@book { ElektronischeZahlungssysteme,
    title = "Elektronische Zahlungssysteme",
    author = "Stephanie {Kretschmar}",
    year = "2005",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832487379",
    url = "https://m.diplom.de/document/223923"
}