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diff --git a/third_party/libwebrtc/pc/external_hmac.h b/third_party/libwebrtc/pc/external_hmac.h
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+++ b/third_party/libwebrtc/pc/external_hmac.h
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+/*
+ *  Copyright 2014 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef PC_EXTERNAL_HMAC_H_
+#define PC_EXTERNAL_HMAC_H_
+
+// External libsrtp HMAC auth module which implements methods defined in
+// auth_type_t.
+// The default auth module will be replaced only when the ENABLE_EXTERNAL_AUTH
+// flag is enabled. This allows us to access to authentication keys,
+// as the default auth implementation doesn't provide access and avoids
+// hashing each packet twice.
+
+// How will libsrtp select this module?
+// Libsrtp defines authentication function types identified by an unsigned
+// integer, e.g. SRTP_HMAC_SHA1 is 3. Using authentication ids, the
+// application can plug any desired authentication modules into libsrtp.
+// libsrtp also provides a mechanism to select different auth functions for
+// individual streams. This can be done by setting the right value in
+// the auth_type of srtp_policy_t. The application must first register auth
+// functions and the corresponding authentication id using
+// crypto_kernel_replace_auth_type function.
+
+#include <stdint.h>
+
+#include "third_party/libsrtp/crypto/include/crypto_types.h"
+#include "third_party/libsrtp/include/srtp.h"
+#include "third_party/libsrtp/include/srtp_priv.h"
+
+#define EXTERNAL_HMAC_SHA1 SRTP_HMAC_SHA1 + 1
+#define HMAC_KEY_LENGTH 20
+
+// The HMAC context structure used to store authentication keys.
+// The pointer to the key  will be allocated in the external_hmac_init function.
+// This pointer is owned by srtp_t in a template context.
+typedef struct {
+  uint8_t key[HMAC_KEY_LENGTH];
+  int key_length;
+} ExternalHmacContext;
+
+srtp_err_status_t external_hmac_alloc(srtp_auth_t** a,
+                                      int key_len,
+                                      int out_len);
+
+srtp_err_status_t external_hmac_dealloc(srtp_auth_t* a);
+
+srtp_err_status_t external_hmac_init(void* state,
+                                     const uint8_t* key,
+                                     int key_len);
+
+srtp_err_status_t external_hmac_start(void* state);
+
+srtp_err_status_t external_hmac_update(void* state,
+                                       const uint8_t* message,
+                                       int msg_octets);
+
+srtp_err_status_t external_hmac_compute(void* state,
+                                        const uint8_t* message,
+                                        int msg_octets,
+                                        int tag_len,
+                                        uint8_t* result);
+
+srtp_err_status_t external_crypto_init();
+
+#endif  // PC_EXTERNAL_HMAC_H_