      if String.length cs mod block <> 0 then
-        invalid_arg "CBC: argument length %u" (String.length cs)
-
-    let next_iv ~iv cs =
-      bounds_check ~iv cs ;
-      if String.length cs > 0 then
+    let check_block_size ~iv len =
+      if String.length iv <> block then
+        invalid_arg "CBC: IV length %u not of block size" (String.length iv);
+      if len mod block <> 0 then
+        invalid_arg "CBC: argument length %u not of block size"
+          len
+    [@@inline]
+
+    let next_iv ?(off = 0) cs ~iv =
+      check_block_size ~iv (String.length cs - off) ;
+      if String.length cs > off then
         String.sub cs (String.length cs - block_size) block_size
       else iv
 
-    let encrypt ~key:(key, _) ~iv src =
-      bounds_check ~iv src ;
-      let dst = Bytes.of_string src in
+    let unsafe_encrypt_into_inplace ~key:(key, _) ~iv dst ~dst_off len =
       let rec loop iv iv_i dst_i = function
-        0 -> ()
-      | b -> Native.xor_into_bytes iv iv_i dst dst_i block ;
-             Core.encrypt ~key ~blocks:1 (Bytes.unsafe_to_string dst) dst_i dst dst_i ;
-             loop (Bytes.unsafe_to_string dst) dst_i (dst_i + block) (b - 1)
+        | 0 -> ()
+        | b ->
+          Native.xor_into_bytes iv iv_i dst dst_i block ;
+          Core.encrypt ~key ~blocks:1 (Bytes.unsafe_to_string dst) dst_i dst dst_i ;
+          (loop [@tailcall]) (Bytes.unsafe_to_string dst) dst_i (dst_i + block) (b - 1)
       in
-      loop iv 0 0 (Bytes.length dst / block) ;
+      loop iv 0 dst_off (len / block)
+
+    let unsafe_encrypt_into ~key ~iv src ~src_off dst ~dst_off len =
+      Bytes.unsafe_blit_string src src_off dst dst_off len;
+      unsafe_encrypt_into_inplace ~key ~iv dst ~dst_off len
+
+    let encrypt_into ~key ~iv src ~src_off dst ~dst_off len =
+      check_block_size ~iv len;
+      check_offset ~tag:"CBC" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"CBC" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      unsafe_encrypt_into ~key ~iv src ~src_off dst ~dst_off len
+
+    let encrypt ~key ~iv src =
+      let dst = Bytes.create (String.length src) in
+      encrypt_into ~key ~iv src ~src_off:0 dst ~dst_off:0 (String.length src);
       Bytes.unsafe_to_string dst
 
-    let decrypt ~key:(_, key) ~iv src =
-      bounds_check ~iv src ;
-      let msg = Bytes.create (String.length src)
-      and b   = String.length src / block in
+    let unsafe_decrypt_into ~key:(_, key) ~iv src ~src_off dst ~dst_off len =
+      let b = len / block in
       if b > 0 then begin
-        Core.decrypt ~key ~blocks:b src 0 msg 0 ;
-        Native.xor_into_bytes iv 0 msg 0 block ;
-        Native.xor_into_bytes src 0 msg block ((b - 1) * block) ;
-      end ;
+        Core.decrypt ~key ~blocks:b src src_off dst dst_off ;
+        Native.xor_into_bytes iv 0 dst dst_off block ;
+        Native.xor_into_bytes src src_off dst (dst_off + block) ((b - 1) * block) ;
+      end
+
+    let decrypt_into ~key ~iv src ~src_off dst ~dst_off len =
+      check_block_size ~iv len;
+      check_offset ~tag:"CBC" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"CBC" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      unsafe_decrypt_into ~key ~iv src ~src_off dst ~dst_off len
+
+    let decrypt ~key ~iv src =
+      let len = String.length src in
+      let msg = Bytes.create len in
+      decrypt_into ~key ~iv src ~src_off:0 msg ~dst_off:0 len;
       Bytes.unsafe_to_string msg
-
   end
 
   module CTR_of (Core : Block.Core) (Ctr : Counters.S) :
@@ -204,30 +286,52 @@ module Modes = struct
     let (key_sizes, block_size) = Core.(key, block)
     let of_secret = Core.e_of_secret
 
-    let stream ~key ~ctr n =
-      let blocks = imax 0 n / block_size in
-      let buf = Bytes.create n in
-      Ctr.unsafe_count_into ctr ~blocks buf ;
-      Core.encrypt ~key ~blocks (Bytes.unsafe_to_string buf) 0 buf 0 ;
-      let slack = imax 0 n mod block_size in
+    let unsafe_stream_into ~key ~ctr buf ~off len =
+      let blocks = imax 0 len / block_size in
+      Ctr.unsafe_count_into ctr buf ~off ~blocks ;
+      Core.encrypt ~key ~blocks (Bytes.unsafe_to_string buf) off buf off ;
+      let slack = imax 0 len mod block_size in
       if slack <> 0 then begin
         let buf' = Bytes.create block_size in
         let ctr = Ctr.add ctr (Int64.of_int blocks) in
-        Ctr.unsafe_count_into ctr ~blocks:1 buf' ;
+        Ctr.unsafe_count_into ctr buf' ~off:0 ~blocks:1 ;
         Core.encrypt ~key ~blocks:1 (Bytes.unsafe_to_string buf') 0 buf' 0 ;
-        Bytes.unsafe_blit buf' 0 buf (blocks * block_size) slack
-      end;
+        Bytes.unsafe_blit buf' 0 buf (off + blocks * block_size) slack
+      end
+
+    let stream_into ~key ~ctr buf ~off len =
+      check_offset ~tag:"CTR" ~buf:"buf" ~off ~len (Bytes.length buf);
+      unsafe_stream_into ~key ~ctr buf ~off len
+
+    let stream ~key ~ctr n =
+      let buf = Bytes.create n in
+      unsafe_stream_into ~key ~ctr buf ~off:0 n;
       Bytes.unsafe_to_string buf
 
