diff --git a/crypto/hkdf/hkdf.c b/crypto/hkdf/hkdf.c
new file mode 100644
index 0000000..be51bc3
--- /dev/null
+++ b/crypto/hkdf/hkdf.c
@@ -0,0 +1,88 @@
+/* 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. */
+
+#include <openssl/hkdf.h>
+
+#include <assert.h>
+
+#include <openssl/err.h>
+#include <openssl/hmac.h>
+
+
+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) {
+  /* https://tools.ietf.org/html/rfc5869#section-2.2 */
+  const size_t digest_len = EVP_MD_size(digest);
+  uint8_t prk[EVP_MAX_MD_SIZE], previous[EVP_MAX_MD_SIZE];
+  size_t n, done = 0;
+  unsigned i, prk_len;
+  int ret = 0;
+  HMAC_CTX hmac;
+
+  /* If salt is not given, HashLength zeros are used. However, HMAC does that
+   * internally already so we can ignore it.*/
+
+  /* Expand key material to desired length. */
+  n = (out_len + digest_len - 1) / digest_len;
+  if (out_len + digest_len < out_len || n > 255) {
+    OPENSSL_PUT_ERROR(HKDF, HKDF, HKDF_R_OUTPUT_TOO_LARGE);
+    return 0;
+  }
+
+  HMAC_CTX_init(&hmac);
+
+  /* Extract input keying material into pseudorandom key |prk|. */
+  if (HMAC(digest, salt, salt_len, secret, secret_len, prk, &prk_len) == NULL) {
+    goto out;
+  }
+  assert(prk_len == digest_len);
+
+  if (!HMAC_Init_ex(&hmac, prk, prk_len, digest, NULL)) {
+    goto out;
+  }
+
+  for (i = 0; i < n; i++) {
+    uint8_t ctr = i + 1;
+    size_t todo;
+
+    if (i != 0 && (!HMAC_Init_ex(&hmac, NULL, 0, NULL, NULL) ||
+                   !HMAC_Update(&hmac, previous, digest_len))) {
+      goto out;
+    }
+    if (!HMAC_Update(&hmac, info, info_len) ||
+        !HMAC_Update(&hmac, &ctr, 1) ||
+        !HMAC_Final(&hmac, previous, NULL)) {
+      goto out;
+    }
+
+    todo = digest_len;
+    if (done + todo > out_len) {
+      todo = out_len - done;
+    }
+    memcpy(out_key + done, previous, todo);
+    done += todo;
+  }
+
+  ret = 1;
+
+out:
+  HMAC_CTX_cleanup(&hmac);
+  if (ret != 1) {
+    OPENSSL_PUT_ERROR(HKDF, HKDF, ERR_R_HMAC_LIB);
+  }
+  return ret;
+}