How can I ensure forward secrecy when storing encrypted messages in a public blockchain? I want to store encrypted sensitive messages in the blockchain.

I was thinking of using public-key cryptography to achieve this, but as the encrypted data will be stored forever in the blockchain and this data will still be sensitive 100 years from now I am afraid there will be a possibility in the future for people to break the encryption (using quantum computing for example).

Is there a way for me to achieve forward secrecy that will likely not be broken in the near future ?

Thank you

EDIT : I can tolerate a few messages being decrypted in the future (out of the tens of thousands I want to store), but I would like to keep that number as low as possible (less than 0.1%)
As long as you use a post-quantum secure encryption scheme the fact that your messages are on the blockchain is not a problem. Traditionally adversaries are assumed to be capable of recording the traffic on an encrypted channel so theoretically they have always been allowed to keep messages for future decryption.

Currently the best way to get the necessary security is probably to use a symmetric scheme because their security is believed to only halve against a quantum computer. Or you pick your favorite post-quantum crypto scheme and combine it with a traditional one. For example Frodo is pretty conservative in its choices and in the paper the authors describe a combination with ECDHE.

Note that forward secrecy is not actually a property of the stored messages but is about the receiver of the message throwing away the key. The goal is that if somebody compromises the sender or receiver in the future they won't find a key that allows them do decrypt those old messages. In this case the difficult part is for the sender to know the current key. That's why this is only used in interactive protocols in which both parties exchange messages. Otherwise they can not agree on a shared secret without one party proposing it and thus having to encrypt it with some long term key.