diff --git a/src/cipher_block.ml b/src/cipher_block.ml
index e196fc06..af958e28 100644
--- a/src/cipher_block.ml
+++ b/src/cipher_block.ml
@@ -152,6 +152,15 @@ module Counters = struct
   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
 
@@ -167,12 +176,8 @@ module Modes = struct
     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;
-      if String.length src - src_off < len then
-        invalid_arg "ECB: source length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "ECB: dst length %u - dst_off %u < len %u"
-          (Bytes.length dst) dst_off 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 =
@@ -209,15 +214,16 @@ module Modes = struct
 
     let of_secret = Core.of_secret
 
-    let bounds_check ?(off = 0) ~iv cs =
+    let block_size_check ?(off = 0) ~iv cs =
       if String.length iv <> block then
         invalid_arg "CBC: IV length %u not of block size" (String.length iv);
       if (String.length cs - off) mod block <> 0 then
         invalid_arg "CBC: argument length %u (off %u) not of block size"
           (String.length cs) off
+    [@@inline]
 
     let next_iv ?(off = 0) cs ~iv =
-      bounds_check ~iv cs ~off ;
+      block_size_check ~iv cs ~off ;
       if String.length cs > off then
         String.sub cs (String.length cs - block_size) block_size
       else iv
@@ -237,13 +243,9 @@ module Modes = struct
       unsafe_encrypt_into_inplace ~key ~iv dst ~dst_off len
 
     let encrypt_into ~key ~iv src ~src_off dst ~dst_off len =
-      bounds_check ~off:src_off ~iv src;
-      if String.length src - src_off < len then
-        invalid_arg "CBC: src has insufficient length (%u - src_off:%u < len %u)"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "CBC: dst has insufficient length (%u - dst_off:%u < len %u)"
-          (Bytes.length dst) dst_off len;
+      block_size_check ~off:src_off ~iv src;
+      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 =
@@ -260,13 +262,9 @@ module Modes = struct
       end
 
     let decrypt_into ~key ~iv src ~src_off dst ~dst_off len =
-      bounds_check ~off:src_off ~iv src;
-      if String.length src - src_off < len then
-        invalid_arg "CBC: src has insufficient length (%u - src_off:%u < len %u)"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "CBC: dst has insufficient length (%u - dst_off:%u < len %u)"
-          (Bytes.length dst) dst_off len;
+      block_size_check ~off:src_off ~iv src;
+      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 =
@@ -302,9 +300,7 @@ module Modes = struct
       end
 
     let stream_into ~key ~ctr buf ~off len =
-      if Bytes.length buf - off < len then
-        invalid_arg "CTR: buffer length %u - off %u < len %u"
-          (Bytes.length 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 =
@@ -317,12 +313,8 @@ module Modes = struct
       Uncommon.unsafe_xor_into src ~src_off dst ~dst_off len
 
     let encrypt_into ~key ~ctr src ~src_off dst ~dst_off len =
-      if String.length src - src_off < len then
-        invalid_arg "CTR: src length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "CTR: dst length %u - dst_off %u < len %u"
-          (Bytes.length 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 =
@@ -416,15 +408,9 @@ module Modes = struct
       unsafe_tag_into ~key ~hkey ~ctr ?adata (Bytes.unsafe_to_string dst) ~off:dst_off ~len dst ~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 "GCM: source length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "GCM: 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 "GCM: dst length %u - tag_off %u < tag_size %u"
-          (Bytes.length dst) tag_off tag_size;
+      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 =
@@ -446,15 +432,9 @@ module Modes = struct
       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 "GCM: source length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "GCM: 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 "GCM: src length %u - tag_off %u < tag_size %u"
-          (String.length src) tag_off tag_size;
+      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 =
@@ -498,15 +478,9 @@ module Modes = struct
         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 =
-      if String.length src - src_off < len then
-        invalid_arg "CCM: source length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "CCM: 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 "CCM: dst length %u - tag_off %u < tag_size %u"
-          (Bytes.length dst) tag_off tag_size;
+      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
 
@@ -525,15 +499,9 @@ module Modes = struct
       Ccm.unsafe_decryption_verification_into ~cipher ~key ~nonce ~maclen:tag_size ~adata src ~src_off ~tag_off dst ~dst_off len
 
     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 "CCM: source length %u - src_off %u < len %u"
-          (String.length src) src_off len;
-      if Bytes.length dst - dst_off < len then
-        invalid_arg "CCM: 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 "CCM: src length %u - tag_off %u < tag_size %u"
-          (String.length src) tag_off tag_size;
+      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