+    let unsafe_encrypt_into ~key ~ctr src ~src_off dst ~dst_off len =
+      unsafe_stream_into ~key ~ctr dst ~off:dst_off len;
+      Uncommon.unsafe_xor_into src ~src_off dst ~dst_off len
+
+    let encrypt_into ~key ~ctr src ~src_off dst ~dst_off len =
+      check_offset ~tag:"CTR" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"CTR" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      unsafe_encrypt_into ~key ~ctr src ~src_off dst ~dst_off len
+
     let encrypt ~key ~ctr src =
-      let res = Bytes.unsafe_of_string (stream ~key ~ctr (String.length src)) in
-      Native.xor_into_bytes src 0 res 0 (String.length src) ;
-      Bytes.unsafe_to_string res
+      let len = String.length src in
+      let dst = Bytes.create len in
+      encrypt_into ~key ~ctr src ~src_off:0 dst ~dst_off:0 len;
+      Bytes.unsafe_to_string dst
 
     let decrypt = encrypt
 
+    let decrypt_into = encrypt_into
+
+    let unsafe_decrypt_into = unsafe_encrypt_into
+
     let add_ctr = Ctr.add
-    let next_ctr ~ctr msg = add_ctr ctr (Int64.of_int @@ String.length msg // block_size)
+    let next_ctr ?(off = 0) msg ~ctr =
+      add_ctr ctr (Int64.of_int @@ (String.length msg - off) // block_size)
     let ctr_of_octets = Ctr.of_octets
   end
 
@@ -235,6 +339,7 @@ module Modes = struct
     type key
     val derive  : string -> key
     val digesti : key:key -> (string Uncommon.iter) -> string
+    val digesti_off_len : key:key -> (string * int * int) Uncommon.iter -> string
     val tagsize : int
   end = struct
     type key = string
@@ -245,15 +350,20 @@ module Modes = struct
       let k = Bytes.create keysize in
       Native.GHASH.keyinit cs k;
       Bytes.unsafe_to_string k
+    let digesti_off_len ~key i =
+      let res = Bytes.make tagsize '\x00' in
+      i (fun (cs, off, len) -> Native.GHASH.ghash key res cs off len);
+      Bytes.unsafe_to_string res
     let digesti ~key i =
       let res = Bytes.make tagsize '\x00' in
-      i (fun cs -> Native.GHASH.ghash key res cs (String.length cs));
+      i (fun cs -> Native.GHASH.ghash key res cs 0 (String.length cs));
       Bytes.unsafe_to_string res
+
   end
 
   module GCM_of (C : Block.Core) : Block.GCM = struct
 
-    let _ = assert (C.block = 16)
+    assert (C.block = 16)
     module CTR = CTR_of (C) (Counters.C128be32)
 
     type key = { key : C.ekey ; hkey : GHASH.key }
@@ -285,43 +395,69 @@ module Modes = struct
         CTR.ctr_of_octets @@
         GHASH.digesti ~key:hkey @@ iter2 nonce (pack64s 0L (bits64 nonce))
 
-    let tag ~key ~hkey ~ctr ?(adata = "") cdata =
-      CTR.encrypt ~key ~ctr @@
-        GHASH.digesti ~key:hkey @@
-          iter3 adata cdata (pack64s (bits64 adata) (bits64 cdata))
+    let unsafe_tag_into ~key ~hkey ~ctr ?(adata = "") cdata ~off ~len dst ~tag_off =
+      CTR.unsafe_encrypt_into ~key ~ctr
+        (GHASH.digesti_off_len ~key:hkey
+           (iter3 (adata, 0, String.length adata) (cdata, off, len)
+              (pack64s (bits64 adata) (Int64.of_int (len * 8)), 0, 16)))
+        ~src_off:0 dst ~dst_off:tag_off tag_size
+
+    let unsafe_authenticate_encrypt_into ~key:{ key; hkey } ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len =
+      let ctr = counter ~hkey nonce in
+      CTR.(unsafe_encrypt_into ~key ~ctr:(add_ctr ctr 1L) src ~src_off dst ~dst_off len);
+      unsafe_tag_into ~key ~hkey ~ctr ?adata (Bytes.unsafe_to_string dst) ~off:dst_off ~len dst ~tag_off
 
-    let authenticate_encrypt_tag ~key:{ key; hkey } ~nonce ?adata data =
-      let ctr   = counter ~hkey nonce in
-      let cdata = CTR.(encrypt ~key ~ctr:(add_ctr ctr 1L) data) in
-      let ctag  = tag ~key ~hkey ~ctr ?adata cdata in
-      cdata, ctag
+    let authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len =
+      check_offset ~tag:"GCM" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"GCM" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      check_offset ~tag:"GCM" ~buf:"dst tag" ~off:tag_off ~len:tag_size (Bytes.length dst);
+      unsafe_authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len
 
     let authenticate_encrypt ~key ~nonce ?adata data =
-      let cdata, ctag = authenticate_encrypt_tag ~key ~nonce ?adata data in
-      cdata ^ ctag
+      let l = String.length data in
+      let dst = Bytes.create (l + tag_size) in
+      unsafe_authenticate_encrypt_into ~key ~nonce ?adata data ~src_off:0 dst ~dst_off:0 ~tag_off:l l;
+      Bytes.unsafe_to_string dst
+
+    let authenticate_encrypt_tag ~key ~nonce ?adata data =
+      let r = authenticate_encrypt ~key ~nonce ?adata data in
+      String.sub r 0 (String.length data),
+      String.sub r (String.length data) tag_size
+
+    let unsafe_authenticate_decrypt_into ~key:{ key; hkey } ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len =
+      let ctr = counter ~hkey nonce in
+      CTR.(unsafe_encrypt_into ~key ~ctr:(add_ctr ctr 1L) src ~src_off dst ~dst_off len);
+      let ctag = Bytes.create tag_size in
+      unsafe_tag_into ~key ~hkey ~ctr ?adata src ~off:src_off ~len ctag ~tag_off:0;
+      Eqaf.equal (String.sub src tag_off tag_size) (Bytes.unsafe_to_string ctag)
 
