dom/quota/EncryptedBlock.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
   5  * You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_dom_quota_EncryptedBlock_h
   8 #define mozilla_dom_quota_EncryptedBlock_h
   9
  10 #include <cstdint>
  11 #include <cstring>
  12 #include <limits>
  13 #include "mozilla/Assertions.h"
  14 #include "mozilla/Span.h"
  15 #include "nsTArray.h"
  16
  17 namespace mozilla::dom::quota {
  18
  19 // An encrypted block has the following format:
  20 // - one basic block containing a uint16_t stating the actual payload length
  21 // - one basic block containing the cipher prefix (tyipically a nonce)
  22 // - encrypted payload up to the remainder of the specified overall size
  23 // We currently assume the basic block size is the same as the cipher prefix
  24 // length.
  25 //
  26 // XXX Actually, we don't need the actual payload length in every block. Only
  27 // the last block may be incomplete. The tricky thing is just that it might be
  28 // incomplete by just one or two bytes.
  29 template <size_t CipherPrefixLength, size_t BasicBlockSize>
  30 class EncryptedBlock {
  31  public:
  32   explicit EncryptedBlock(const size_t aOverallSize) {
  33     MOZ_RELEASE_ASSERT(aOverallSize >
  34                        CipherPrefixOffset() + CipherPrefixLength);
  35     MOZ_RELEASE_ASSERT(aOverallSize <= std::numeric_limits<uint16_t>::max());
  36     // XXX Do we need this to be fallible? Then we need a factory/init function.
  37     // But maybe that's not necessary as the block size is not user-provided and
  38     // small.
  39     mData.SetLength(aOverallSize);
  40
  41     // Bug 1867394: Making sure to zero-initialize first block as there might
  42     // be some unused bytes in it which could expose sensitive data.
  43     // Currently, only sizeof(uint16_t) bytes gets used in the first block.
  44     std::fill(mData.begin(), mData.begin() + CipherPrefixOffset(), 0);
  45     SetActualPayloadLength(MaxPayloadLength());
  46   }
  47
  48   size_t MaxPayloadLength() const {
  49     return mData.Length() - CipherPrefixLength - CipherPrefixOffset();
  50   }
  51
  52   void SetActualPayloadLength(uint16_t aActualPayloadLength) {
  53     memcpy(mData.Elements(), &aActualPayloadLength, sizeof(uint16_t));
  54   }
  55   size_t ActualPayloadLength() const {
  56     return *reinterpret_cast<const uint16_t*>(mData.Elements());
  57   }
  58
  59   using ConstSpan = Span<const uint8_t>;
  60   using MutableSpan = Span<uint8_t>;
  61
  62   ConstSpan CipherPrefix() const {
  63     return WholeBlock().Subspan(CipherPrefixOffset(), CipherPrefixLength);
  64   }
  65   MutableSpan MutableCipherPrefix() {
  66     return MutableWholeBlock().Subspan(CipherPrefixOffset(),
  67                                        CipherPrefixLength);
  68   }
  69
  70   ConstSpan Payload() const {
  71     return WholeBlock()
  72         .SplitAt(CipherPrefixOffset() + CipherPrefixLength)
  73         .second.First(RoundedUpToBasicBlockSize(ActualPayloadLength()));
  74   }
  75   MutableSpan MutablePayload() {
  76     return MutableWholeBlock()
  77         .SplitAt(CipherPrefixOffset() + CipherPrefixLength)
  78         .second.First(RoundedUpToBasicBlockSize(ActualPayloadLength()));
  79   }
  80
  81   ConstSpan WholeBlock() const { return mData; }
  82   MutableSpan MutableWholeBlock() { return mData; }
  83
  84  private:
  85   static constexpr size_t CipherPrefixOffset() {
  86     return RoundedUpToBasicBlockSize(sizeof(uint16_t));
  87   }
  88
  89   static constexpr size_t RoundedUpToBasicBlockSize(const size_t aValue) {
  90     return (aValue + BasicBlockSize - 1) / BasicBlockSize * BasicBlockSize;
  91   }
  92
  93   nsTArray<uint8_t> mData;
  94 };
  95
  96 }  // namespace mozilla::dom::quota
  97
  98 #endif