642965c678
No-Typo-Check: Not related to this change. Bug: 161477208 Change-Id: I99e4780f6855b7045aa0cd5a49c13d2d0d51ed64
47 lines
1.6 KiB
C++
47 lines
1.6 KiB
C++
// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
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// source code may only be used and distributed under the Widevine License
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// Agreement.
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#ifndef WVCDM_UTIL_ADVANCE_IV_CTR_H_
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#define WVCDM_UTIL_ADVANCE_IV_CTR_H_
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#include <stdint.h>
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#include <string.h>
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#include "string_conversions.h"
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namespace wvutil {
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// Advance an IV according to ISO-CENC's CTR modes. The lower half of the IV is
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// split off and treated as an unsigned 64-bit integer, then incremented by the
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// number of complete crypto blocks decrypted. The resulting value is then
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// copied back into the IV over the previous lower half.
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inline void AdvanceIvCtr(uint8_t (*subsample_iv)[16], size_t bytes) {
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constexpr size_t kAesBlockSize = 16;
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constexpr size_t kIvSize = kAesBlockSize;
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constexpr size_t kCounterIndex = kIvSize / 2;
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constexpr size_t kCounterSize = kIvSize / 2;
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uint64_t counter;
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static_assert(
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sizeof(*subsample_iv) == kIvSize,
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"The subsample_iv field is no longer the length of an AES-128 IV.");
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static_assert(sizeof(counter) == kCounterSize,
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"A uint64_t failed to be half the size of an AES-128 IV.");
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// Defensive copy because the elements of the array may not be properly
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// aligned
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memcpy(&counter, &(*subsample_iv)[kCounterIndex], kCounterSize);
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const size_t increment =
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bytes / kAesBlockSize; // The truncation here is intentional
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counter = htonll64(ntohll64(counter) + increment);
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memcpy(&(*subsample_iv)[kCounterIndex], &counter, kCounterSize);
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}
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} // namespace wvutil
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#endif // WVCDM_UTIL_ADVANCE_IV_CTR_H_
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