Tham khảo tài liệu 'hacker professional ebook part 189', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | see Question the set of validating hash values cannot be forged. The time associated with the document by the time-stamp is the date of publication. The use of a DTS would appear to be extremely important if not essential for maintaining the validity of documents over many years see Question . Suppose a landlord and tenant sign a twenty-year lease. The public keys used to sign the lease will expire after say two years solutions such as recertifying the keys or resigning every two years with new keys require the cooperation of both parties several years after the original signing. If one party becomes dissatisfied with the lease he or she may refuse to cooperate. The solution is to register the lease with the DTS at the time of the original signing both parties would then receive a copy of the time-stamp which can be used years later to enforce the integrity of the original lease. In the future it is likely that a DTS will be used for everything from long-term corporate contracts to personal diaries and letters. Today if an historian discovers some lost letters of Mark Twain their authenticity is checked by physical means. But a similar find 100 years from now may consist of an author s computer files digital time-stamps may be the only way to authenticate the find. 4 Factoring and Discrete Log What is a one-way function A one-way function is a mathematical function that is significantly easier to perform in one direction the forward direction than in the opposite direction the inverse direction . One might for example compute the function in minutes but only be able to compute the inverse in months or years. A trap-door one-way function is a one-way function where the inverse direction is easy if you know a certain piece of information the trap door but difficult otherwise. What is the significance of one-way functions for cryptography Public-key cryptosystems are based on presumed trap-door one-way functions. The public key gives information .
