Update some comments and code to reflect the fact that HMAC() is no longer hostile to NULL key data.

crypto/hmac_openssl.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 "crypto/hmac.h"
 
 #include <openssl/hmac.h>
 
 #include <algorithm>
 #include <vector>
@@ skipping 13 matching lines @@
     : hash_alg_(hash_alg), plat_(new HMACPlatformData()) {
   // Only SHA-1 and SHA-256 hash algorithms are supported now.
   DCHECK(hash_alg_ == SHA1 || hash_alg_ == SHA256);
 }
 
 bool HMAC::Init(const unsigned char* key, size_t key_length) {
   // Init must not be called more than once on the same HMAC object.
   DCHECK(plat_->key.empty());
 
   plat_->key.assign(key, key + key_length);
   if (key_length == 0) {

[davidben, 2015/03/04 19:40:59]

Hrm. This seems to conflict with the documentation. Given we have passing tests
for it, I suspect this comment is just a lie.

https://code.google.com/p/chromium/codesearch#chromium/src/crypto/hmac.h&q=HM...

-    // Special-case: if the key is empty, use a key with one zero
-    // byte. OpenSSL's HMAC function breaks when passed a NULL key. (It calls
-    // HMAC_Init_ex which treats a NULL key as having already been initialized
-    // with a key previously.) HMAC pads keys with zeros, so this key is
-    // equivalent.
+    // HMAC::Sign() assumes that plat_->key is not empty.
+    // HMAC pads keys with zeros, so this key is equivalent.

[davidben, 2015/03/04 19:40:59]

Hrm. This seems to only be relevant for the DCHECK. What if we instead defer
initializing plat_ until HMAC::Init? Then we can DCHECK on plat_ existing
without extra state.

     plat_->key.push_back(0);
   }
   return true;
 }
 
 HMAC::~HMAC() {
   // Zero out key copy.
   plat_->key.assign(plat_->key.size(), 0);
   STLClearObject(&plat_->key);
 }
 
 bool HMAC::Sign(const base::StringPiece& data,
                 unsigned char* digest,
                 size_t digest_length) const {
   DCHECK(!plat_->key.empty());  // Init must be called before Sign.
 
   ScopedOpenSSLSafeSizeBuffer<EVP_MAX_MD_SIZE> result(digest, digest_length);
   return !!::HMAC(hash_alg_ == SHA1 ? EVP_sha1() : EVP_sha256(),
                 &plat_->key[0], plat_->key.size(),
                 reinterpret_cast<const unsigned char*>(data.data()),
                 data.size(),
                 result.safe_buffer(), NULL);
 }
 
 }  // namespace crypto