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I have a well-protected masterseed that I have not memorized, and memorizing it won't help since it could already be compromised. My solution is to use a clever passphrase on top of that masterseed. I've been looking around in Mycelium because I read somewhere that you can appear to access your bitcoin wallet and show any bad guys that it's empty by using an incorrect passphrase. This makes so much sense that I want to do it, at least just for fun.

As I understand it, the masterseed encodes an integer which is the private key for a bitcoin address, and also a chunk of entropy that is used to generate the paths in the resulting HD wallet. When a passphrase is used, the effect is to change the masterseed so that an entirely different HD wallet is the result. So with one masterseed, you can have as many (invisible!) HD wallets as you want. The feature required is the ability to restore from the masterseed plus passphrase. Indeed, if the wallet is locked with the passphrase, then entering the wrong passphrase gives you a new wallet (and you'd have to realize on your own that you entered the wrong passphrase).

That is my dream: to be able to run Mycelium in front of a bad guy, enter my passphrase, and show that I have zero-balance. An additional feature would be the option for old transactions to show up so that it still looks real, but like everything has been spent.

I have an Android with Mycelium, and I've looked at the cold storage and backup features (and the "Advanced" add-a-key feature) and I can't find a place where I can enter my 12 masterseed words PLUS passphrase and get a new wallet. The only place I found like that is for cold-storage spending, and then it says "no funds found" (as expected). It could at least give me a btc address so I could send some funds to my special secret passphrase address.

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Yes! You can absolutely do this.

Trezor has this exact ability, on top of that you can actually use the device with your phone in Mycelium.

https://shop.trezor.io/

The setting must be activated, last firmware did not have it activated by default. This may have changed though.

  • If the Trezor is destroyed, do you have to buy a new one, or can you restore the wallet to Mycelium and then just use Mycelium? – Dave Scotese Mar 24 '17 at 21:27
  • You can restore the words to a different wallet. I am not sure which wallets can do the password on top of the seed though, you may have difficulty finding this extra feature. – MaxSan Mar 27 '17 at 10:39
  • Thanks MaxSan. I did find that feature in Mycelium, but when I try to use it, it asks me to plug in my Trezor. The only place in Mycelium where I found the ability to enter the words and a passphrase was in spending from cold storage, but it doesn't tell me the address from which it would spend, so I can't place funds in "cold storage" that I can spend that way, which is my ultimate goal. – Dave Scotese Mar 29 '17 at 3:53
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You could also load it into other clients such as Bitcoin Core if you convert the seed phrase to a WIF-encoded extended private master key (112 byte base58 string starting with 'xprv')

Not sure if it offers a way to easily do that, such as dumpwallet in core, I wouldn't just go entering it into any webpage offering to convert it for you, but it's pretty easy, you can just checkout this code https://github.com/trezor/python-mnemonic and run something like this from the python3 console (this is the seed from the unit test so, if someone did send actual funds to one of its keys, it sure wasn't me, have at it)

$ python3
Python 3.8.1 (default, Dec 21 2019, 20:57:38) 
>>> from mnemonic import mnemonic
>>> m = mnemonic.Mnemonic('english')
>>> sd = m.to_seed('bless cloud wheel regular tiny venue bird web grief security dignity zoo')
>>> m.to_hd_master_key(sd)
'xprv9s21ZrQH143K2oZcxUYy3fQGuVvxyVfs32imTuxjpVxzXMDA1phX18HGn8Ycsb1LjY9BuZmE4sqCJimiM7y6TVeh5feDcg6rHW8K5d8KNJb'

Then in Bitcoin Core, you could create a new wallet, run sethdseed true "xprv9s21ZrQH143K2oZcxUYy3fQGuVvxyVfs32imTuxjpVxzXMDA1phX18HGn8Ycsb1LjY9BuZmE4sqCJimiM7y6TVeh5feDcg6rHW8K5d8KNJb" in the console with the new wallet selected, and then set a password for it.

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