The worlds most valuable information travels through the Internet today. Be it via e-mails, chats, video conferences, online banking, eBay, Amazon, or through other channels. The way in which we protect this data is by usage of public-key cryptography and digital signatures. For now, the systems of choice are RSA, Diffe-Hellman, DSA, and ECDSA. It is conjectured, that in about 20 years quantum computers will be able to break all these systems, rendering them useless. Therefore, new public-key systems and signature schemes are under extreme scrutiny, because the most secure and efficient of them may one day be responsible for our secrets. In this book, we have analyzed a particularly promising, quantum-secure digital signature scheme, namely NTRUSign. We explain this scheme in detail and give an overview of all relevant attacks, which in turn enables us to assess the security.