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2. German federal network agency: A presentation on validity models, http://www.bundesnetzagentur.de/media/archive/1343.pps
3. Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., Polk, W.: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. IETF Request For Comments, 5280 (May 2008)
4. Dierks, T., Rescorla, E.: The Transport Layer Security (TLS) Protocol Version 1.2. IETF Request For Comments, 5246 (August 2008)
5. ETSI. Electronic Signatures and Infrastructures (ESI): Electronic Signature Formats. TS 101 733 V1.7.4 (July 2008)