-    let authenticate_decrypt_tag ~key:{ key; hkey } ~nonce ?adata ~tag:tag_data cipher =
-      let ctr  = counter ~hkey nonce in
-      let data = CTR.(encrypt ~key ~ctr:(add_ctr ctr 1L) cipher) in
-      let ctag = tag ~key ~hkey ~ctr ?adata cipher in
-      if Eqaf.equal tag_data ctag then Some data else None
+    let authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len =
+      check_offset ~tag:"GCM" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"GCM" ~buf:"src tag" ~off:tag_off ~len:tag_size (String.length src);
+      check_offset ~tag:"GCM" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      unsafe_authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len
 
     let authenticate_decrypt ~key ~nonce ?adata cdata =
       if String.length cdata < tag_size then
         None
       else
-        let cipher, tag =
-          String.sub cdata 0 (String.length cdata - tag_size),
-          String.sub cdata (String.length cdata - tag_size) tag_size
-        in
-        authenticate_decrypt_tag ~key ~nonce ?adata ~tag cipher
+        let l = String.length cdata - tag_size in
+        let data = Bytes.create l in
+        if unsafe_authenticate_decrypt_into ~key ~nonce ?adata cdata ~src_off:0 ~tag_off:l data ~dst_off:0 l then
+          Some (Bytes.unsafe_to_string data)
+        else
+          None
+
+    let authenticate_decrypt_tag ~key ~nonce ?adata ~tag:tag_data cipher =
+      let cdata = cipher ^ tag_data in
+      authenticate_decrypt ~key ~nonce ?adata cdata
   end
 
   module CCM16_of (C : Block.Core) : Block.CCM16 = struct
 
-    let _ = assert (C.block = 16)
+    assert (C.block = 16)
 
-    let tag_size = 16
+    let tag_size = C.block
 
     type key = C.ekey
 
@@ -330,29 +466,58 @@ module Modes = struct
     let (key_sizes, block_size) = C.(key, block)
 
     let cipher ~key src ~src_off dst ~dst_off =
-      if String.length src - src_off < block_size || Bytes.length dst - dst_off < block_size then
-        invalid_arg "src len %u, dst len %u" (String.length src - src_off) (Bytes.length dst - dst_off);
       C.encrypt ~key ~blocks:1 src src_off dst dst_off
 
-    let authenticate_encrypt_tag ~key ~nonce ?(adata = "") cs =
-      Ccm.generation_encryption ~cipher ~key ~nonce ~maclen:tag_size ~adata cs
+    let unsafe_authenticate_encrypt_into ~key ~nonce ?(adata = "") src ~src_off dst ~dst_off ~tag_off len =
+      Ccm.unsafe_generation_encryption_into ~cipher ~key ~nonce ~adata
+        src ~src_off dst ~dst_off ~tag_off len
+
+    let valid_nonce nonce =
+      let nsize = String.length nonce in
+      if nsize < 7 || nsize > 13 then
+        invalid_arg "CCM: nonce length not between 7 and 13: %u" nsize
+
+    let authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len =
+      check_offset ~tag:"CCM" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"CCM" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      check_offset ~tag:"CCM" ~buf:"dst tag" ~off:tag_off ~len:tag_size (Bytes.length dst);
+      valid_nonce nonce;
+      unsafe_authenticate_encrypt_into ~key ~nonce ?adata src ~src_off dst ~dst_off ~tag_off len
 
     let authenticate_encrypt ~key ~nonce ?adata cs =
-      let cdata, ctag = authenticate_encrypt_tag ~key ~nonce ?adata cs in
-      cdata ^ ctag
+      valid_nonce nonce;
+      let l = String.length cs in
+      let dst = Bytes.create (l + tag_size) in
+      unsafe_authenticate_encrypt_into ~key ~nonce ?adata cs ~src_off:0 dst ~dst_off:0 ~tag_off:l l;
+      Bytes.unsafe_to_string dst
+
+    let authenticate_encrypt_tag ~key ~nonce ?adata cs =
+      let res = authenticate_encrypt ~key ~nonce ?adata cs in
+      String.sub res 0 (String.length cs), String.sub res (String.length cs) tag_size
+
+    let unsafe_authenticate_decrypt_into ~key ~nonce ?(adata = "") src ~src_off ~tag_off dst ~dst_off len =
+      Ccm.unsafe_decryption_verification_into ~cipher ~key ~nonce ~adata src ~src_off ~tag_off dst ~dst_off len
 
