使用AES / Crypto++解密

Question

我在想如何用AES解密一段密文。我正在使用crypto++库--或者至少尝试使用这个库。但我一点进展都没有。我假设只需要几行代码就可以运行这个解密算法，但是我搞不清楚。这就是我所写的。现在开始笑吧：

#include <stdio.h>
#include <cstdlib>
#include <rijndael.h>
#include <sha.h>

using namespace std;

int main()
{

    // Decryption
    CTR_Mode< AES >::Decryption decryptor;
    decryptor.SetKeyWithIV( cbcKey, AES::DEFAULT_KEYLENGTH, cbcCipher );
}

有没有人能给我一个简短的教程，告诉我如何使用crypto++在给定解密密钥的情况下“简单地”解密一个16字节的密文块？他们的文档比你在上面看到的密文更隐秘(至少对我来说是这样)，我通过搜索找到了一点帮助。非常感谢。

Answer 1

crypto++库网页上的常见问题解答包含指向“教程”的指针，请访问over there。

Answer 2

谁能给我一个简短的教程，告诉我如何使用crypto++在给定解密密钥的情况下“简单地”解密一个16字节的密文块？

这是来自Crypto++维基的。它提供了一个CTR mode example。

AutoSeededRandomPool prng;

SecByteBlock key(AES::DEFAULT_KEYLENGTH);
prng.GenerateBlock( key, key.size() );

byte ctr[ AES::BLOCKSIZE ];
prng.GenerateBlock( ctr, sizeof(ctr) );

string plain = "CTR Mode Test";
string cipher, encoded, recovered;

/*********************************\
\*********************************/

try
{
    cout << "plain text: " << plain << endl;

    CTR_Mode< AES >::Encryption e;
    e.SetKeyWithIV( key, key.size(), ctr );

    // The StreamTransformationFilter adds padding
    //  as required. ECB and CBC Mode must be padded
    //  to the block size of the cipher. CTR does not.
    StringSource ss1( plain, true, 
        new StreamTransformationFilter( e,
            new StringSink( cipher )
        ) // StreamTransformationFilter      
    ); // StringSource
}
catch( CryptoPP::Exception& e )
{
    cerr << e.what() << endl;
    exit(1);
}

/*********************************\
\*********************************/

// Pretty print cipher text
StringSource ss2( cipher, true,
    new HexEncoder(
        new StringSink( encoded )
    ) // HexEncoder
); // StringSource
cout << "cipher text: " << encoded << endl;

/*********************************\
\*********************************/

try
{
    CTR_Mode< AES >::Decryption d;
    d.SetKeyWithIV( key, key.size(), ctr );

    // The StreamTransformationFilter removes
    //  padding as required.
    StringSource ss3( cipher, true, 
        new StreamTransformationFilter( d,
            new StringSink( recovered )
        ) // StreamTransformationFilter
    ); // StringSource

    cout << "recovered text: " << recovered << endl;
}
catch( CryptoPP::Exception& e )
{
    cerr << e.what() << endl;
    exit(1);
}

下面是运行示例程序的结果：

$ ./crytpopp-test.exe 
key: F534FC7F0565A8CF1629F01DB31AE3CA
counter: A4D16CBC010DACAA2E54FA676B57A345
plain text: CTR Mode Test
cipher text: 12455EDB41020E6D751F207EE6
recovered text: CTR Mode Test

Answer 3

实际上，Crypto++教程很难理解。一般来说，为了使用Crypto++应用程序接口，你至少需要有一些关于密码学的基础知识，我建议你使用this Cryptography course。

话虽如此，让我来回答你的问题。因为它有点模棱两可，我假设你想要解密一个AES密码，使用CTR模式。作为输入，假设您有一个密码、IV和密钥(在std::string中都以十六进制表示)。

#include <iostream>
#include <string>

#include "crypto++/modes.h" // For CTR_Mode
#include "crypto++/filters.h" //For StringSource
#include "crypto++/aes.h" // For AES
#include "crypto++/hex.h" // For HexDecoder

int main(int argc, char* argv[]) {
    // Our input:
    // Note: the input was previously generated by the same cipher 
    std::string iv_string = "37C6D22FADE22B2D924598BEE2455EFC";
    std::string cipher_string = "221DF9130F0E05E7E87C89EE6A";
    std::string key_string = "7D9BB722DA2DC8674E08C3D44AAE976F";

    std::cout << "Cipher text: " << cipher_string << std::endl;
    std::cout << "Key: " << key_string << std::endl;
    std::cout << "IV: " << iv_string << std::endl;

    // 1. Decode iv: 
    // At the moment our input is encoded in string format...
    // we need it in raw hex: 
    byte iv[CryptoPP::AES::BLOCKSIZE] = {};
    // this decoder would transform our std::string into raw hex:
    CryptoPP::HexDecoder decoder;
    decoder.Put((byte*)iv_string.data(), iv_string.size());
    decoder.MessageEnd();
    decoder.Get(iv, sizeof(iv));

    // 2. Decode cipher:
    // Next, we do a similar trick for cipher, only here we would leave raw hex
    //  in a std::string.data(), since it is convenient for us to pass this
    // std::string to the decryptor mechanism:
    std::string cipher_raw;
    {
        CryptoPP::HexDecoder decoder;
        decoder.Put((byte*)cipher_string.data(), cipher_string.size());
        decoder.MessageEnd();

        long long size = decoder.MaxRetrievable();
        cipher_raw.resize(size);       
        decoder.Get((byte*)cipher_raw.data(), cipher_raw.size());
        // If we print this string it's completely rubbish: 
        // std::cout << "Raw cipher: " << cipher_raw << std::endl;
    }

    // 3. Decode the key:
    // And finally the same for the key:
    byte key[CryptoPP::AES::DEFAULT_KEYLENGTH];
    {
        CryptoPP::HexDecoder decoder;
        decoder.Put((byte*)key_string.data(), key_string.size());
        decoder.MessageEnd();
        decoder.Get(key, sizeof(key));
    }

    // 4. Decrypt:
    std::string decrypted_text;
    try {
            CryptoPP::CTR_Mode<CryptoPP::AES>::Decryption d;
            d.SetKeyWithIV(key, sizeof(key), iv);

            CryptoPP::StringSource ss(
                cipher_raw, 
                true, 
                new CryptoPP::StreamTransformationFilter(
                    d,
                    new CryptoPP::StringSink(decrypted_text)
                ) // StreamTransformationFilter
            ); // StringSource

            std::cout << "Decrypted text: " << decrypted_text << std::endl;
    }
    catch( CryptoPP::Exception& e ) {
            std::cerr << e.what() << std::endl;
            exit(1);
    }

    return 0;
}

我使用Crypto++562在Ubutunu14.04上编译了它：

g++ -Wall -std=c++0x -o prog practicalAES.cpp -lcryptopp

如果我运行这个程序，我会得到这样的输出：

Cipher text: 221DF9130F0E05E7E87C89EE6A Key: 7D9BB722DA2DC8674E08C3D44AAE976F IV: 37C6D22FADE22B2D924598BEE2455EFC Decrypted text: CTR Mode Test

它在这里并不真正可见，但是密钥和IV都有相同的长度- 16字节(或128位)。这是块大小，所以这个密码是AES-128。因为它是CTR模式，所以不添加填充，并且密码和明文具有相同的字节数。

还要注意，60%的代码涉及将字符串解码为十六进制，而解密本身只是最后一步(因此，如果您的输入数据是原始十六进制，则不需要解码)。