BIP39 Split seed

Modified on Thu, 24 Dec, 2020 at 1:19 PM

To create a BIP39 Split seed, the idea is to manually split a 24-words seed into 3 different segments and create redundancy.

Each share contains 2/3^rd of the whole seed and is 16 words long. 8 words are missing in each share.

Initial seed	Split seed
ABC	AB \| BC \| CA
24 words	16 words \| 16 words \| 16 words

In the case of a BIP39 Split, the shares are not independent. With one share, enough information is available to know that 8 words are missing and in which position they are. The security of one share is reduced compared to a whole 24 words seed.

If someone finds a 24 words BIP39 share, the overall security comes down to 8 words to guess:

Initial seed length	BIP39 share missing words	Encryption level of one share
12 words	4 words	44 bits
15 words	5 words	55 bits
18 words	6 words	66 bits
21 words	7 words	77 bits
24 words	8 words	88 bits

To keep a reasonable level of encryption per share, HODLR Disks BIP39 Split 2-of-3 recovery scheme should never be used with less than a 24-words seed.
According to Ian Coleman (Source: https://iancoleman.io/bip39/#english), brute forcing a 24 words BIP39 Mnemonic split with only one share could take up to 3’830’854 years for a single CPU.

If you think you could be targeted by such a resourceful crypto expert thief, you should better review your security strategy from scratch and definitely use Shamir Secret Sharing security scheme.

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