Implement HKDF.

See https://tools.ietf.org/html/rfc5869.

Change-Id: I6d012d8c38806df80aa44350d3a86eaff499ac05
Reviewed-on: https://boringssl-review.googlesource.com/2533
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/err.h

diff --git a/include/openssl/err.h b/include/openssl/err.h
index b9c48c3..ca1f6c4 100644
--- a/include/openssl/err.h
+++ b/include/openssl/err.h
@@ -403,6 +403,7 @@
   ERR_LIB_DIGEST,
   ERR_LIB_CIPHER,
   ERR_LIB_USER,
+  ERR_LIB_HKDF,
   ERR_NUM_LIBS
 };
 
@@ -442,6 +443,7 @@
 #define ERR_R_USER_LIB ERR_LIB_USER
 #define ERR_R_DIGEST_LIB ERR_LIB_DIGEST
 #define ERR_R_CIPHER_LIB ERR_LIB_CIPHER
+#define ERR_R_HKDF_LIB ERR_LIB_HKDF
 
 /* Global reasons. */
 #define ERR_R_FATAL 64

include/openssl/hkdf.h

diff --git a/include/openssl/hkdf.h b/include/openssl/hkdf.h
new file mode 100644
index 0000000..4091b84
--- /dev/null
+++ b/include/openssl/hkdf.h
@@ -0,0 +1,45 @@
+/* Copyright (c) 2014, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#ifndef OPENSSL_HEADER_HKDF_H
+#define OPENSSL_HEADER_HKDF_H
+
+#include <openssl/base.h>
+
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
+
+/* Computes HKDF (as specified by RFC 5869) of initial keying material |secret|
+ * with |salt| and |info| using |digest|, and outputs |out_len| bytes to
+ * |out_key|. It returns one on success and zero on error.
+ *
+ * HKDF is an Extract-and-Expand algorithm. It does not do any key stretching,
+ * and as such, is not suited to be used alone to generate a key from a
+ * password. */
+OPENSSL_EXPORT int HKDF(uint8_t *out_key, size_t out_len, const EVP_MD *digest,
+                        const uint8_t *secret, size_t secret_len,
+                        const uint8_t *salt, size_t salt_len,
+                        const uint8_t *info, size_t info_len);
+
+
+#if defined(__cplusplus)
+}  /* extern C */
+#endif
+
+#define HKDF_F_HKDF 100
+#define HKDF_R_OUTPUT_TOO_LARGE 100
+
+#endif  /* OPENSSL_HEADER_HKDF_H */