Triple mnemonic

In this site created by Stepan Snigirev, anyone can generate what he called “double mnemonic”. 

Do not use that link to handle real funds. Never use seeds exposed to any online device.

Quoting Stepan:

“24-word mnemonic generated by this tool consists of two valid 12-word mnemonics.

So you can use both, or combined. Have fun and make everyone confused :)”

Starting with v1.4, Seedmate can generate this kind of mnemonic, which I considered that should be called 'Triple mnemonic' instead. Why triple? Let’s count:

This kind of mnemonic can be used to split in half a valid 24 word mnemonic into 2 valid mnemonic of 12 words, and keep them physically separated. Then you could place decoy funds on the small ones, that could be handed under duress, or can provide an early warning of data leakage of one part if funds move. To access main wallet just enter both parts in the right order (only 2 possibilities).

This technique is often criticized as a bad practice, because the attacker obtains part of your seed when reaching one part. But being 12 words each, the amount of missing information is similar to what would be missing using just a 12 words mnemonic: impossible to brute force. 

But of course, if you lose one then you lose access to your funds, it is a 2/2 scheme. Redundancy (multiple copies) is mandatory.

Now let’s compare this strategy to the standard passphrase, which is generally considered a good practice. Look at this table:

So if you compare a strong passphrase to a triple mnemonic split (good entropy):

• Higher brute force protection

• Easier to use

• Lower chance of typo error, since passphrases are case sensitive and can use special characters

• More wallet compatibility

Small disadvantages:

• Passphrases allow multiple wallet creation, using multiple passphrases

• A few entropy bits are lost: 4 from the first checksum and 4 from the last checksum. So the mnemonic would have 248 bits of entropy rather than 256. Still astronomically big (128 bits is already considered as impossible to guess)

 Triple mnemonic in Seedmate:

• create a 24 word mnemonic using the input method that you prefer, and write it down or store on an SD slot

• go to Create new seed words, triple mnemonic, and enter the mnemonic previously created or load from SD 

• Seedmate will modify it to meet checksums rules on all three mnemonics, displaying the details of the checksums and marking in green the modified words of the final list.

• Verify that only word numbers 12, 24 and possibly 23 were modified, comparing to the original mnemonic.

• Write down the 24 word list in two part, verify that 3 mnemonics (24w + 2x12w) are valid, using load function or entering them in any wallet.

• Clear SD card

• Store parts separately and redundantly.