To upgrade the Bitcoin network to an anti quantum algorithm, hard fork is currently the most direct way, but it is not the only way. A hard fork will cause significant changes to the Bitcoin protocol, requiring all users and miners to upgrade their software to support new quantum resistant encryption algorithms. However, the implementation of quantum resistant encryption does not necessarily rely on hard forks, and other forms of upgrade or transition schemes can also be considered, although hard forks are still the most likely choice. Is only a hard fork feasible? Hard fork is not the only option, but it is the most effective and direct way. To ensure that all users can smoothly transition to the new anti quantum encryption algorithm, the following methods may be taken: Soft fork: Soft fork is usually used for protocol upgrades that are backward compatible, where updated nodes can still communicate with old version nodes. If the anti quantum encryption algorithm is designed as a soft fork, it may allow the old version of the private key to continue to be used, but the new version of the private key will adopt the quantum resistant algorithm. Although soft forks can make the upgrade process smoother, implementing anti quantum encryption usually requires more complex designs, so hard forks are more suitable in this situation. Double signature mechanism: During the transition period, the Bitcoin network may use both traditional ECDSA signatures and new anti quantum signatures simultaneously. This can allow users to gradually transition to new algorithms without losing access to existing private keys. • Choice of forked chain: Bitcoin can have a quantum resistant forked chain, where users can choose to continue using the old chain or migrate to the new chain. This method may not require excessive intervention in the existing Bitcoin blockchain, but it will result in the existence of two parallel blockchains, and users must choose one of them. 2. If a person is imprisoned, can they still use their old private key to obtain the latest version of Bitcoin after being released? This depends on how the Bitcoin network implements quantum resistant encryption algorithms. If a hard fork is implemented: If Bitcoin switches to quantum resistant encryption algorithm through a hard fork, all old private keys remain valid but can only be used on the old Bitcoin chain. In other words, if you have a private key for a Bitcoin address and owned Bitcoin before the hard fork, you can continue to use your original private key for transactions on the old chain after the hard fork, provided that you still hold the private key and it has not been broken by a quantum computer. If a hard fork causes two chains to coexist: If the quantum resistance upgrade is achieved through a hard fork (causing two chains to coexist), you can still retrieve Bitcoin from the old chain using the old private key. However, if you wish to use Bitcoin on a new chain (i.e. quantum resistant chain), you will need to migrate to the new chain and operate with a new quantum resistant private key. There is usually a 'replication' mechanism (similar to the fork events of Bitcoin Cash and Bitcoin), where users can access assets on the new chain by verifying a new quantum resistant private key. If the quantum resistant private key is compatible with the existing private key: If Bitcoin adopts a transition scheme that makes the newly generated private key (quantum resistant key) compatible with the existing private key, then even if the old private key is still valid, the new Bitcoin can be mapped onto the old private key. This means that users do not need to worry about being separated by a hard fork and can continue to access and control assets on the new chain through the original private key. 3. Quantum resistance and long-term validity of private keys If the Bitcoin network is upgraded to anti quantum encryption (whether through hard forks or other schemes), theoretically, old private keys can still access assets on the old blockchain. The private keys of Bitcoin (currently based on ECDSA) are very secure under existing technological conditions, but if quantum computers are powerful enough to crack ECDSA, then all private keys that rely on the algorithm (including those that have existed for many years) may become insecure. If a person is absent for a long time (such as going to jail), and during this period the Bitcoin network upgrades to anti quantum encryption algorithms, they can still access old versions of Bitcoin using their old private keys. But to access the new version of Bitcoin (i.e. assets on the quantum resistance chain), they may need to map the old private key to the new quantum resistance system through some means, such as migration or private key update. summary Hard fork is the most direct and common way to upgrade Bitcoin to quantum resistant encryption, but it can also be done through methods such as soft fork, double signature, or fork chain. If a person goes to jail, theoretically they can still access Bitcoin on the old chain using their old private key after a hard fork, but cannot directly access Bitcoin on the new chain unless they participate in the new chain by migrating or updating their private key. For Bitcoin users, keeping their private keys safe is the most important thing. Whether it's anti quantum encryption or any other changes, it is necessary to ensure that the private keys can be safely used in future network upgrades.