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

crypto/hmac_openssl.cc

Index: crypto/hmac_openssl.cc
diff --git a/crypto/hmac_openssl.cc b/crypto/hmac_openssl.cc
index 92eea19d780270cde6fa593986ed2015e9687f85..ef20290e223200959674baf2d5f4db647b9d2cba 100644
--- a/crypto/hmac_openssl.cc
+++ b/crypto/hmac_openssl.cc
@@ -20,45 +20,37 @@ struct HMACPlatformData {
   std::vector<unsigned char> key;
 };
 
-HMAC::HMAC(HashAlgorithm hash_alg)
-    : hash_alg_(hash_alg), plat_(new HMACPlatformData()) {
+HMAC::HMAC(HashAlgorithm hash_alg) : hash_alg_(hash_alg) {
   // 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());
-
+  DCHECK(!plat_);
+  plat_.reset(new HMACPlatformData());
   plat_->key.assign(key, key + key_length);
-  if (key_length == 0) {
-    // 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.
-    plat_->key.push_back(0);
-  }
   return true;
 }
 
 HMAC::~HMAC() {
-  // Zero out key copy.
-  plat_->key.assign(plat_->key.size(), 0);
-  STLClearObject(&plat_->key);
+  if (plat_) {
+    // Zero out key copy.
+    plat_->key.assign(plat_->key.size(), 0);
+    STLClearObject(&plat_->key);

[davidben, 2015/03/05 00:31:29]

(While you're here, I'm not sure line 40 does anything useful. Line 39 may be
less pointless, if the compile isn't clever enough to optimize it away. *shrug*)

[eroman, 2015/03/05 00:41:11]

Yeah I agree doesn't seem useful. I will propose that separately though to keep
this CL strictly a refactor.

+  }
 }
 
 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.
+  DCHECK(plat_);  // 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);
+                  vector_as_array(&plat_->key), plat_->key.size(),
+                  reinterpret_cast<const unsigned char*>(data.data()),
+                  data.size(), result.safe_buffer(), NULL);
 }
 
 }  // namespace crypto