c++ - 如何获取/设置CFB_Mode加密对象的内部状态？

是否可以创建一个与以前创建的Encryption对象具有相同内部状态的新Encryption对象(我需要这样做，以便仅在函数调用之间保留IV，而不是整个对象)？

我认为使用GetNextIV函数应该有可能，但没有得到正确的结果。在以下示例中，字符串This is the text得到编码，正确的编码文本为:

94-41-d2-d4-06-05-f6-cd-d0-25-d6-f4-f6-52-55-7b-7c-

但是我得到:

94-a8-a9-b3-e0-a9-b3-e0-b4-a8-a5-e0-b4-a5-b8-b4-c0-

如您所见，仅第一个字节(94)被正确编码。您可以使用以下代码示例进行尝试:

#include <iostream>
#include <iomanip>
#include <crypto++/modes.h>
#include <crypto++/aes.h>
#include <crypto++/osrng.h>

using namespace CryptoPP;

void printHex(const byte in) {
    std::cout << std::setfill('0') << std::setw(2) << std::hex << (int)in << "-";
}

int main() {

    // Generate a random key and IV
    AutoSeededRandomPool rnd;
    SecByteBlock key(0x00, AES::DEFAULT_KEYLENGTH);
    rnd.GenerateBlock(key, key.size());
    byte iv[AES::BLOCKSIZE];
    rnd.GenerateBlock(iv, AES::BLOCKSIZE);

    // Encrypt byte by byte
    // this results in the correct encrypted text
    byte text[] = "This is the text";
    int msgLen = sizeof(text);
    CFB_Mode<AES>::Encryption cfbEncB(key, key.size(), iv);
    for (byte* beg = &text[0]; beg != &text[msgLen]; ++beg) {
        printHex(cfbEncB.ProcessByte(*beg));
    }
    std::cout << std::endl;

    // Encrypt byte by byte only keeping IV for each iteration
    // This is not the expected output, how to get it right?
    byte nextiv[AES::BLOCKSIZE];
    std::copy(&iv[0], &iv[AES::BLOCKSIZE], &nextiv[0]);
    for (byte* beg = &text[0]; beg != &text[msgLen]; ++beg) {
        CFB_Mode<AES>::Encryption cfbEncC(key, key.size(), nextiv);
        printHex(cfbEncC.ProcessByte(*beg));
        cfbEncC.GetNextIV(nextiv);
    }
    std::cout << std::endl;
}

最佳答案

可悲的是没有。这是GetNextIV对cryptlib.h的评论:

//! get a secure IV for the next message
/*! This method should be called after you finish encrypting one message and are ready to start the next one.
    After calling it, you must call SetKey() or Resynchronize() before using this object again.
    This method is not implemented on decryption objects. */
virtual void GetNextIV(RandomNumberGenerator &rng, byte *IV);

因此，GetNextIV用于获取满足不同消息的密码IV要求的IV，而不是当前消息的内部状态。

它不显示内部状态的第二个线索是PRNG参数。如果使用NullRNG()，则应引发异常。内部状态不应随机:)



我认为您正在寻找的内部状态在m_register和m_temp中。它们受到保护并且缺少访问器，因此您将需要修改库。

从modes.h:

class CRYPTOPP_DLL CRYPTOPP_NO_VTABLE CFB_ModePolicy : public ModePolicyCommonTemplate<CFB_CipherAbstractPolicy>
{
public:
    IV_Requirement IVRequirement() const {return RANDOM_IV;}
    static const char * CRYPTOPP_API StaticAlgorithmName() {return "CFB";}

protected:
    unsigned int GetBytesPerIteration() const {return m_feedbackSize;}
    byte * GetRegisterBegin() {return m_register + BlockSize() - m_feedbackSize;}
    bool CanIterate() const {return m_feedbackSize == BlockSize();}
    void Iterate(byte *output, const byte *input, CipherDir dir, size_t iterationCount);
    void TransformRegister();
    void CipherResynchronize(const byte *iv, size_t length);
    void SetFeedbackSize(unsigned int feedbackSize);
    void ResizeBuffers();

    SecByteBlock m_temp;
    unsigned int m_feedbackSize;
};

从modes.cpp:

void CFB_ModePolicy::Iterate(byte *output, const byte *input, CipherDir dir, size_t iterationCount)
{
    assert(m_cipher->IsForwardTransformation());    // CFB mode needs the "encrypt" direction of the underlying block cipher, even to decrypt
    assert(m_feedbackSize == BlockSize());

    unsigned int s = BlockSize();
    if (dir == ENCRYPTION)
    {
        m_cipher->ProcessAndXorBlock(m_register, input, output);
        m_cipher->AdvancedProcessBlocks(output, input+s, output+s, (iterationCount-1)*s, 0);
        memcpy(m_register, output+(iterationCount-1)*s, s);
    }
    else
    {
        memcpy(m_temp, input+(iterationCount-1)*s, s);  // make copy first in case of in-place decryption
        m_cipher->AdvancedProcessBlocks(input, input+s, output+s, (iterationCount-1)*s, BlockTransformation::BT_ReverseDirection);
        m_cipher->ProcessAndXorBlock(m_register, input, output);
        memcpy(m_register, m_temp, s);
    }
}

void CFB_ModePolicy::TransformRegister()
{
    assert(m_cipher->IsForwardTransformation());    // CFB mode needs the "encrypt" direction of the underlying block cipher, even to decrypt
    m_cipher->ProcessBlock(m_register, m_temp);
    unsigned int updateSize = BlockSize()-m_feedbackSize;
    memmove_s(m_register, m_register.size(), m_register+m_feedbackSize, updateSize);
    memcpy_s(m_register+updateSize, m_register.size()-updateSize, m_temp, m_feedbackSize);
}

这个比较容易。只需调用SetKeyWithIV或Resynchronize即可。