libcryfs/test/implementations/encrypted/testutils/FakeAuthenticatedCipher.h

#pragma once
#ifndef MESSMER_BLOCKSTORE_TEST_IMPLEMENTATIONS_ENCRYPTED_TESTUTILS_FAKEAUTHENTICATEDCIPHER_H_
#define MESSMER_BLOCKSTORE_TEST_IMPLEMENTATIONS_ENCRYPTED_TESTUTILS_FAKEAUTHENTICATEDCIPHER_H_

#include "../../../../implementations/encrypted/ciphers/Cipher.h"
#include <messmer/cpp-utils/data/FixedSizeData.h>

struct FakeKey {
  FakeKey(uint8_t value_):value(value_) {}
  static FakeKey CreateOSRandom() {
    return FakeKey(rand());
  }
  static FakeKey FromBinary(const void *data) {
    return FakeKey(*(uint8_t*)data);
  }
  static constexpr unsigned int BINARY_LENGTH = 1;

  uint8_t value;
};

// This is a fake cipher that uses an indeterministic caesar chiffre and a 4-byte parity for a simple authentication mechanism
class FakeAuthenticatedCipher {
public:
  BOOST_CONCEPT_ASSERT((blockstore::encrypted::CipherConcept<FakeAuthenticatedCipher>));

  using EncryptionKey = FakeKey;

  static EncryptionKey Key1() {
    return FakeKey(5);
  }
  static EncryptionKey Key2() {
    return FakeKey(63);
  }

  static constexpr unsigned int ciphertextSize(unsigned int plaintextBlockSize) {
    return plaintextBlockSize + 5;
  }

  static constexpr unsigned int plaintextSize(unsigned int ciphertextBlockSize) {
    return ciphertextBlockSize - 5;
  }

  static cpputils::Data encrypt(const byte *plaintext, unsigned int plaintextSize, const EncryptionKey &encKey) {
    cpputils::Data result(ciphertextSize(plaintextSize));

    //Add a random IV
    uint8_t iv = rand();
    std::memcpy(result.data(), &iv, 1);

    //Use caesar chiffre on plaintext
    _caesar((byte*)result.data() + 1, plaintext, plaintextSize, encKey.value + iv);

    //Add parity information
    int32_t parity = _parity((byte*)result.data(), plaintextSize + 1);
    std::memcpy((byte*)result.data() + plaintextSize + 1, &parity, 4);

    return result;
  }

  static boost::optional<cpputils::Data> decrypt(const byte *ciphertext, unsigned int ciphertextSize, const EncryptionKey &encKey) {
    //We need at least 5 bytes (iv + parity)
    if (ciphertextSize < 5) {
      return boost::none;
    }

    //Check parity
    int32_t expectedParity = _parity(ciphertext, plaintextSize(ciphertextSize) + 1);
    int32_t actualParity = *(int32_t*)(ciphertext + plaintextSize(ciphertextSize) + 1);
    if (expectedParity != actualParity) {
      return boost::none;
    }

    //Decrypt caesar chiffre from ciphertext
    int32_t iv = *(int32_t*)ciphertext;
    cpputils::Data result(plaintextSize(ciphertextSize));
    _caesar((byte*)result.data(), ciphertext + 1, plaintextSize(ciphertextSize), -(encKey.value+iv));
    return std::move(result);
  }

private:
  static int32_t _parity(const byte *data, unsigned int size) {
    int32_t parity = 34343435; // some init value
    int32_t *intData = (int32_t*)data;
    unsigned int intSize = size / sizeof(int32_t);
    for (unsigned int i = 0; i < intSize; ++i) {
      parity += intData[i];
    }
    unsigned int remainingBytes = size - 4 * intSize;
    for (unsigned int i = 0; i < remainingBytes; ++i) {
      parity += (data[4*intSize + i] << (24 - 8*i));
    }
    return parity;
  }

  static void _caesar(byte *dst, const byte *src, unsigned int size, uint8_t key) {
    for (unsigned int i = 0; i < size; ++i) {
      dst[i] = src[i] + key;
    }
  }
};

#endif