In this talk an important tool from cryptography is explained: digital signature. It is shown how digital signatures function and why they are fundamental for the security of modern information systems. The traditional realization of digital signatures is presented. Their security relies on the difficulty of certain number theoretic problems. This difficulty is not guaranteed. It is shown that quantum computers may render traditional digital signatures insecure. The minimum security requirements for digital signatures are discussed and a scheme is presented that only relies on these requirements. This scheme, the extended Merkle Signature Scheme XMSS, can solve the problem of providing future digital signatures.