-    let authenticate_decrypt_tag ~key ~nonce ?(adata = "") ~tag cs =
-      Ccm.decryption_verification ~cipher ~key ~nonce ~maclen:tag_size ~adata ~tag cs
+    let authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len =
+      check_offset ~tag:"CCM" ~buf:"src" ~off:src_off ~len (String.length src);
+      check_offset ~tag:"CCM" ~buf:"src tag" ~off:tag_off ~len:tag_size (String.length src);
+      check_offset ~tag:"CCM" ~buf:"dst" ~off:dst_off ~len (Bytes.length dst);
+      valid_nonce nonce;
+      unsafe_authenticate_decrypt_into ~key ~nonce ?adata src ~src_off ~tag_off dst ~dst_off len
 
     let authenticate_decrypt ~key ~nonce ?adata data =
       if String.length data < tag_size then
         None
       else
-        let data, tag =
-          String.sub data 0 (String.length data - tag_size),
-          String.sub data (String.length data - tag_size) tag_size
-        in
-        authenticate_decrypt_tag ~key ~nonce ?adata ~tag data
+        let dlen = String.length data - tag_size in
+        let dst = Bytes.create dlen in
+        if authenticate_decrypt_into ~key ~nonce ?adata data ~src_off:0 ~tag_off:dlen dst ~dst_off:0 dlen then
+          Some (Bytes.unsafe_to_string dst)
+        else
+          None
+
+    let authenticate_decrypt_tag ~key ~nonce ?adata ~tag cs =
+      authenticate_decrypt ~key ~nonce ?adata (cs ^ tag)
   end
 end
 
diff --git a/src/cipher_stream.ml b/src/cipher_stream.ml
index 67ee0a63..69bbae3f 100644
--- a/src/cipher_stream.ml
+++ b/src/cipher_stream.ml
@@ -26,7 +26,7 @@ module ARC4 = struct
           let j = (j + si + x) land 0xff in
           let sj = s.(j) in
           s.(i) <- sj ; s.(j) <- si ;
-          loop j (succ i)
+          (loop [@tailcall]) j (succ i)
     in
     ( loop 0 0 ; (0, 0, s) )
 
@@ -44,7 +44,7 @@ module ARC4 = struct
           s.(i) <- sj ; s.(j) <- si ;
           let k  = s.((si + sj) land 0xff) in
           Bytes.set_uint8 res n (k lxor String.get_uint8 buf n);
-          mix i j (succ n)
+          (mix [@tailcall]) i j (succ n)
     in
     let key' = mix i j 0 in
     { key = key' ; message = Bytes.unsafe_to_string res }
diff --git a/src/mirage_crypto.mli b/src/mirage_crypto.mli
index d33b8420..d85797e6 100644
--- a/src/mirage_crypto.mli
+++ b/src/mirage_crypto.mli
@@ -74,8 +74,13 @@ module Poly1305 : sig
   (** [maci ~key iter] is the all-in-one mac computation:
       [get (feedi (empty ~key) iter)]. *)
 
-  val macl : key:string -> string list -> string
-  (** [macl ~key datas] computes the [mac] of [datas]. *)
+  val mac_into : key:string -> (string * int * int) list -> bytes -> dst_off:int -> unit
+  (** [mac_into ~key datas dst dst_off] computes the [mac] of [datas]. *)
+
+  (**/**)
+  val unsafe_mac_into : key:string -> (string * int * int) list -> bytes -> dst_off:int -> unit
+  (** [unsafe_mac_into ~key datas dst dst_off] is {!mac_into} without bounds checks. *)
+  (**/**)
 end
 
 (** {1 Symmetric-key cryptography} *)
@@ -141,6 +146,65 @@ module type AEAD = sig
       returned.
 
       @raise Invalid_argument if [nonce] is not of the right size. *)
+
+  (** {1 Authenticated encryption and decryption into existing buffers} *)
+
+  val authenticate_encrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> bytes -> dst_off:int ->
+    tag_off:int -> int -> unit
+  (** [authenticate_encrypt_into ~key ~nonce ~adata msg ~src_off dst ~dst_off ~tag_off len]
+      encrypts [len] bytes of [msg] starting at [src_off] with [key] and [nonce]. The output
+      is put into [dst] at [dst_off], the tag into [dst] at [tag_off].
+
+      @raise Invalid_argument if [nonce] is not of the right size.
+      @raise Invalid_argument if [String.length msg - src_off < len].
+      @raise Invalid_argument if [Bytes.length dst - dst_off < len].
+      @raise Invalid_argument if [Bytes.length dst - tag_off < tag_size].
+  *)
+
+  val authenticate_decrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> tag_off:int -> bytes ->
+    dst_off:int -> int -> bool
+  (** [authenticate_decrypt_into ~key ~nonce ~adata msg ~src_off ~tag_off dst ~dst_off len]
+      computes the authentication tag using [key], [nonce], and [adata], and
+      decrypts the [len] bytes encrypted data from [msg] starting at [src_off] into [dst]
+      starting at [dst_off]. If the authentication tags match, [true] is
+      returned, and the decrypted data is in [dst].
+
+      @raise Invalid_argument if [nonce] is not of the right size.
+      @raise Invalid_argument if [String.length msg - src_off < len].
+      @raise Invalid_argument if [Bytes.length dst - dst_off < len].
+      @raise Invalid_argument if [String.length msg - tag_off < tag_size]. *)
+
+  (**/**)
+  val unsafe_authenticate_encrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> bytes -> dst_off:int ->
+    tag_off:int -> int -> unit
+  (** [unsafe_authenticate_encrypt_into] is {!authenticate_encrypt_into}, but
+      without bounds checks.
+
+      @raise Invalid_argument if [nonce] is not of the right size.
+
+      This may cause memory issues if an invariant is violated:
+      {ul
+      {- [String.length msg - src_off >= len].}
+      {- [Bytes.length dst - dst_off >= len].}
+      {- [Bytes.length dst - tag_off >= tag_size].}} *)
+
+  val unsafe_authenticate_decrypt_into : key:key -> nonce:string ->
+    ?adata:string -> string -> src_off:int -> tag_off:int -> bytes ->
+    dst_off:int -> int -> bool
+  (** [unsafe_authenticate_decrypt_into] is {!authenticate_decrypt_into}, but
+      without bounds checks.
+
+      @raise Invalid_argument if [nonce] is not of the right size.
+
+      This may cause memory issues if an invariant is violated:
+      {ul
+      {- [String.length msg - src_off >= len].}
+      {- [Bytes.length dst - dst_off >= len].}
+      {- [String.length msg - tag_off >= tag_size].}} *)
+  (**/**)
 end
 
