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Split a Seed Phrase (2-of-M) / Guide

Why?

While a 2-of-3 split is a good enough backup strategy, the 2-of-M generalization is sometimes needed for sophisticated use cases such as social recovery. With this method, up to 26 shares shares can be generated and any 2 of these can be used to recover the seed phrase.

Steps

The process is very similar to that of the 2-of-3 guide. This document only outlines the parts that differ.

1. Decide how many shares are needed:
This is the "M" in "2-of-M". The maximum number of shares is 26. Shares A, B and C are on the first sheet. Label each consecutive share using letters in alphabetical order starting from D. On each share, write the "2-of-__" part with the total number of shares (M).
2. Choose a group identifier:
This is the same as the 2-of-3-guide. (e.g.: "Brand Model MM/DD")
3. Write the seed phrase:
Identical to the 2-of-3 guide.
4. Generate the random share A:
Identical to the 2-of-3 guide.
5. Generate the checksums for the seed phrase and share A:
Identical to the 2-of-3 guide.
6. Derive as many shares as needed:
This follows the same process as explained in the 2-of-3 guide, except one needs to continue to derive more shares after "Share C". This process is explained in "A) Deriving Shares" shown below.
7. Verify the derived checksums:
Verify all of the shares' checksums as explained in the 2-of-3 guide, except for share A.
8. Recover the first seed word:
For first time users, recover the first seed word from any two shares. The recovery process is explained below in "B) Recovering the Seed Phrase".

A) Deriving Shares

  1. Place the pointer on the first character of the seed phrase.
  2. On the outer row, look for the first character of share A.
  3. Write the corresponding inner-row character as the first character of share B.
  4. Without moving the wheel, now look for that share B character on the outer row.
  5. Write the corresponding inner-row character as the first character of share C.
  6. Similarly, look for that share C character on the outer row to find share D character. Keep going until the first character of each share has been found.
  7. Repeat steps 1 to 6 using the second character of the seed phrase, and so on until both white and gray fields are completed for all shares.

B) Recovering the Seed Phrase

This follows the same process as explained in the 2-of-3 guide, except that the outer share, the inner share and the solving window have to be determined using the 2-of-M wheel:

  1. Pick two shares of the same group.
  2. Start with the share with the lower alphabetically ordered index. This is likely to be the outer share.
  3. Look for that share's index on the outer row.
  4. Look for the other share's index on the inner row.
  5. Turn the wheel so that both letters align.
  6. Look at the window under the "Recovery" label. If it shows a number, then it is the solving window.
  7. If it does not show a number, try again from step 2 but swap the outer and the inner share.

Now that the outer share, the inner share and the solving window have been determined, proceed as explained in the 2-of-3 guide.