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Let's assume a dishonest Alice who sends, encrypts & digital signs a message to Bob.

Bob stores the decrypted message and the digital signature in a database.

However Alice is a bad girl and erases all data from her side, including her private/public keys. She then claims she never sent the message.

Is there a solution to this case?

In my case, Alice uses a communication tool that Bob has provided, where the private/public key are generated within the context of the tool (with no 'public key' advertised somewhere else, hence making it even more difficult to prove anything)

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  • $\begingroup$ Who's the dishonest third party? Alice isn't a third party; she's a normal party to the communication, whereas a third party is someone who's not supposed to be directly involved with the communication. $\endgroup$ – cpast Feb 1 '15 at 17:13
  • $\begingroup$ correct - I have changed the title $\endgroup$ – user3684457 Feb 1 '15 at 17:41
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Yes, the solution is that her public key needs to be stored in a place that is and will be accessible to Bob.
(such as in Bob's database)

I had not realized that importance of that before reading your question.

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  • $\begingroup$ however, if Alice dishonestly deletes her public key from her device, she can always claim that whatever public key is stored on the server was not hers :( $\endgroup$ – user3684457 Feb 1 '15 at 22:19
  • $\begingroup$ Yes. $\:$ That issue is also fundamental for nonrepudiation, but it's independent $\hspace{1.75 in}$ of the issue raised in your opening post. $\;\;\;\;$ $\endgroup$ – user991 Feb 1 '15 at 22:34
  • $\begingroup$ So how can I solve this aspect of nonrepudiation? $\endgroup$ – user3684457 Feb 1 '15 at 22:50
  • $\begingroup$ The most generic way is a (physically) signed contract stating that, in return for something, Alice will be bound to a certain extent by signed messages that are compatible with [specific public key]. $\:$ One potential alternative is a (video-and-audio) recording of Alice stating that she will be bound to a certain extent by signed messages that are compatible with [specific public key]. $\;\;\;\;$ $\endgroup$ – user991 Feb 1 '15 at 22:59
  • $\begingroup$ Ok - I am not sure yet about my math there, but does that also applies to any ephemeral key form a ECDHE? (the long public key binding therefore any ephemeral key?) $\endgroup$ – user3684457 Feb 1 '15 at 23:02

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