 (** Block ciphers.
@@ -157,12 +221,68 @@ module Block : sig
   module type ECB = sig
 
     type key
+
     val of_secret : string -> key
+    (** Construct the encryption key corresponding to [secret].
+
+        @raise Invalid_argument if the length of [secret] is not in
+        {{!key_sizes}[key_sizes]}. *)
 
     val key_sizes  : int array
+    (** Key sizes allowed with this cipher. *)
+
     val block_size : int
+    (** The size of a single block. *)
+
     val encrypt : key:key -> string -> string
+    (** [encrypt ~key src] encrypts [src] into a freshly allocated buffer of the
+        same size using [key].
+
+        @raise Invalid_argument if the length of [src] is not a multiple of
+        {!block_size}. *)
+
     val decrypt : key:key -> string -> string
+    (** [decrypt ~key src] decrypts [src] into a freshly allocated buffer of the
+        same size using [key].
+
+        @raise Invalid_argument if the length of [src] is not a multiple of
+        {!block_size}. *)
+
+    val encrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    (** [encrypt_into ~key src ~src_off dst dst_off len] encrypts [len] octets
+        from [src] starting at [src_off] into [dst] starting at [dst_off].
+
+        @raise Invalid_argument if [len] is not a multiple of {!block_size}.
+        @raise Invalid_argument if [src_off < 0 || String.length src - src_off < len].
+        @raise Invalid_argument if [dst_off < 0 || Bytes.length dst - dst_off < len]. *)
+
+    val decrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    (** [decrypt_into ~key src ~src_off dst dst_off len] decrypts [len] octets
+        from [src] starting at [src_off] into [dst] starting at [dst_off].
+
+        @raise Invalid_argument if [len] is not a multiple of {!block_size}.
+        @raise Invalid_argument if [src_off < 0 || String.length src - src_off < len].
+        @raise Invalid_argument if [dst_off < 0 || Bytes.length dst - dst_off < len]. *)
+
+    (**/**)
+    val unsafe_encrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    (** [unsafe_encrypt_into] is {!encrypt_into}, but without bounds checks.
+
+        This may cause memory issues if an invariant is violated:
+        {ul
+        {- [len] must be a multiple of {!block_size},}
+        {- [src_off >= 0 && String.length src - src_off >= len],}
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len].}} *)
+
+    val unsafe_decrypt_into : key:key -> string -> src_off:int -> bytes -> dst_off:int -> int -> unit
+    (** [unsafe_decrypt_into] is {!decrypt_into}, but without bounds checks.
+
+        This may cause memory issues if an invariant is violated:
+        {ul
+        {- [len] must be a multiple of {!block_size},}
+        {- [src_off >= 0 && String.length src - src_off >= len],}
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len].}} *)
+    (**/**)
   end
 
   (** {e Cipher-block chaining} mode. *)
@@ -195,22 +315,79 @@ module Block : sig
         @raise Invalid_argument if [iv] is not [block_size], or [msg] is not
         [k * block_size] long. *)
 
-    val next_iv : iv:string -> string -> string
-    (** [next_iv ~iv ciphertext] is the first [iv] {e following} the
+    val next_iv : ?off:int -> string -> iv:string -> string
+    (** [next_iv ~iv ciphertext ~off] is the first [iv] {e following} the
         encryption that used [iv] to produce [ciphertext].
 
         For protocols which perform inter-message chaining, this is the [iv]
         for the next message.
 
-        It is either [iv], when [len ciphertext = 0], or the last block of
-        [ciphertext]. Note that
+        It is either [iv], when [String.length ciphertext - off = 0], or the
+        last block of [ciphertext]. Note that
 
 {[encrypt ~iv msg1 || encrypt ~iv:(next_iv ~iv (encrypt ~iv msg1)) msg2
   == encrypt ~iv (msg1 || msg2)]}
 
-        @raise Invalid_argument if the length of [iv] is not [block_size], or
-        the length of [ciphertext] is not [k * block_size] for some [k]. *)
-  end
+        @raise Invalid_argument if the length of [iv] is not [block_size].
+        @raise Invalid_argument if the length of [ciphertext] is not a multiple
+        of [block_size]. *)
+
+    val encrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [encrypt_into ~key ~iv src ~src_off dst dst_off len] encrypts [len]
+        octets from [src] starting at [src_off] into [dst] starting at [dst_off].
+
+        @raise Invalid_argument if the length of [iv] is not {!block_size}.
+        @raise Invalid_argument if [len] is not a multiple of {!block_size}.
+        @raise Invalid_argument if [src_off < 0 || String.length src - src_off < len].
+        @raise Invalid_argument if [dst_off < 0 || Bytes.length dst - dst_off < len]. *)
+
+    val decrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [decrypt_into ~key ~iv src ~src_off dst dst_off len] decrypts [len]
+        octets from [src] starting at [src_off] into [dst] starting at [dst_off].
+
+        @raise Invalid_argument if the length of [iv] is not {!block_size}.
+        @raise Invalid_argument if [len] is not a multiple of {!block_size}.
+        @raise Invalid_argument if [src_off < 0 || String.length src - src_off < len].
+        @raise Invalid_argument if [dst_off < 0 || Bytes.length dst - dst_off < len]. *)
+
+    (**/**)
+    val unsafe_encrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [unsafe_encrypt_into] is {!encrypt_into}, but without bounds checks.
+
+        This may casue memory issues if an invariant is violated:
+        {ul
+        {- the length of [iv] must be {!block_size},}
+        {- [len] must be a multiple of {!block_size},}
+        {- [src_off >= 0 && String.length src - src_off >= len],}
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len].}} *)
+
+    val unsafe_decrypt_into : key:key -> iv:string -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [unsafe_decrypt_into] is {!decrypt_into}, but without bounds checks.
+
+        This may casue memory issues if an invariant is violated:
+        {ul
+        {- the length of [iv] must be {!block_size},}
+        {- [len] must be a multiple of {!block_size},}
+        {- [src_off >= 0 && String.length src - src_off >= len],}
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len].}} *)
+
+    val unsafe_encrypt_into_inplace : key:key -> iv:string ->
+      bytes -> dst_off:int -> int -> unit
+    (** [unsafe_encrypt_into_inplace] is {!unsafe_encrypt_into}, but assumes
+        that [dst] already contains the mesage to be encrypted.
+
+        This may casue memory issues if an invariant is violated:
+        {ul
+        {- the length of [iv] must be {!block_size},}
+        {- [len] must be a multiple of {!block_size},}
+        {- [src_off >= 0 && String.length src - src_off >= len],}
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len].}} *)
+    (**/**)
+end
 
