Digital signatures are based on cryptographic algorithms. National laws aim for the evidentiary equivalence of digital and electronic signatures with handwritten signatures. The forgery resistance of digital signatures is a crucial requirement. The book presents the known methods for validating digital signatures, analyzes problems, and proposes new solutions. It takes into account the provisions of the Signature Act and X.509. The theory regarding the submission and access of declarations under the German Civil Code (BGB) is contrasted with the process of signed and electronically transmitted messages, identifying existing risk potentials and developing a model for the fair acknowledgment of digitally signed messages as well as for the realization of multi-party secure e-commerce applications.