Depending on how time travel works that might have a huge hole.
1. You pick a hash of the headline from the January 14th 2030 of the Springfield Shopper as your key and encrypt some data.
2. You use your time machine to send the encrypted data to someone in 2024 with instructions telling them how to recover the key, expecting that they will have to wait until January 14th 2030.
3. Some rich person decides they do not want to wait. They notice that January 14th is National Hot Pastrami Sandwich Day [1].
4. They buy the Springfield Shopper and institute a policy that every year on January 14th the headline will be "Happy National Hot Pastrami Sandwich Day!".
5. If we are in the kind of universe where time travel to the past can affect the time that the time traveler came from their actions in #4 make it so "Happy National Hot Pastrami Sandwich Day!" is the headline you used.
6. In 2024 they try to decrypt the data using a hash of "Happy National Hot Pastrami Sandwich Day!" and it works.
1. You pick a hash of the headline from the January 14th 2030 of the Springfield Shopper as your key and encrypt some data.
2. You use your time machine to send the encrypted data to someone in 2024 with instructions telling them how to recover the key, expecting that they will have to wait until January 14th 2030.
3. Some rich person decides they do not want to wait. They notice that January 14th is National Hot Pastrami Sandwich Day [1].
4. They buy the Springfield Shopper and institute a policy that every year on January 14th the headline will be "Happy National Hot Pastrami Sandwich Day!".
5. If we are in the kind of universe where time travel to the past can affect the time that the time traveler came from their actions in #4 make it so "Happy National Hot Pastrami Sandwich Day!" is the headline you used.
6. In 2024 they try to decrypt the data using a hash of "Happy National Hot Pastrami Sandwich Day!" and it works.
[1] https://www.daysoftheyear.com/days/hot-pastrami-sandwich-day...