1. Electronic signature Validity Model 1

2. Shell model Certificate 1 Certificate 2 Certificate 3 Signed document Generate valid signature validCheck invalidCheck Time 2 Electronic signature

3. Signed document Chain model Generate valid signature valid Certificate 1 Certificate 3 Certificate 2 Check Time 3 Electronic signature

4. Chain model: multiple verification Document A Document B Document C Verify signature: Certificate 1 Certificate 3 Certificate 2 Document A Time Document B Document C ? ! 4 Electronic signature

5. Algorithms Certificate 1 Certificate 2 Shell model Chain model Hybrid model Time Generate valid signature Signature validSignature invalid Generate valid signature Signature valid Generate valid signature Signature valid 5 Electronic signature

6. Root CA CA User Chain model Hybrid model Time [a] Generate valid signature (max. 3a) Signature valid Generate valid signature (max. 1a) Signature valid 123456 6 Electronic signature