ECIES: add HKDF (specified in RFC 5869) using HMAC-SHA256
Change-Id: I18cf63b6454d3d11386e9de93d934d759e0abc0e
diff --git a/hmac.h b/hmac.h
new file mode 100644
index 0000000..09ae5e8
--- /dev/null
+++ b/hmac.h
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef SYSTEM_KEYMASTER_HMAC_H_
+#define SYSTEM_KEYMASTER_HMAC_H_
+
+#include <keymaster/serializable.h>
+
+namespace keymaster {
+
+
+// Only HMAC-SHA256 is supported.
+class HmacSha256 {
+ public:
+ HmacSha256() {};
+
+ // DigestLength returns the length, in bytes, of the resulting digest.
+ size_t DigestLength() const;
+
+ // Initializes this instance using |key|. Call Init only once. It returns
+ // false on the second of later calls.
+ bool Init(const uint8_t* key, size_t key_length);
+ bool Init(const Buffer& key);
+
+ // Sign calculates the HMAC of |data| with the key supplied to the Init
+ // method. At most |digest_len| bytes of the resulting digest are written
+ // to |digest|.
+ bool Sign(const Buffer& data, uint8_t* digest, size_t digest_len) const;
+ bool Sign(const uint8_t* data, size_t data_len, uint8_t* digest, size_t digest_len) const;
+
+ // Verify returns true if |digest| is a valid HMAC of |data| using the key
+ // supplied to Init. |digest| must be exactly |DigestLength()| bytes long.
+ // Use of this method is strongly recommended over using Sign() with a manual
+ // comparison (such as memcmp), as such comparisons may result in
+ // side-channel disclosures, such as timing, that undermine the cryptographic
+ // integrity.
+ bool Verify(const Buffer& data, const Buffer& digest) const;
+ bool Verify(const uint8_t* data, size_t data_len, const uint8_t* digest,
+ size_t digest_len) const;
+
+ private:
+ Buffer key_;
+};
+
+} // namespace keymaster
+
+#endif // SYSTEM_KEYMASTER_HMAC_H_