base::MD5IntermediateFinal() generates digest non-destructively.

base/md5.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The original file was copied from sqlite, and was in the public domain.
 
 /*
  * This code implements the MD5 message-digest algorithm.
  * The algorithm is due to Ron Rivest.  This code was
  * written by Colin Plumb in 1993, no copyright is claimed.
@@ skipping 242 matching lines @@
         /* Append length in bits and transform */
         ((uint32 *)ctx->in)[ 14 ] = ctx->bits[0];
         ((uint32 *)ctx->in)[ 15 ] = ctx->bits[1];
 
         MD5Transform(ctx->buf, (uint32 *)ctx->in);
         byteReverse((unsigned char *)ctx->buf, 4);
         memcpy(digest->a, ctx->buf, 16);
         memset(ctx, 0, sizeof(*ctx));    /* In case it's sensitive */
 }
 
+/*
+ * Clone the context and use the clone to generate the digest.

[awong, 2014/03/31 20:22:19]

This should be an impl comment, not a function comment. Also explain why cloning
is necessary.

[Scott Hess - ex-Googler, 2014/03/31 20:48:26]

I'll try, but the commenting style is breaking my brain.  [Dear lord!  It was
copied from sqlite?]  WDYT about simply having no comment at all?

[awong, 2014/03/31 21:04:32]

I think it's critical to understand the MD5Final mutates the MD5Context* and
that this memcpy is "good enough" to fix things.

What about
/* MD5Final mutates the MD5Context*. Make a copy for generating the intermediate
value. */

[Scott Hess - ex-Googler, 2014/03/31 21:29:59]

Done.

+ */
+void MD5IntermediateFinal(MD5Digest* digest, const MD5Context* context) {
+  MD5Context context_copy;
+  memcpy(&context_copy, context, sizeof(context_copy));
+  MD5Final(digest, &context_copy);
+}
+
 std::string MD5DigestToBase16(const MD5Digest& digest) {
   static char const zEncode[] = "0123456789abcdef";
 
   std::string ret;
   ret.resize(32);
 
   int j = 0;
   for (int i = 0; i < 16; i ++) {
     int a = digest.a[i];
     ret[j++] = zEncode[(a>>4)&0xf];
@@ skipping 10 matching lines @@
   MD5Final(digest, &ctx);
 }
 
 std::string MD5String(const StringPiece& str) {
   MD5Digest digest;
   MD5Sum(str.data(), str.length(), &digest);
   return MD5DigestToBase16(digest);
 }
 
 }  // namespace base