QSslKeyPrivate::decodePem(const QByteArray &pem, const QByteArray &passPhrase, bool deepClear) 
calls
q_PEM_read_bio_RSAPrivateKey(bio, &rsa, 0, phrase); 
The third parameter is the openssl passphrase callback. If it is NULL the default openssl passphrase callback asks through standard input the good passphrase and the application stalls.