Well then that means you can change nothing and not time travel so all. Or you are saying “big events” are fixed? I do not understand how the universe would make that designation. Pause your grandfather on the street for one second, and your genetic heritage is completely different as a different sperm was the progenitor of your parent.
No you can still time travel, but yes you cannot change things. It has to be self consistent.
For example, if you plan to go back and kill your grandfather, you'd perhaps be run over by a car two blocks before you get to him, but if you had read past newspapers carefully before your trip, you would have noticed a short article at the end of the newspaper about some mysterious John Doe who wore funny clothes and got run over by a car just on the day you planned to kill your grandfather, which ended up being you, the time traveler.
In that case, there is no point in the time travel at all. If you do not already live in a universe where a future Oracle gave technological secrets to elevate humanity, you know the plan fails.
If you can send things to the past, just deliver your one time pad on the date that you want things to be decryptable. Or, depending on how exactly the time travel works, just use the encryption scheme that you know gets broken on exactly your target date.
