Is it safe if I kept sign transaction file .txn for long time?

[Not your key not your BTC]

As, title above, is it safe if I kept sign transaction file .txn for long time?

because I was waiting for the right time to broadcast it.

I was set it to high fee about 20 sat/b.

I just worry if any hacker out there can hack it if I kept it for long time.

[Mia Chloe]

I won't say yes but it doesn't matter much as long as your keys are not compromised you are still good and safe. That high fee will make sure that if the transaction eventually gets broadcasted it will be confirmed in no time so its more like a fail safe in case the sending wallet keys get compromised and they intend to quickly drop the transaction.
Anyways hope you know that big fee rate is also a huge disadvantage for you if the receiving wallet keys get compromised because a hacker would have the advantage of moving the funds before you even get to drop the transaction. All he would have to do is a CPFP with an even bigger fee.

[Hatchy]

You can store it in your offline device and encrypt the file so no one can access it.. but regarding the fees, I think it will be better to just sign a new transaction when you are ready and use the current fee rate at that present time.. two reason is that so you don't end up using a fee that might be lower(though 20sat/vbyte is high and will be able to process any transactions currently) or spending too much fee on a transaction.. just make sure you don't let the transaction have any access to the Internet till you want to broadcast it..

[hugeblack]

You can keep the transaction .txn file and broadcast it after a while but there is a condition which is that the transaction is valid so that there is no double spending or spending from an address with zero bitcoin or in simpler words: input addresses were not used after signing that transaction.

The best thing is to prepare a new transaction when you want to broadcast it using watchonly wallet, send it to airgapped wallet where you sign it and then return it to watchonly wallet to broadcast it.

I just worry if any hacker out there can hack it if I kept it for long time.

hacker can only broadcast it but modifying it will corrupt the signature.

[Not your key not your BTC]

I won't say yes but it doesn't matter much as long as your keys are not compromised you are still good and safe. That high fee will make sure that if the transaction eventually gets broadcasted it will be confirmed in no time so its more like a fail safe in case the sending wallet keys get compromised and they intend to quickly drop the transaction.
Anyways hope you know that big fee rate is also a huge disadvantage for you if the receiving wallet keys get compromised because a hacker would have the advantage of moving the funds before you even get to drop the transaction. All he would have to do is a CPFP with an even bigger fee.

while I still have access the key, I will bump it if fee rate change.

just make sure you don't let the transaction have any access to the Internet till you want to broadcast it..

all I did using airgapped device.

The best thing is to prepare a new transaction when you want to broadcast it using watch-only wallet, send it to airgapped wallet where you sign it and then return it to watch-only wallet to broadcast it.

It's exactly what I did. I just not broadcast it yet.

hacker can only broadcast it but modifying it will corrupt the signature.

That make me calm.

[pooya87]

Signed bitcoin transactions can and should be public, that is how Bitcoin works. The entire blockchain is made of similar signed transactions. That means as long as you are using a wallet without bugs (in this case Electrum) there is no security concerns about someone getting their hands on your signed transaction.

The only concern is that they can broadcast it and have it confirmed, as someone else pointed out.

[Not your key not your BTC]

Signed bitcoin transactions can and should be public, that is how Bitcoin works. The entire blockchain is made of similar signed transactions. That means as long as you are using a wallet without bugs (in this case Electrum) there is no security concerns about someone getting their hands on your signed transaction.

The only concern is that they can broadcast it and have it confirmed, as someone else pointed out.

I am just concerned if they can modified received address.

[pooya87]

I am just concerned if they can modified received address.

You just created your transaction using Electrum UI without manually modifying anything and then signed it, right?
In that case it is impossible to change anything in that signed transaction. Even changing a single byte in it would invalidate the signature and the tx becomes invalid (can not be mined).

The only scenarios where the receiving address (better said the outputs since that includes the amount as well) can change is if you manually changed the sighash type used for each input you were spending. Keep in mind that this is not something that can change in the UI or when you just click "spend".

There are multiple different types but the 3 that you should know of here are:

• SIGHASH_ALL (0x01) this is the default value that Electrum sets for everything. In this sighash type you are signing everything and as I explained nothing in the tx can change without making it invalid.
  
• SIGHASH_NONE (0x02) as the name suggests none of the outputs are signed which means the outputs can be changed without invalidating the transaction itself.
  
• SIGHASH_SINGLE (0x03) as the name suggests you are only signing a "single" output, the output at the same index as the input that has the signature with the said sighash type (eg. second input signs the second output). The rest of your outputs can be change without invalidating the transaction itself.