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I was wondering if there's any option to create sort of a time capsule in a Word document, i.e. not encrypted by password rather than a date. Let's say I send the document today but the receiving side can open the document only the next month on a specific date.

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I know no feature like this built into any word processor. And, despite work on so-called time-capsule cryptography, we know no practical[*] way to implement one without relying on trusted hardware or trusted third party.

To achieve this, we can use the help of a trusted third party/server. It can generate and publish public keys for which it will publicly release the private keys at a set date. The sender enciphers the message with the public key selected per the desired release date. The receiver can't decipher until that date. To do so, s/he must get online, get the private key (now published) from the third party, then decipher.


[*] Problems with proposed time-capsule cryptography systems requiring no trusted external hardware/party are that their delay of release is too imprecise, and they would be prohibitively costly. They achieve timed release by requiring sequential iteration of a certain computation, assumed to require at least a certain time $T_0$. To obtain a delay $T$, the receiver is forced to perform $t=T/T_0$ operations, at full rate, during the whole timed release. But $T_0$ varies a lot with the technology, and the fasted high-speed logic is prohibitively expensive. That used to be GaAs, now perhaps InP, or is it unobtainium-on-diaphire immersed is liquid nitronium.

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    $\begingroup$ I'm saying that the method of using $x^{2^t}\bmod{pq}$ as a key is not practical. Problem is, we do not know how to map $t$ to a clock duration precisely enough. Ask different people and you'll get wildly varying estimates of how many modular squarings per second is achievable. And a release delays of 1 year becoming 6 month is a hell of a practical issue. Also, people receiving a document to be read in 1 year do not want to bear the cost of running a high-end ASIC full steam for that time, or worse purchasing a new one after 6 month and move the computation to this newer, faster gear. $\endgroup$ – fgrieu Mar 10 at 19:17
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    $\begingroup$ Wonder what the progress on LCS35 is. $\endgroup$ – Squeamish Ossifrage Mar 10 at 20:15
  • $\begingroup$ We have a question about the status of LCS35. $\endgroup$ – fgrieu Mar 11 at 7:59

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