First thing is first, I am not a cryptographer and therefore this may weaken your 12-24 seed for all I know. I am not responsible for anything that goes wrong if you decide to do this.
The Trezor Model T wallet has a cool feature called shamir backup and is not available in any other hardware or software wallet that I know of but this backup is better since you can split up your words and give them to people you trust for safekeeping. I found a way to convert any 12-24 word seed into a shamir backup.
This method can save money by not needing to buy a Trezor Model T and instead buying a Ledger or a Trezor One. Or you can not buy a hardware wallet and do this with any software wallet but I recommend getting a hardware wallet.
Generate Shamir Backup
1 - Generate or take your current 12-24 seed. In this example we will use this 12 word seed
trigger glance skirt welcome spread radar blast artefact earth adult wink polar
2 - Get the word list in your language (link below) and print them out for security. Find the words from your seed in the word list and write down the number the word is in the list. For example word 1 is “abandon”, word 5 is “above” and word 1730 is “subway”. In this example if you take my seed from above this will be the numbers of each word in order.
1861 0790 1622 1995 1689 1414 0188 0104 0556 0031 2015 1340
3 - Go to a SLIP39 mnemonic shares generator (link below) or to its github (link below) and download the website and run it offline which is recommended for security. For extra security use this on an air gapped computer.
4 - Under create in the master secret field. you will put the numbers in there but the length of the master secret field has to be either 64 or 128 characters in length. There are a few ways to achieve this. Only hexadecimal characters are allowed which means only 0, 1, 2, 3, 4, 5, 6, 7, 9, 9, A, B, C, D, E, F are valid characters. Therefore it is best to add a letter character like “F” between each number and at the beginning or the end to set the character length to be 64 or 128. By using my 12 numbers above, we can put it in the field in the following ways
1861f0790f1622f1995f1689f1414f0188f0104f0556f0031f2015f1340fffff
f1861f0790f1622ff1995f1689f1414ff0188f0104f0556ff0031f2015f1340f
1861a0790b1622c1995d1689e1414f0188a0104b0556c0031d2015e1340fffff
5 - Change the total shares and threshold for how many shares you want and how many share you need to regenerate the master secret.
6 - Write down each of the shares onto a piece of paper. By using the first example above of my master secret, this is what is generated for shares when I have the total shares set to 3 and the threshold set to 2.
ugly academic acrobat leader amount shadow discuss shame script airline switch editor skin human scholar brother email pulse public best headset vanish boundary scroll necklace submit round curly traveler fawn craft group aide
ugly academic beard leader afraid invasion staff lair broken raspy dwarf quiet papa pink response rumor twin garbage ruin erode papa hazard hand twin again ranked hamster playoff burden hawk sniff always valid
ugly academic ceramic leader armed deal worthy parking stilt tidy academic single raspy charity diminish pistol angel trip family slap unfold academic twice omit punish liberty machine escape response humidity process marvel aviation
7 - Now it is highly recommended that you verify your written down shares to ensure you written down the words correctly and in the right order. Under combine enter in each share to ensure it will work in regenerating your master secret. In this example we will the shares from above which will give me this as a master secret.
1861f0790f1622f1995f1689f1414f0188f0104f0556f0031f2015f1340fffff
8 - This is optional but you can print out the BIP39 word list to put with your shamir backup. Make sure when you print it out it has the number index next to the word. This will allow you to easily retrieve your 12-24 word seed from the master secret.
Recover 12-24 Seed From Shamir Backup
1 - Go to a SLIP39 mnemonic shares generator (link below) or to its github (link below) and download the website and run it offline which is recommended for security. For extra security use this on an air gapped computer.
2 - Under combine enter in your shares. In this example we will use these two shamir backups
ugly academic acrobat leader amount shadow discuss shame script airline switch editor skin human scholar brother email pulse public best headset vanish boundary scroll necklace submit round curly traveler fawn craft group aide
ugly academic beard leader afraid invasion staff lair broken raspy dwarf quiet papa pink response rumor twin garbage ruin erode papa hazard hand twin again ranked hamster playoff burden hawk sniff always valid
3 - You will see master secret was generated which contains the numbers you need to restore your 12-24 word seed. By using the shares above this is the master secret I get
1861f0790f1622f1995f1689f1414f0188f0104f0556f0031f2015f1340fffff
4 - Remove the letters from the master secret and so you will end up with 12-24 numbers. Using the master secret above this is what I end up with
1861 0790 1622 1995 1689 1414 0188 0104 0556 0031 2015 1340
5 - Get the word list in your language (link below) and print them out for security. Use the numbers to index the words. You will convert each number to a word from the word list. In this example if you take my numbers from above this will be word seed which you need to recover your wallet.
trigger glance skirt welcome spread radar blast artefact earth adult wink polar
Links
Mnemonic word lists
https://github.com/bitcoin/bips/tree/master/bip-0039
SLIP39 mnemonic shares
SLIP39 mnemonic shares github
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Thanks for this! I wasn’t aware that you could convert a sharded seed back into a plain 12-word seed. I’ll have to give this a go.
