Porting of HKDF changes from server.

crypto/hkdf.cc

Index: crypto/hkdf.cc
diff --git a/crypto/hkdf.cc b/crypto/hkdf.cc
new file mode 100644
index 0000000000000000000000000000000000000000..18bba6a983a1aed1d32d6e8714c9a99830a3c95e
--- /dev/null
+++ b/crypto/hkdf.cc
@@ -0,0 +1,92 @@
+// Copyright (c) 2013 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/hkdf.h"
+
+#include "base/logging.h"
+#include "crypto/hmac.h"
+
+namespace crypto {
+
+const size_t kSHA256HashLength = 32;
+
+HKDF::HKDF(const base::StringPiece& secret,
+           const base::StringPiece& salt,
+           const base::StringPiece& info,
+           size_t key_bytes_to_generate,
+           size_t iv_bytes_to_generate) {
+  // https://tools.ietf.org/html/rfc5869#section-2.2
+  base::StringPiece actual_salt = salt;
+  char zeros[kSHA256HashLength];
+  if (actual_salt.empty()) {
+    // If salt is not given, HashLength zeros are used.
+    memset(zeros, 0, sizeof(zeros));
+    actual_salt.set(zeros, sizeof(zeros));
+  }
+
+  // Perform the Extract step to transform the input key and
+  // salt into the pseudorandom key (PRK) used for Expand.
+  HMAC prk_hmac(HMAC::SHA256);
+  bool result = prk_hmac.Init(actual_salt);
+  DCHECK(result);
+
+  // |prk| is a pseudorandom key (of kSHA256HashLength octets).
+  uint8 prk[kSHA256HashLength];
+  DCHECK_EQ(sizeof(prk), prk_hmac.DigestLength());
+  result = prk_hmac.Sign(secret, prk, sizeof(prk));
+  DCHECK(result);
+
+  // https://tools.ietf.org/html/rfc5869#section-2.3
+  // Perform the Expand phase to turn the pseudorandom key
+  // and info into the output keying material.
+  const size_t material_length =
+      2*key_bytes_to_generate + 2*iv_bytes_to_generate;
+  const size_t n = (material_length + kSHA256HashLength-1) /
+                   kSHA256HashLength;
+  DCHECK_LT(n, 256u);
+
+  output_.resize(n * kSHA256HashLength);
+  base::StringPiece previous;
+
+  char* buf = new char[kSHA256HashLength + info.size() + 1];

[wtc, 2013/02/25 22:18:00]

I think it is better to use scoped_ptr<char[]> here. This is
common practice in the Chromium source tree.

[ramant (doing other things), 2013/02/26 22:39:46]

Done.

+  uint8 digest[kSHA256HashLength];
+
+  HMAC hmac(HMAC::SHA256);
+  result = hmac.Init(prk, sizeof(prk));
+  DCHECK(result);
+
+  for (size_t i = 0; i < n; i++) {
+    memcpy(buf, previous.data(), previous.size());
+    size_t j = previous.size();
+    memcpy(buf + j, info.data(), info.size());
+    j += info.size();
+    buf[j++] = static_cast<char>((i + 1) & 0xFF);

[wtc, 2013/02/25 22:18:00]

Please remove & 0xFF.

It implies i + 1 can be greater than 255 (0xFF),
but the RFC requires that i + 1  <= 255. This requirement
comes from the requirement on L (<= 255*HashLen) and is
checked by the DCHECK on line 47.

So i + 1 won't overflow an unsigned char. We can add a
static_cast if compilers warn about it.

[ramant (doing other things), 2013/02/26 22:39:46]

Done.

+
+    result = hmac.Sign(base::StringPiece(buf, j), digest, sizeof(digest));
+    DCHECK(result);
+
+    memcpy(&output_[i*sizeof(digest)], digest, sizeof(digest));
+    previous = base::StringPiece(reinterpret_cast<char*>(digest),
+                                 sizeof(digest));
+  }
+
+  size_t j = 0;
+  client_write_key_ = base::StringPiece(reinterpret_cast<char *>(&output_[j]),
+                                        key_bytes_to_generate);
+  j += key_bytes_to_generate;
+  server_write_key_ = base::StringPiece(reinterpret_cast<char *>(&output_[j]),
+                                        key_bytes_to_generate);
+  j += key_bytes_to_generate;
+  client_write_iv_ = base::StringPiece(reinterpret_cast<char *>(&output_[j]),
+                                       iv_bytes_to_generate);
+  j += iv_bytes_to_generate;
+  server_write_iv_ = base::StringPiece(reinterpret_cast<char *>(&output_[j]),
+                                       iv_bytes_to_generate);
+  delete[] buf;
+}
+
+HKDF::~HKDF() {
+}
+
+}  // namespace crypto