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6

In general, no. You can prove a message was written no later than time T by hashing the message with a collision-resistant hash, and communicating the hash to some trusted third party that records it was aware of the hash at time T. The bitcoin blockchain can be used as a third party here. You don't need to put the whole message in the blockchain, just a ...


1

If B needs to prove to A that B knows (or wrote) the message M at a certain time, this simple protocol can be used: A sends to B a uniformly distributed random NONCE, named R, of at least 128 bits. A records this date D1 from a clock that A trusts. B signs {M,R}. Let S be this signature. B sends M and S to A. A validates the signature S, the nonce R and the ...


-2

Gpg2 does not specifically exist to produce the time a message was composed, but it does record and transmit the time a file was made or updated. If you create a .txt file, for example, and you encrypt it with Gpg2, say using RSA and AES256, the literal data packet in the encrypted file will include the Unix time of when the file was created. You can check ...


0

One can prove that a message has been known for at least a certain amount of time. This can be done using a proof of sequential work or a (verifiable) delay function. The core idea is that the prover does a long (in wall clock time) computation on the message which proves that it must have been known to the prover for at least some time. These methods, ...


0

The algorithm you describe is accurate and was essentially described in the original Byzantine Generals Problem paper as the SM protocol - signed messages. One of the assumptions in the SM protocol (and in fact the entire paper) is that messages must be delivered, in other words, if a node chooses not to answer, that is detectible. This is only possible in a ...


1

Your warrant just needs a simple digital signature, which provides the properties of authenticity and non-repudiation. That is very basic. Of course you need a proper public key infrastructure (PKI) along that, to ensure that everyone actually knows which verification keys are real, etc. Regarding your different authorization levels, you might want to look ...


2

An additional example to complement the other answers: the classical zero-knowledge protocol of Goldwasser, Micali, and Rackoff, based on quadratic residuosity (here), is perfectly zero-knowledge (and its statistical soundness can be made negligibly small via sequential repetition). A long standing open problem has been to understand whether it remains zero-...


2

Theorem 3.2 of the following paper presents a constructive counterexample for ZK concurrent composition about the DL problem. It is a very interesting construction. Uriel Feige, Adi Shamir. Witness Indistinguishable and Witness Hiding Protocols (STOC'90). https://www.isical.ac.in/~rcbose/internship/lectures2016/rt02feigeshamir.pdf


10

Consider the function $f : \{L,R\} \times \{ U,D \} \to \{0,1,2\}$ defined by the following table: $$ \begin{array}{c|cc} f & L & R \\ \hline U & 0 & 0 \\ D & 1 & 2 \end{array} $$ Let's say Alice has input from $\{L,R\}$ (she chooses a column) and Bob has input from $\{U,D\}$ (he chooses a row). A ...


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