diff --git a/third_party/libuweave/src/crypto_hmac.c b/third_party/libuweave/src/crypto_hmac.c
new file mode 100644
index 0000000..56bb754
--- /dev/null
+++ b/third_party/libuweave/src/crypto_hmac.c
@@ -0,0 +1,64 @@
+// Copyright 2015 The Weave 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 "src/crypto_hmac.h"
+
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <openssl/evp.h>
+#include <openssl/hmac.h>
+
+size_t uw_crypto_hmac_required_buffer_size_() {
+  return sizeof(HMAC_CTX);
+}
+
+bool uw_crypto_hmac_init_(uint8_t* state_buffer,
+                          size_t state_buffer_len,
+                          const uint8_t* key,
+                          size_t key_len) {
+  if (sizeof(HMAC_CTX) > state_buffer_len) {
+    return false;
+  }
+  HMAC_CTX* context = (HMAC_CTX*)state_buffer;
+  HMAC_CTX_init(context);
+  return HMAC_Init(context, key, key_len, EVP_sha256()) ? 0 : sizeof(HMAC_CTX);
+}
+
+bool uw_crypto_hmac_update_(uint8_t* state_buffer,
+                            size_t state_buffer_len,
+                            const uint8_t* data,
+                            size_t data_len) {
+  if (sizeof(HMAC_CTX) > state_buffer_len) {
+    return false;
+  }
+  HMAC_CTX* context = (HMAC_CTX*)state_buffer;
+  return HMAC_Update(context, data, data_len);
+}
+
+bool uw_crypto_hmac_final_(uint8_t* state_buffer,
+                           size_t state_buffer_len,
+                           uint8_t* truncated_digest,
+                           size_t truncated_digest_len) {
+  if (sizeof(HMAC_CTX) > state_buffer_len) {
+    return false;
+  }
+  HMAC_CTX* context = (HMAC_CTX*)state_buffer;
+
+  const size_t kFullDigestLen = (size_t)EVP_MD_size(EVP_sha256());
+  if (truncated_digest_len > kFullDigestLen) {
+    return false;
+  }
+
+  uint8_t digest[kFullDigestLen];
+  uint32_t len = kFullDigestLen;
+
+  bool result = HMAC_Final(context, digest, &len) && kFullDigestLen == len;
+  HMAC_CTX_cleanup(context);
+  if (result) {
+    memcpy(truncated_digest, digest, truncated_digest_len);
+  }
+  return result;
+}