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

base/md5.h

Index: base/md5.h
diff --git a/base/md5.h b/base/md5.h
index fba02bd11635d63ac2057e25cf150bc25d84f012..b4bfef4bfd53a8133c2fcdcd2cccd5e37a0c595f 100644
--- a/base/md5.h
+++ b/base/md5.h
@@ -58,6 +58,10 @@ 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);