   (** {e Counter} mode. *)
   module type CTR = sig
@@ -231,6 +408,27 @@ module Block : sig
 
     type ctr
 
+    val add_ctr : ctr -> int64 -> ctr
+    (** [add_ctr ctr n] adds [n] to [ctr]. *)
+
+    val next_ctr : ?off:int -> string -> ctr:ctr -> ctr
+    (** [next_ctr ~off msg ~ctr] is the state of the counter after encrypting or
+        decrypting [msg] at offset [off] with the counter [ctr].
+
+        For protocols which perform inter-message chaining, this is the
+        counter for the next message.
+
+        It is computed as [C.add ctr (ceil (len msg / block_size))]. Note that
+        if [len msg1 = k * block_size],
+
+{[encrypt ~ctr msg1 || encrypt ~ctr:(next_ctr ~ctr msg1) msg2
+  == encrypt ~ctr (msg1 || msg2)]}
+
+    *)
+
+    val ctr_of_octets : string -> ctr
+    (** [ctr_of_octets buf] converts the value of [buf] into a counter. *)
+
     val stream : key:key -> ctr:ctr -> int -> string
     (** [stream ~key ~ctr n] is the raw keystream.
 
@@ -249,31 +447,51 @@ module Block : sig
 
     val encrypt : key:key -> ctr:ctr -> string -> string
     (** [encrypt ~key ~ctr msg] is
-          [stream ~key ~ctr ~off (len msg) lxor msg]. *)
+          [stream ~key ~ctr (len msg) lxor msg]. *)
 
     val decrypt : key:key -> ctr:ctr -> string -> string
     (** [decrypt] is [encrypt]. *)
 
-    val add_ctr : ctr -> int64 -> ctr
-    (** [add_ctr ctr n] adds [n] to [ctr]. *)
+    val stream_into  : key:key -> ctr:ctr -> bytes -> off:int -> int -> unit
+    (** [stream_into ~key ~ctr dst ~off len] is the raw key stream put into
+        [dst] starting at [off].
 
-    val next_ctr : ctr:ctr -> string -> ctr
-    (** [next_ctr ~ctr msg] is the state of the counter after encrypting or
-        decrypting [msg] with the counter [ctr].
+        @raise Invalid_argument if [Bytes.length dst - off < len]. *)
 
-        For protocols which perform inter-message chaining, this is the
-        counter for the next message.
+    val encrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [encrypt_into ~key ~ctr src ~src_off dst ~dst_off len] produces the
+        key stream into [dst] at [dst_off], and then xors it with [src] at
+        [src_off].
 
-        It is computed as [C.add ctr (ceil (len msg / block_size))]. Note that
-        if [len msg1 = k * block_size],
+        @raise Invalid_argument if [dst_off < 0 || Bytes.length dst - dst_off < len].
+        @raise Invalid_argument if [src_off < 0 || String.length src - src_off < len]. *)
 
-{[encrypt ~ctr msg1 || encrypt ~ctr:(next_ctr ~ctr msg1) msg2
-  == encrypt ~ctr (msg1 || msg2)]}
+    val decrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [decrypt_into] is {!encrypt_into}. *)
 
-    *)
+    (**/**)
+    val unsafe_stream_into  : key:key -> ctr:ctr -> bytes -> off:int -> int -> unit
+    (** [unsafe_stream_into] is {!stream_into}, but without bounds checks.
 
-    val ctr_of_octets : string -> ctr
-    (** [ctr_of_octets buf] converts the value of [buf] into a counter. *)
+        This may cause memory issues if the invariant is violated:
+        {ul
+        {- [off >= 0 && Bytes.length buf - off >= len].}} *)
+
+    val unsafe_encrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [unsafe_encrypt_into] is {!encrypt_into}, but without bounds checks.
+
+        This may cause memory issues if an invariant is violated:
+        {ul
+        {- [dst_off >= 0 && Bytes.length dst - dst_off >= len],}
+        {- [src_off >= 0 && String.length src - src_off >= len].}} *)
+
+    val unsafe_decrypt_into : key:key -> ctr:ctr -> string -> src_off:int ->
+      bytes -> dst_off:int -> int -> unit
+    (** [unsafe_decrypt_into] is {!unsafe_encrypt_into}. *)
+    (**/**)
   end
 
