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diff --git a/src/core/tsi/alts/frame_protector/alts_counter.h b/src/core/tsi/alts/frame_protector/alts_counter.h
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+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * 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 GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_ALTS_COUNTER_H
+#define GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_ALTS_COUNTER_H
+
+#include <grpc/support/port_platform.h>
+
+#include <stdbool.h>
+#include <stdlib.h>
+
+#include <grpc/grpc.h>
+
+/* Main struct for a crypter counter managed within seal/unseal operations. */
+typedef struct alts_counter {
+  size_t size;
+  size_t overflow_size;
+  unsigned char* counter;
+} alts_counter;
+
+/**
+ * This method creates and initializes an alts_counter instance.
+ *
+ * - is_client: a flag indicating if the alts_counter instance will be used
+ *   at client (is_client = true) or server (is_client = false) side.
+ * - counter_size: size of buffer holding the counter value.
+ * - overflow_size: overflow size in bytes. The counter instance can be used
+ *   to produce at most 2^(overflow_size*8) frames.
+ * - crypter_counter: an alts_counter instance to be returned from the method.
+ * - error_details: a buffer containing an error message if the method does not
+ *   function correctly. It is legal to pass nullptr into error_details and
+ *   otherwise, the parameter should be freed with gpr_free.
+ *
+ * On success, the method returns GRPC_STATUS_OK. Otherwise,
+ * it returns an error status code along with its details specified in
+ * error_details (if error_details is not nullptr).
+ */
+grpc_status_code alts_counter_create(bool is_client, size_t counter_size,
+                                     size_t overflow_size,
+                                     alts_counter** crypter_counter,
+                                     char** error_details);
+
+/**
+ * This method increments the internal counter.
+ *
+ * - crypter_counter: an alts_counter instance.
+ * - is_overflow: after incrementing the internal counter, if an overflow
+ *   occurs, is_overflow is set to true, and no further calls to
+ *   alts_counter_increment() should be made. Otherwise, is_overflow is set to
+ *   false.
+ * - error_details: a buffer containing an error message if the method does not
+ *   function correctly. It is legal to pass nullptr into error_details and
+ *   otherwise, the parameter should be freed with gpr_free.
+ *
+ * On success, the method returns GRPC_STATUS_OK. Otherwise,
+ * it returns an error status code along with its details specified in
+ * error_details (if error_details is not nullptr).
+ */
+grpc_status_code alts_counter_increment(alts_counter* crypter_counter,
+                                        bool* is_overflow,
+                                        char** error_details);
+
+/**
+ * This method returns the size of counter buffer.
+ *
+ * - crypter_counter: an alts_counter instance.
+ */
+size_t alts_counter_get_size(alts_counter* crypter_counter);
+
+/**
+ * This method returns the counter buffer.
+ *
+ * - crypter_counter: an alts_counter instance.
+ */
+unsigned char* alts_counter_get_counter(alts_counter* crypter_counter);
+
+/**
+ * This method de-allocates all memory allocated to an alts_coutner instance.
+ * - crypter_counter: an alts_counter instance.
+ */
+void alts_counter_destroy(alts_counter* crypter_counter);
+
+#endif /* GRPC_CORE_TSI_ALTS_FRAME_PROTECTOR_ALTS_COUNTER_H */