diff -uNr a/vtools/manifest b/vtools/manifest
--- a/vtools/manifest 6e0728447dd05a8e1d4dd21a9647ac87e1e66b16e1cb15dca1a9e775907767fcae8b4db6be152aba863bee3bf15cfb665512a45f001593bfd990b43e4087d3ef
+++ b/vtools/manifest 1f6a6754f578117a08bc3b4428801c6dbcfd04e5266de39188dc9401ed1b2cf0d33d41a47fb37480ccae92e3a5d4adde71b398f538a6d1e92effb23801b584d8
@@ -8,3 +8,4 @@
 517100 phf vtools_vpatch_newline Vpatch tool support for "No newline at end of file" directive.
 543200 phf vtools_ksum Ksum standalone keccak hasher
 547300 bvt vtools_tempfile_standalone_notmp Standalone temporary file generation code; creates temporary files in the vpatch work directory.
+584341 bvt vdiff_blockwise_read Fix stack overflow in vdiff when hashing large files.
diff -uNr a/vtools/src/keccak_c.adb b/vtools/src/keccak_c.adb
--- a/vtools/src/keccak_c.adb 8b4ed2ecae1c6fee1cc25f280351f57e4329d048617754f580ccc2df61734ede8d64d26dff20f29810a345b3e376ed6455f78e792eb02af14d1e5c6c757771fd
+++ b/vtools/src/keccak_c.adb 130484edcb794a78db04eeb6293a21ef5ea70ac04c55a7641e67fac4879935fb1a51624081d583ae47bc30948610eaa6a3d6639ce53d0be53d7f315406d22e9a
@@ -18,20 +18,30 @@
    procedure C_Hash(Ctx: in C_Context_Access;
                     Input: Char_Star;
                     Len: Interfaces.C.size_t) is
+      Buffer_Size: constant Natural := 2048;
+      Byte_Size: constant Natural := 8;
+      N: Natural := 0;
+      I: Natural := 0;
       L: Natural := Natural(Len);
-      S: String(1..L);
-      B: Bitstream(1..S'Length*8);
+      Buf: String(1..Buffer_Size);
+      B: Bitstream(1..Buf'Length*Byte_Size);
       Ptr: Char_Star := Input;
    begin
       if Input = null then
          raise Strings.Dereference_Error;
       end if;
-      for Chr of S loop
-         Chr := Character(Ptr.all);
-         Char_Ptrs.Increment(Ptr);
+      while L > I loop
+         N := 0;
+         for Chr of Buf loop
+               exit when L <= I;
+               Chr := Character(Ptr.all);
+               Char_Ptrs.Increment(Ptr);
+               N := N + 1;
+               I := I + 1;
+         end loop;
+         ToBitstream(Buf(1..N), B(1..N*Byte_Size));
+         KeccakHash(Ctx.all, B(1..N*Byte_Size));
       end loop;
-      ToBitstream(S, B);
-      KeccakHash(Ctx.all, B);
    end C_Hash;
 
    procedure C_End(Ctx: C_Context_Access;