#include "Poco/HMACEngine.h"
#include "Poco/SHA1Engine.h"
using Poco::DigestEngine;
using Poco::HMACEngine;
using Poco::SHA1Engine;
int main(int argc, char** argv)
{
std::string message1("This is a top-secret message.");
std::string message2("Don't tell anyone!");
std::string passphrase("s3cr3t"); // HMAC needs a passphrase
HMACEngine hmac(passphrase); // we'll compute a HMAC-SHA1
hmac.update(message1);
hmac.update(message2);
const DigestEngine::Digest& digest = hmac.digest();
// finish HMAC computation and obtain digest
std::string digestString(DigestEngine::digestToHex(digest));
// convert to a string of hexadecimal numbers
return 0;
}

2.3 与DigestEngine类相关的流(DigestInputStream/DigestOutputStream)
可以通过Poco::DigestInputStream和Poco::DigestOutputStream类对DigestEngine类进行输入输出操作。过程很简单，只要在在构造Stream时，把相关的DigestEngine类传入即可。需要注意的是，在向DigestOutputStream类写入后，要及时调用flush函数，以确保Stream把所有数据都输入进DigestEngine类。

下面是相关的一个例子：

[cpp]
#include "Poco/DigestStream.h"
#include "Poco/MD5Engine.h"
using Poco::DigestOutputStream;
using Poco::DigestEngine;
using Poco::MD5Engine;
int main(int argc, char** argv)
{
MD5Engine md5;
DigestOutputStream ostr(md5);
ostr << "This is some text";
ostr.flush(); // Ensure everything gets passed to the digest engine
const DigestEngine::Digest& digest = md5.digest(); // obtain result
std::string result = DigestEngine::digestToHex(digest);
return 0;
}