@book { DiedigitaleSignatur,
    title = "Die digitale Signatur",
    author = "Heiko {Heibel}",
    year = "2003",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832470678",
    url = "https://m.diplom.de/document/222386"
}