   (** {e Galois/Counter Mode}. *)
diff --git a/src/native.ml b/src/native.ml
index 55437a70..f6d59da3 100644
--- a/src/native.ml
+++ b/src/native.ml
@@ -20,8 +20,8 @@ end
 
 module Poly1305 = struct
   external init     : bytes -> string -> unit = "mc_poly1305_init" [@@noalloc]
-  external update   : bytes -> string -> int -> unit = "mc_poly1305_update" [@@noalloc]
-  external finalize : bytes -> bytes -> unit = "mc_poly1305_finalize" [@@noalloc]
+  external update   : bytes -> string -> int -> int -> unit = "mc_poly1305_update" [@@noalloc]
+  external finalize : bytes -> bytes -> int -> unit = "mc_poly1305_finalize" [@@noalloc]
   external ctx_size : unit -> int = "mc_poly1305_ctx_size" [@@noalloc]
   external mac_size : unit -> int = "mc_poly1305_mac_size" [@@noalloc]
 end
@@ -29,7 +29,7 @@ end
 module GHASH = struct
   external keysize : unit -> int = "mc_ghash_key_size" [@@noalloc]
   external keyinit : string -> bytes -> unit = "mc_ghash_init_key" [@@noalloc]
-  external ghash : string -> bytes -> string -> int -> unit = "mc_ghash" [@@noalloc]
+  external ghash : string -> bytes -> string -> int -> int -> unit = "mc_ghash" [@@noalloc]
   external mode : unit -> int = "mc_ghash_mode" [@@noalloc]
 end
 
@@ -37,9 +37,9 @@ end
  * Unsolved: bounds-checked XORs are slowing things down considerably... *)
 external xor_into_bytes : string -> int -> bytes -> int -> int -> unit = "mc_xor_into_bytes" [@@noalloc]
 
-external count8be   : bytes -> bytes -> blocks:int -> unit = "mc_count_8_be"    [@@noalloc]
-external count16be  : bytes -> bytes -> blocks:int -> unit = "mc_count_16_be"   [@@noalloc]
-external count16be4 : bytes -> bytes -> blocks:int -> unit = "mc_count_16_be_4" [@@noalloc]
+external count8be   : ctr:bytes -> bytes -> off:int -> blocks:int -> unit = "mc_count_8_be"    [@@noalloc]
+external count16be  : ctr:bytes -> bytes -> off:int -> blocks:int -> unit = "mc_count_16_be"   [@@noalloc]
+external count16be4 : ctr:bytes -> bytes -> off:int -> blocks:int -> unit = "mc_count_16_be_4" [@@noalloc]
 
 external misc_mode : unit -> int = "mc_misc_mode" [@@noalloc]
 
diff --git a/src/native/ghash_ctmul.c b/src/native/ghash_ctmul.c
index 7788fd05..bb1a2b05 100644
--- a/src/native/ghash_ctmul.c
+++ b/src/native/ghash_ctmul.c
@@ -290,8 +290,8 @@ CAMLprim value mc_ghash_init_key_generic (value key, value m) {
   return Val_unit;
 }
 
-CAMLprim value mc_ghash_generic (value m, value hash, value src, value len) {
-  br_ghash_ctmul(Bp_val(hash), Bp_val(m), _st_uint8(src), Int_val(len));
+CAMLprim value mc_ghash_generic (value m, value hash, value src, value off, value len) {
+  br_ghash_ctmul(Bp_val(hash), Bp_val(m), _st_uint8_off(src, off), Int_val(len));
   return Val_unit;
 }
 
diff --git a/src/native/ghash_generic.c b/src/native/ghash_generic.c
index 2cc49532..68768c85 100644
--- a/src/native/ghash_generic.c
+++ b/src/native/ghash_generic.c
@@ -101,9 +101,9 @@ CAMLprim value mc_ghash_init_key_generic (value key, value m) {
 }
 
 CAMLprim value
-mc_ghash_generic (value m, value hash, value src, value len) {
+mc_ghash_generic (value m, value hash, value src, value off, value len) {
   __ghash ((__uint128_t *) Bp_val (m), (uint64_t *) Bp_val (hash),
-           _st_uint8 (src), Int_val (len) );
+           _st_uint8_off (src, off), Int_val (len) );
   return Val_unit;
 }
 
diff --git a/src/native/ghash_pclmul.c b/src/native/ghash_pclmul.c
index 58ca02ea..7c7ea95b 100644
--- a/src/native/ghash_pclmul.c
+++ b/src/native/ghash_pclmul.c
@@ -204,11 +204,11 @@ CAMLprim value mc_ghash_init_key (value key, value m) {
 }
 
 CAMLprim value
-mc_ghash (value k, value hash, value src, value len) {
+mc_ghash (value k, value hash, value src, value off, value len) {
   _mc_switch_accel(pclmul,
-    mc_ghash_generic(k, hash, src, len),
+    mc_ghash_generic(k, hash, src, off, len),
     __ghash ( (__m128i *) Bp_val (k), (__m128i *) Bp_val (hash),
-      (__m128i *) _st_uint8 (src), Int_val (len) ))
+              (__m128i *) _st_uint8_off (src, off), Int_val (len) ))
   return Val_unit;
 }
 
diff --git a/src/native/mirage_crypto.h b/src/native/mirage_crypto.h
index 0542db2f..5496a965 100644
--- a/src/native/mirage_crypto.h
+++ b/src/native/mirage_crypto.h
@@ -105,7 +105,7 @@ CAMLprim value mc_ghash_key_size_generic (__unit ());
 CAMLprim value mc_ghash_init_key_generic (value key, value m);
 
 CAMLprim value
-mc_ghash_generic (value m, value hash, value src, value len);
+mc_ghash_generic (value m, value hash, value src, value off, value len);
 
 CAMLprim value
 mc_xor_into_generic (value b1, value off1, value b2, value off2, value n);
@@ -114,6 +114,6 @@ CAMLprim value
 mc_xor_into_bytes_generic (value b1, value off1, value b2, value off2, value n);
 
