请用qt写出对字符串加解密的例子

当然可以，以下是一个使用Qt实现字符串加解密的例子：

#include <QtCore/QCoreApplication>
#include <QtCore/QDebug>
#include <QtCore/QCryptographicHash>

QString encrypt(QString plaintext, QString key)
{
    QByteArray keyBytes = key.toUtf8();
    QByteArray plaintextBytes = plaintext.toUtf8();
    QByteArray hash = QCryptographicHash::hash(keyBytes, QCryptographicHash::Sha256);
    QByteArray iv = QCryptographicHash::hash(keyBytes, QCryptographicHash::Md5);
    QAESEncryption encryption(QAESEncryption::AES_256, QAESEncryption::CBC);
    encryption.setKey(hash);
    encryption.setIV(iv);
    QByteArray ciphertextBytes = encryption.encode(plaintextBytes);
    QString ciphertext = QString::fromUtf8(ciphertextBytes.toBase64());
    return ciphertext;
}

QString decrypt(QString ciphertext, QString key)
{
    QByteArray keyBytes = key.toUtf8();
    QByteArray ciphertextBytes = QByteArray::fromBase64(ciphertext.toUtf8());
    QByteArray hash = QCryptographicHash::hash(keyBytes, QCryptographicHash::Sha256);
    QByteArray iv = QCryptographicHash::hash(keyBytes, QCryptographicHash::Md5);
    QAESEncryption decryption(QAESEncryption::AES_256, QAESEncryption::CBC);
    decryption.setKey(hash);
    decryption.setIV(iv);
    QByteArray plaintextBytes = decryption.decode(ciphertextBytes);
    QString plaintext = QString::fromUtf8(plaintextBytes);
    return plaintext;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QString plaintext = "Hello, world!";
    QString key = "mysecretkey";

    QString ciphertext = encrypt(plaintext, key);
    qDebug() << "Ciphertext:" << ciphertext;

    QString decryptedText = decrypt(ciphertext, key);
    qDebug() << "Decrypted text:" << decryptedText;

    return a.exec();
}

这个例子使用了Qt的QCryptographicHash和QAESEncryption类来实现AES-256加解密。在加密时，我们使用SHA-256哈希算法和MD5哈希算法来生成密钥和初始化向量。在解密时，我们使用相同的密钥和初始化向量来解密密文。