Widevine MediaCas client code that works with Android R

wvutil/include/advance_iv_ctr.h

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+// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+
+#ifndef WV_UTIL_ADVANCE_IV_CTR_H_
+#define WV_UTIL_ADVANCE_IV_CTR_H_
+
+#include <stdint.h>
+#include <string.h>
+
+#include "string_conversions.h"
+
+namespace wvutil {
+
+// Advance an IV according to ISO-CENC's CTR modes. The lower half of the IV is
+// split off and treated as an unsigned 64-bit integer, then incremented by the
+// number of complete crypto blocks decrypted. The resulting value is then
+// copied back into the IV over the previous lower half.
+inline void AdvanceIvCtr(uint8_t (*subsample_iv)[16], size_t bytes) {
+  constexpr size_t kAesBlockSize = 16;
+  constexpr size_t kIvSize = kAesBlockSize;
+  constexpr size_t kCounterIndex = kIvSize / 2;
+  constexpr size_t kCounterSize = kIvSize / 2;
+
+  uint64_t counter;
+
+  static_assert(
+      sizeof(*subsample_iv) == kIvSize,
+      "The subsample_iv field is no longer the length of an AES-128 IV.");
+  static_assert(sizeof(counter) == kCounterSize,
+                "A uint64_t failed to be half the size of an AES-128 IV.");
+
+  // Defensive copy because the elements of the array may not be properly
+  // aligned
+  memcpy(&counter, &(*subsample_iv)[kCounterIndex], kCounterSize);
+
+  const size_t increment =
+      bytes / kAesBlockSize;  // The truncation here is intentional
+  counter = htonll64(ntohll64(counter) + increment);
+
+  memcpy(&(*subsample_iv)[kCounterIndex], &counter, kCounterSize);
+}
+
+}  // namespace wvutil
+
+#endif  // WV_UTIL_ADVANCE_IV_CTR_H_