Nelson da Silva,  Thiago Acórdi Ramos,  Ricardo Felipe Custódio. 

Digital signatures are usually employed as the digital counterpart of handwritten signatures, allowing the authentication of electronic documents. These signatures, however, may quickly lose its validity, creating a preservation challenge for those documents that must be kept for a longer period of time. In this paper, we improve the efficiency and reliability of the usual approach for this problem, through a new time-stamp scheme. Such time-stamps carries a Certificate of Authenticity, with reduces its storage and validation costs, while protecting the signature even in the presence of an adversary able to compromise the Time Stamping Authority's private key or its signature scheme.

http://www.lbd.dcc.ufmg.br/colecoes/sbseg/2011/006.pdf

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