Given the increased speculation about Bitcoin Unlimited splitting away from the current version of Bitcoin, we felt like it would be reassuring to provide you with an updated article on the possible fallouts.

In the article from Feb 17, 2016 about the Core/Classic hard fork possibility, we explained that at the time of hard fork, we would provide you with two separate wallets, a core-wallet and a classic-wallet. Since then, we have implemented multicurrency support in TREZOR Wallet, so a separation at the domain level will no longer be necessary. Should Bitcoin Unlimited (BTU) split away and be implemented in TREZOR Wallet, it would be provided as another option in the currency selector.

Therefore, in terms of the security of your coins, a little has changed — no action is required from you & your coins remain safe. You will not magically lose coins because of a fork.

I don’t understand, what is a hard fork?

To explain this simply, let’s forget the bitcoin-specific terms.

Everyone within the Bitcoin network is currently adhering to a set of rules, laid out by Bitcoin Core. Citing alleged limitations, a competing group, Bitcoin Unlimited, has proposed a change to the rules. The change is, however, incompatible with the current Bitcoin and would therefore create a “new coin,” while carrying over the entire transaction history of the currency.

Under normal circumstances, a change of rules would only occur if the whole network would agree on the new rules. Only one coin would emerge, causing no problem. However, as this split is caused by the very disagreement on the rules, initiated by Bitcoin Unlimited, it is very unlikely that Bitcoin Core would give in and would move to Unlimited. As a result, at the moment of the split, there would be two coins, each adhering to their own rules (also known as: Contentious Hard Fork).

So I will have both Bitcoin Core and Bitcoin Unlimited coins?

Yes. If, at the time of the fork, you own a certain amount of bitcoins, after the fork you will hold the same amount in BTC and BTU (doubling the amount). After the fork, these coins will be incompatible. They will effectively become separate currencies.

(Note that: double the amount ≠ double the value)

How safe are my coins?

TREZOR holds your private keys, which are used to sign transactions. Having these keys makes you the true owner of the coins — only the one who possesses the keys can spend the coins. With the hard fork, the ownership of the keys does not change. On the contrary, you will gain additional coins, controlling both the keys of Bitcoin and Bitcoin Unlimited.

Will TREZOR (the device) support Bitcoin Unlimited?

As Bitcoin Unlimited will be a fork of Bitcoin, from a technological point of view there is no reason for TREZOR not to support BTU.

Will TREZOR Wallet support Bitcoin Unlimited?

This will be contingent on the availability of a Bitcoin Unlimited backend. The support will be announced at a later stage, after the fork, due to the necessary updates (see below). However, this does not mean that your BTU coins will be inaccessible. You can use your TREZOR with a TREZOR-compatible third party wallet that supports Bitcoin Unlimited coins.

If you need your BTU coins right at the moment of the fork, you can restore your recovery seed in a BTU-compatible wallet. Note that through this method you will compromise your seed and therefore we strongly recommend you to move all of your coins, including altcoins, to a different seed.

(Advanced) What about replay attacks?

A replay attack is when a transaction on one chain is retransmitted on the second chain, without the owner’s knowledge. For instance, if you wanted to send 2 bitcoins to a certain address, an attacker could reuse your signed transaction on the BTU chain, causing you to spend coins on both chains, even though you only wanted to spend coins on one.

TREZOR developers are working on updating the TREZOR Wallet to protect your wallet from replay attacks. This is an important task, required to keep your coins safe.