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

base/md5.h

 // 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.
 
 #ifndef BASE_MD5_H_
 #define BASE_MD5_H_
 
 #include "base/base_export.h"
 #include "base/strings/string_piece.h"
 
@@ skipping 40 matching lines @@
 BASE_EXPORT void MD5Init(MD5Context* context);
 
 // For the given buffer of |data| as a StringPiece, updates the given MD5
 // context with the sum of the data. You can call this any number of times
 // during the computation, except that MD5Init() must have been called first.
 BASE_EXPORT void MD5Update(MD5Context* context, const StringPiece& data);
 
 // Finalizes the MD5 operation and fills the buffer with the digest.
 BASE_EXPORT void MD5Final(MD5Digest* digest, MD5Context* context);
 
+// MD5Final() modifies the context.  This clones the context to allow generating
+// an intermediate digest without affecting the original context.

[Ryan Sleevi, 2014/03/26 17:49:46]

Comment nit: This doesn't really clone (at least, not from the point of view of
the caller)

// Because MD5Final() destructively finalizes the context, preventing further
data from being hashed,
// MD5IntermediateFinal() is provided to allow callers to obtain the hash of all
the data so far, while
// permitting the further hashing of additional data with the original context.

[Scott Hess - ex-Googler, 2014/03/27 19:22:34]

Yeah, I first had both in a single CL and let the context influence me.

I didn't like your comment, so I randomly rewrote it, and threw in a const to
make it clear.  Feel free to tell me "No, my version", because I'm pretty sure
that anyone who's reading the comment is already lost :-).

+BASE_EXPORT void MD5IntermediateFinal(MD5Digest* digest, MD5Context* context);
+
 // Converts a digest into human-readable hexadecimal.
 BASE_EXPORT std::string MD5DigestToBase16(const MD5Digest& digest);
 
 // Returns the MD5 (in hexadecimal) of a string.
 BASE_EXPORT std::string MD5String(const StringPiece& str);
 
 }  // namespace base
 
 #endif  // BASE_MD5_H_