 CAMLprim value
-mc_count_16_be_4_generic (value ctr, value dst, value blocks);
+mc_count_16_be_4_generic (value ctr, value dst, value off, value blocks);
 
 #endif /* H__MIRAGE_CRYPTO */
diff --git a/src/native/misc.c b/src/native/misc.c
index ba9590f8..dea76e18 100644
--- a/src/native/misc.c
+++ b/src/native/misc.c
@@ -60,9 +60,9 @@ mc_xor_into_bytes_generic (value b1, value off1, value b2, value off2, value n)
 }
 
 #define __export_counter(name, f)                                        \
-  CAMLprim value name (value ctr, value dst, value blocks) {  \
+  CAMLprim value name (value ctr, value dst, value off, value blocks) {  \
     f ( (uint64_t*) Bp_val (ctr),                                        \
-        (uint64_t*) _bp_uint8 (dst), Long_val (blocks) );       \
+        (uint64_t*) _bp_uint8_off (dst, off), Long_val (blocks) );       \
     return Val_unit;                                                     \
   }
 
diff --git a/src/native/misc_sse.c b/src/native/misc_sse.c
index 1f2265da..c155d468 100644
--- a/src/native/misc_sse.c
+++ b/src/native/misc_sse.c
@@ -48,11 +48,11 @@ mc_xor_into_bytes (value b1, value off1, value b2, value off2, value n) {
 }
 
 #define __export_counter(name, f)                                        \
-  CAMLprim value name (value ctr, value dst, value blocks) {  \
-    _mc_switch_accel(ssse3,                                     \
-      name##_generic (ctr, dst, blocks),                            \
+  CAMLprim value name (value ctr, value dst, value off, value blocks) {  \
+    _mc_switch_accel(ssse3,                                              \
+      name##_generic (ctr, dst, off, blocks),                            \
       f ( (uint64_t*) Bp_val (ctr),                                      \
-          (uint64_t*) _bp_uint8 (dst), Long_val (blocks) ))     \
+          (uint64_t*) _bp_uint8_off (dst, off), Long_val (blocks) ))     \
     return Val_unit;                                                     \
   }
 
diff --git a/src/native/poly1305-donna.c b/src/native/poly1305-donna.c
index 567649ab..46991dc2 100644
--- a/src/native/poly1305-donna.c
+++ b/src/native/poly1305-donna.c
@@ -59,13 +59,13 @@ CAMLprim value mc_poly1305_init (value ctx, value key) {
   return Val_unit;
 }
 
-CAMLprim value mc_poly1305_update (value ctx, value buf, value len) {
-  poly1305_update ((poly1305_context *) Bytes_val(ctx), _st_uint8(buf), Int_val(len));
+CAMLprim value mc_poly1305_update (value ctx, value buf, value off, value len) {
+  poly1305_update ((poly1305_context *) Bytes_val(ctx), _st_uint8_off(buf, off), Int_val(len));
   return Val_unit;
 }
 
-CAMLprim value mc_poly1305_finalize (value ctx, value mac) {
-  poly1305_finish ((poly1305_context *) Bytes_val(ctx), Bytes_val(mac));
+CAMLprim value mc_poly1305_finalize (value ctx, value mac, value off) {
+  poly1305_finish ((poly1305_context *) Bytes_val(ctx), _bp_uint8_off(mac, off));
   return Val_unit;
 }
 
diff --git a/src/poly1305.ml b/src/poly1305.ml
index eb571b82..0a2cb72d 100644
--- a/src/poly1305.ml
+++ b/src/poly1305.ml
@@ -11,7 +11,8 @@ module type S = sig
 
   val mac : key:string -> string -> string
   val maci : key:string -> string iter -> string
-  val macl : key:string -> string list -> string
+  val mac_into : key:string -> (string * int * int) list -> bytes -> dst_off:int -> unit
+  val unsafe_mac_into : key:string -> (string * int * int) list -> bytes -> dst_off:int -> unit
 end
 
 module It : S = struct
@@ -31,7 +32,7 @@ module It : S = struct
     ctx
 
   let update ctx data =
-    P.update ctx data (String.length data)
+    P.update ctx data 0 (String.length data)
 
   let feed ctx cs =
     let t = dup ctx in
@@ -45,7 +46,7 @@ module It : S = struct
 
   let final ctx =
     let res = Bytes.create mac_size in
-    P.finalize ctx res;
+    P.finalize ctx res 0;
     Bytes.unsafe_to_string res
 
   let get ctx = final (dup ctx)
@@ -54,8 +55,25 @@ module It : S = struct
 
   let maci ~key iter = feedi (empty ~key) iter |> final
 
-  let macl ~key datas =
+  let unsafe_mac_into ~key datas dst ~dst_off =
     let ctx = empty ~key in
-    List.iter (update ctx) datas;
-    final ctx
+    List.iter (fun (d, off, len) -> P.update ctx d off len) datas;
+    P.finalize ctx dst dst_off
+
+  let mac_into ~key datas dst ~dst_off =
+    if Bytes.length dst - dst_off < mac_size then
+      Uncommon.invalid_arg "Poly1305: dst length %u - off %u < len %u"
+        (Bytes.length dst) dst_off mac_size;
+    if dst_off < 0 then
+      Uncommon.invalid_arg "Poly1305: dst_off %u < 0" dst_off;
+    let ctx = empty ~key in
+    List.iter (fun (d, off, len) ->
+        if off < 0 then
+          Uncommon.invalid_arg "Poly1305: d off %u < 0" off;
+        if String.length d - off < len then
+          Uncommon.invalid_arg "Poly1305: d length %u - off %u < len %u"
+            (String.length d) off len;
+        P.update ctx d off len)
+      datas;
+    P.finalize ctx dst dst_off
 end