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

base/md5_unittest.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.
 
 #include <string.h>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/md5.h"
@@ skipping 186 matching lines @@
 
   MD5Digest digest;
   MD5Final(&digest, &ctx);
 
   std::string actual = MD5DigestToBase16(digest);
   std::string expected = "900150983cd24fb0d6963f7d28e17f72";
 
   EXPECT_EQ(expected, actual);
 }
 
+// Test that a digest generated by MD5IntermediateFinal() gives the same results
+// as an independently-calculated digest, and also does not modify the context.
+TEST(MD5, IntermediateFinal) {
+  // Independent context over the header.
+  MD5Context check_header_context;
+  MD5Init(&check_header_context);
+
+  // Independent context over entire input.
+  MD5Context check_full_context;
+  MD5Init(&check_full_context);
+
+  // Context intermediate digest will be calculated from.
+  MD5Context context;
+  MD5Init(&context);
+
+  const char kHeader[] = "header data";

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

static const char blah[]?

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

Done.

+  const char kBody[] = "payload data";
+
+  MD5Update(&context, kHeader);
+  MD5Update(&check_header_context, kHeader);
+  MD5Update(&check_full_context, kHeader);
+
+  MD5Digest check_header_digest;
+  MD5Final(&check_header_digest, &check_header_context);
+
+  MD5Digest header_digest;
+  MD5IntermediateFinal(&header_digest, &context);
+
+  MD5Update(&context, kBody);
+  MD5Update(&check_full_context, kBody);
+
+  MD5Digest check_full_digest;
+  MD5Final(&check_full_digest, &check_full_context);
+
+  MD5Digest digest;
+  MD5Final(&digest, &context);
+
+  // The header and full digest pairs are the same, and they aren't the same as
+  // each other.
+  EXPECT_TRUE(!memcmp(&header_digest, &check_header_digest,
+                      sizeof(header_digest)));
+  EXPECT_TRUE(!memcmp(&digest, &check_full_digest, sizeof(digest)));
+  EXPECT_FALSE(!memcmp(&digest, &header_digest, sizeof(digest)));
+}
+
 }  // namespace base